1. General introduction to Animal with particular emphasis on external features
  2. Histology (microscopic anatomy) (Tissues)
  3. Embryology
  4. Structures and functions of various systems of animal body
  5. Poultry anatomy and physiology

Text: (1) Anatomy and physiology of farm animals by R.D Frandson and E.H. Whitten

(2) Biology – A functional Approach by M.B.V Roberts


Definition – This is the microscopic study of cells and tissues of all living things (plants and animals).

CELL – A cell is a unit mass of protoplasm containing a nucleus. A protoplasm is surrounded by an outer membrane known as plasma membrane, plasmalema or cell membrane.

The study of cell is known as cytology (through electron microscope). A typical cell consists of four (4) main parts namely:

  1. The cytoplasm
  2. The nucleus
  3. The cell membrane
  4. Cell inclusion
A typical cell as seen with the optical or light microscope

Detailed structure of a cell is studied in cytology from which a generalized animal cell is obtained

A generalized animal cell (as seen with a n electron microscope)

Components of Animal cell

The important cellular components of an animal cell include the followings:

  1. Microsomes – These are smallest particles seen in the cytoplasm and cannot be seen with an ordinary light microscope. Some microsomes are termed ribosomes and they consist of chiefly of ribonucleic acid (RNA) and are associated with the manufacture of protein within the cell.
  2. Cytoplasm – This is the living plate that houses all other cellular inclusions. Food materials are also stored in the cytoplasm and it is there that complex chemical reactions take place, building up materials and supplying energy for the cell’s activities.
  3. Endoplasmic Reticulum (ER) – This consists of a system of tubes of flattened sac located in the cytoplasm of practically all cells. There are the types.
    1. Rough Endoplasmic Reticulum – This has ribosomes like breads along the retiala membrane which makes it to appear rough on the surface. The RER is involved with protein synthesis by virtue of its ribosomal activities and the more RER found in a cell, the greater the amount of proteins and enzymes the cell is capable of synthesizing.
    2. Smooth Endoplasmic Reticulum (SER) is involves in the syntehsising and transportation of glycogen, lipids and steroids.
  4. Mitochondria – These are organelles in which all the enzymes associated with oxidation of nutrients, C02, ATP and H20 are found.
  5. Nucleus – Within the nuclear membrane is the clear nuclear sap with a variety of bodies suspended in it. In the nuclear sap are found dense – like structures termed nucleoli. Also seen during cell division are the chromosomes. Each is composed of an extremely thin spiralised axial filaments bearing pairs of thicker and denser regions called the chromomeres.
  6. Golgi Apparatus – These function as a site of the final stages of synthesis of secretive products of the cell and their packages.
  7. Centrosome(s) – These refer to the paired centrioles located near the nucleus in all cells. They function in cell division forming the two poles of the spindle from which the asters radiate.
  8. Lysosomes – These are dark spherical bodies that are very prominent in the cytoplasm. There is enough evidence that the lysosomes contain potent enzymes responsible for splitting complex chemical compounds into simpler sub-units.

Another important function of lysosomes is that they destroy worn-out organelles within the cell.

  1. Plasma membrane – This is the actual membrane that surrounds the protoplasm. It contains three layers – the outermost layer, the innermost layer both are composed by protein, while the middle layer is a bimolecular layer of primarily phospholipids.


Definition – Tissue is defined as an aggregate of similar cells, together with any intercellular substance secreted by them, coordinated to perform one or more functions. eg. Muscle, blood, bones, tissues etc.


There are primarily four types of tissues

  1. Epithelial tissues
  2. Connective tissues
  3. Muscle tissues
  4. Nervous tissues


These tissue cover the surfaces of the animal body, line the body cavities and form the active parts of glands. They are primarily protective and often secretive in their functions.


These tissues join and pack firmly together all the parts of an organism and lasso connect the organs themselves together.


These are contractive and enable the animal or parts of it to move.


These possess the controlling and coordinating mechanism by which stimuli are perceived impulses transmitted and effectors exited to respond.


The cells of an epithelial tissue form a sheet covering a fine surface externally or internally. In an epithelial tissue there is always a very small amount of the intercellular substance hence the cells are closed together. These cells usually rest on basement membrane.

Types of Epithelial Tissue

  1. Simple Epithelium Tissues

These are usually one celled-thick. The five types of epithelial tissues mentioned above are:

  1. Simple squamous epithelial tissue – This is sometimes called pavement epithelium because the cell are very thin and flat. They are very close together and are found where a smooth surface is required in order to reduce friction eg the endothelium of blood vessels.
  2. Simple cuboidal epithelial tissue – These cells are cuboidal in shape. They are approximately equal in all dimensions. On the free surface these cells appear as close-filting hexagons, but a vertical section shows the cell as a row of squares. This type of epithelium can be found in some ducts and in passage ways in the kidneys, thyroid vesicle and sweat glands.

    Simple cuboidal epithelial tissue
    Simple cuboidal epithelial tissue
  3. Simple columnar epithelial tissue (or glandular) – The cells of these tissue are fall, and cylindrical in shape with the inner end tapeing or are arranged in somewhat like the cells in a honey comb or cartridges in a box. The nucleus are usually located at the base of the cells. They are bathed in mucous produced by globlet cells which lie among columnar cells. Examples of columnar tissue can be found in small intestine and gall bladder.

    Simple columnar epithelial tissue
  4. Simple ciliated columnar epithelial tissue

Some columnar cells have whip like protections called cilia extending from the free extremity. At intervals between the ciliated cells are mucus secreting cells which taper towards the free end.

Simple ciliated columnar epithelial tissue

Examples of simple ciliated columnar epithelial tissue include the cells lining the trachea (ie the windpipe). The cilia wave in such a manner as to move any foreign materials in the trachea toward the mouth where it can be coughed out or swallowed. Others are the epithelia along the hair passages, in the oviduct, the ventricles of the brain and spinal cord.

  1. Pseudostralified columnar epithelium – This is composed of columnar cells. However, they vary in length, thereby giving the appearance of more than one layer or stratum though is one cell thick, it is usually columnar or ciliated. Example of this epithelium is found in the trachea where the lining cells are ciliated.


These are epithelia that consist of more than one stratum or layer of epithelial cells. They are usually referred to as stratified epithelia. They include stratified squamous, stratified columnar and transitional epithelia.

  1. Stratified squamous epithelium – This forms the outer layer of the skin and the lining of the first part of the digestive tract as far as the stomach. It also lines the fore stomach of the ruminants (ie rumen, reticulum and omasum). It is the thickest and toughest of the epithelia, consisting of many layers of cells. The deepest layer is known as the stratum germination and constrains the actively growing and multiply cells. These cells are somewhat cuboidal in shape, but as they are punched toward the surface, away from the food supply, they became flattened and lifeless and are constantly in the process of peeling off. This layer of dead cells bluns very thick in areas that subjected to friction. Eg buccal cavity, esophagus and virgina.
  2. Stratified columnar epithelium – This is composed of more than one layer of columnar cells and is found living part of pharynx and salvary ducts.
  3. Transitional epithelium – This forms the lining of portions of the urinary system that are subjected to stretching. These areas include the bladder and ureters. The transitional epithelium has only three or four layers of cells. Renewal of the cells takes place at the lowest layer where mitosis is frequent the cells are small and polyhedral. The cells at the free surface are somewhat flattened to allow for the distension of organs that are subjected to stretching.

    Stratified and Transitional Epithelium


Connective tissues as the name implies, serve to connect other tissues. They give form and strength to many organs and often serve for protection and leverage. Connective tissues include the followings:

  1. Connective tissue proper
    1. Aneolar connective tissue
    2. Yellow elastic connective tissue
    3. White fibrous (collagenous) tissue
    4. Adipose tissue
  2. Skeletal tissue
    1. Bone
    2. Cartilages
      1. Hyaline cartilage
      2. Elastic cvartilage
      3. Fibro cartilage
  3. Blood

Connective Tissue Division

  1. Connective tissue proper – are characterized by the possession of considerable numbers of fibres in the in the intercellular substances which in life is a fluid or semi-fluid in nature.
    1. Areolar connective tissue – This contains a great proportion of transparent semi-fluid matrix which contains abundant mucin scartened throughout the matrix or body and numerous wavy – white fibres cemented together by mucin. These fibres largely consist of the protein collagen. They are found in the body whenever protective cushioning and flexibility are needed. For example, blood vessels are surrounded by a sheath of areolar C.T. which permits the vessels to move and yet protects them. Beneath the dermis is a layer of loosely arranged areolar C.T. fibres which attach the skin to underlying muscles. This attachment is flexible enough to permit the movement of the skin. The following cells are found in the areolar C.T.
      1. Fibroblasts – These often lie alongside the fibres they secret (ie they secret the fibres). They are flattened cells often branched with oval nucli.
      2. Histiocytes – These are wondering phagocytic cells. They may be rounded or amoeboid in shape with small spherical nucleus.
      3. Mast cells – These are small oval cells with granular cytoplasm.
      4. Plasma cells – These are also seen in areolar connective tissue.
    2. Yellow elastic connective tissue – This consists mainly of parallel yellow elastic fibres with interspersed fibroblasts and intervening boundles of very fine white fibres. It contains kinked fibres which tend to regain their original shape after being stretched. This type of C.T is seen in many ligaments, forming the bone sin the wall of arteries and the bronchioles, in the lungs and the ligamentous nuchea (a strong band that helps to support the head)
    3. White fibrous (collagenous) tissue – This consists of almost entirely of closed-packed white collegen fibre occasionally interspersed with continuous rows of fibroblasts. The bundles of white fibres are bound together by areola. They are found in locations where great strength with limited flexibility is desirable eg tendous of muscle, the duramata of the brain, the sclera and the cornea, the kidney capsule, the perichondrium of cartilage and periosteal of bones.


    1. Adipose connective tissue – This is also called fat and forms when connective tissue cells take up fat for storage as inclusions within the cytoplasm of the cell. As more fat is taken up for storage, the cell eventually becomes so filled with fat that the nucleus is puched to one side of the cell which as a result, becomes spherical.

Bones and cartilages provide firm areas for attachment of the tendous and muscles. They give a large measure of support to the body forming a framework to which soft parts of the body can be fastened and in conjunction with the muscles they enable locomotion.

  1. Cartilages – is a special type of connective tissue commonly called gristle. It is firmer than fibrous tissue, yet not as hard as bone. The nature of cartilage is due to the structure of the intercellular material found between the chondrocytes (ie cartilage cells). The 3 types of cartilage known are hyaline, elastic and fibrous.
    1. Hyaline cartilage – is the glass-like covering of bones within joints. This type of cartilage forms a smooth surface that reduces friction, so that one bone easily glides over another. The actively growing areas near the ends of long bones (ie epiphysis) also consist of hyaline cartilage.
    2. Elastic cartilage – This consists of a micture of cartilage substance and elastic fibres. This type of cartilage forms the basis of the external ear, the epiglottis, the external auditory meatus and the Eustachian tube and nose.
    3. Fibrocartilage – This consists of a mixture of cartilage and collagenous fibers which forms a semi-elastic cushion of great strength. The intervertebral discs found between the bodies of adjacent vertebrae are composed of fibrocartilage.
  2. Bone – is a connective tissue with a large amount of intercellular substances consisting of mineral salts. It is very strong and rigid tissue providing adequate protection for delicate organs such as BRAIN and the SPINAL CORD. About 15% of the body weight of an adult manmal is bone. Each bone is formed by bone-forming cells called osteoblasts. These cells produce a substance known as osteoid tissue which later becomes calcified to form bone. The bone is enclosed in a tough sheath of fibrious connective tissue called the periosteum. Underneath the periosteum is a very dense layer of compact bone, beneath with is a thicker zone of cancellous or spongy bories. In the long bone a marrow cavity is present. This is the site of formation of most of the blood corpusules.
    1. Cancellous bone – is found in the extremities of long bones where resistance to compression without excessive weight is needed. It is also found between two layers of compact bone as in the skull. This arrangement is called diptoc. The plates and spicules of bone are arranged in a manner to best resist stresses and strains imposed on the bone by weight or pull of muscus.
    2. Compact bone –is found in the shafts of long bones that consist of many laminated tubes known as Haversian system. Each Haversian system consists of one canal containing vessels and nerves surrounded by circular plates of bone forming the laminated cyclinder. These plates of bones are laid down in a centripetal fashion (ie from the periphery toward the centre). After the bone is formed, the osteoblasts became embedded within the bone substances and are called osteocytes (bone cells), rather than osteoblasts (bone-forming cells). The spaces in which the osteocyles are found are called lacunae (ie little lakes), and the spaces where the cell processes are located are called canaliculi (ie small canals).


Diagram of a bone

  1. BLOOD – Blood consists of fluid matrix (liquid portion) called plasma, a variety of cells proteins, monosaccharides (simple sugars), products of fat degradation and other circulating nutrients plus wastes electrolytes for acid-base balance and chemical intermediates of cellular metabolism. Blood is often considered to be a connective tissue because of the origin of some of its components.


  1. Red blood cells (RBC) – These are called erythrocytes. In most domestic mammals they are non-nucleated biconcave discs that contain the substance hemoglobin. The main function of erythrocytes or RBC is to carry haemoglobin, while haemoglobin in turn has the primary function of carrying oxygen from the lungs to all the tissues of the animal. At the tissue level oxygen is released to the cells, while CO2, which is produced by the cells, diffuses into the blood to be carried back to the lungs where it can be eliminated during breathing. A condition called anemia results from a loos of red cells or an insufficient amount of hemoglobin synthesis, or deficiencies of folic acid or vit B12 (or intrinsic factor) or premature degradation of the red cells, or if the cells became sickled (crescents-shaped). The term amenia refers to a reduced concentration of functional red cells in the blood.
  2. White blood cells (WBC) –These are also called leucocytes and are one of the first lines of defence of the body against infection. They include granulocytes and granulocytes
    1. Agramulocytes – are of two kinds
      • Monocytes which are large
      • Lymphocytes which usually are smaller. An excess of these cells in the body tends to be associated with chronic types of disease.
    2. Granulocytes (or polymorphomuclear leucocytes)

These are of 3 types and are classified according to their affinity for different stains.

      • Neutrophils granules. This stain indifferentialy (ie is no colour change when the granules are introduced to stain)
      • Basophils. These have dark-staining granules when stained with common blood stains.
      • Acidophils (eosinophils). These have red-stains granules with common blood stains.
  1. Blood platelets – These are small irregularly shaped particles that are associated with the clothing of the blood.
  2. Plasma –This refers to the fluid part of unclottted blood. Plasma is particularly useful as a substitute for blood in transfusion because the proteins give it the same osmotic pressure as in whole blood. Plasma, therefore, will not escape from blood vessels as readily as a salt solution will. (why? – because plasma and salt solution do not have the same osmotic pressure).
  3. Serum – This is the supernatant yellow fluid that is expressed out when blood clots and retracts. It is plasma but without most of the clothing factors. It is used for prevention and treatment of diseases because it contains the antibody fractions of the blood


The contractile fibrious band that produce motion in animal body is called muscle. There are 3 types of muscle tissues. These are

  1. Striated/skeletal/voluntary muscle
  2. Smooth/unstriated/unvoluntary/visceral muscle
  3. Cardiac/striafted/involuntary muscle
  4. Striated/skeletal/voluntary muscle – The cells of this muscle consist of long fibres which show under the microscope characteristic cross striations, many peripherally located nuclei and a cell membrane called sarcolemma. Each striated muscle fibre cell has its own nerve supply in order to contract, and when stimulated the whole fibre will contract. This is the “all-or-none” law of muscle contraction.

The essential function of the striated muscle is to cause movement of the skeleton by contraction of the fibers. This contraction exert a pull on the tendon which is transmitted to the bone by the tend on insertion thus, infecting movemention. Skeletal/striated muscle works in pairs – a flexor which causes bending at a joint and – an extensorwhich causes straightening. However, the force of contraction depends on the state of the fibers at any one moment. Is it already fatigued? Warmed up? (supply low, is it stretched). Striated muscle tissue plus some connective tissues make up the flesh of meat producing animals.

  1. Smooth/unstriated/unvoluntary/visceral muscle – The cells of this muscle are spindle-shaped containing one centrally located nucleus per cell. They are found in the walls of the digestive tract, in the walls of blood vessels and in the walls of urinary and reproductive organs. These cells contract more slowly than striated muscle and respond to a variety of stimuli. The main function of the smooth muscle is that of opening and closing of cavities. For example, in the gut, they cause the narrowing of the cavity through the circular muscles while the longitudinal muscles open the gut cavity.
  2. Cardiac/striated/unvoluntarily muscle – It is also called unvolutnary striated muscle because it is not usually under conscious control, yet it does have cross striation. The heart is composed of a complex, branched arrangement of cardia muscles cells. This consists of cylindrical, short cells arranged in columns. Each cell has a central nucleus, myotibrils and faint transverse striations. Adjacent columns are joined by many oblique cross connections. In structure, it is an intermediate between voluntary and involuntary muscles.

Muscle Tissue


(striated, voluntary)


(unstriated/involuntary visceral

Cardiac (striated involuntary)

Small piece section of cardiac muscle


The cells which constitute the Nervous tissue always show branching protoplasmic processes some of which may be very long.


Nerve cell (neurone)


Cell body

Nerve fibre

Neurons/ (Nerve cells) – These are the actual nerve cells each always have at least 2 branched protoplasmic processes one those called the AXON which carries impulses away from the cell body. The other process is called DENDRITES which carries impulses into the cell body.

NERVE FIBRES – There are of 2 types

  1. The myelinated and
  2. The non-myclinated fibres

In both types of fibre the axis cylinder is an essential component

Myelinated fibres – These have a central core the axis cylinder is surrounded by a white lipid myelin sheath and enclosed in a delicate membrane called NEURILEMMA. The axis cylinder is a continuation of the cytoplasm of the Neurone. The myelin sheath is continuous with the central nervous system and is interrupted in the peripheral fibres by constriction called “the Node of Ranvier” .

Non myelinated nerve fibre – There is no myelinated sheath covering the axis cylinder, but only the axis cylinder enclosed by the neunclemma and connectictiice tissue is present.

NERVES – A nerve consists of a bundle of fibres bound up by a connective tissue sheet called the epineurium.

T/s of a whole nerve

FIBRES – Are divided into smaller bundles each enclosed by a perineurium, each covered by endoneurium. In the connective tissues are found blood vessels and lymphatic vessels. Nerves are described according to the direction in which impulses are conducted by the fibres. Nerves containing only fibres which carry impulses into the CNS are called sensory or afferent (SAN) in nerve. Those which contain fibres carrying only outward impulses are called motor or efferent (MEN) – out nerves many of the nerves have mixed nerves of afferent and efferent fibre.



Gametogenesis is the formation of the garmetes which takes place in the gonads (ie the reproductive organs). Game to genesis consists of two processes.

      1. Spermatogenesis – The production of sperm by the festis in males.
      2. Oogenesis – which is the production of eggs by the ovary in females.

Cells in a particular region of the gonad divide repeatedly. The daughter cells are formed, grow, divide again and the differentiate into the appropriate gametes eggs or sperm as the case may be. The original cells, called the primordial germ cells are diploid, but the gametes are haploid, thus gametogenesis involves meiosis.

Spermatogenesis – In spermatogenesis the premordial germ cells divide repeatedly by mitosis to form diploid spermatogonia. There then follows a brief period of growth during which each spermatogonium increases in size to form a primary spermatocyte. This then divides meiotically, the first meiotic division giving two secondary spermatocytes, and the second a group of four spermatids. Finally, each spermatid, at this stage a seemingly unspecialized cell, differentiates into a spermatozoon with characteristic head and tail. Clearly a large number of spermatozoa are formed from a single primordial germ cell.

Oogenesis – Is basically similar to spermatogenesis. A primordial cell proliferates mitotically to form oogonia but only one of these grows into a primary oocyte. The others degenerate the amount of growth that takes place at this stage is much greater than in spermatogenesis, this is what makes the egg so much larger than the spermatozoa. The primary oocyte now undergoes meiosis, but the divisions are unequal resulting in the products differing gr4eatly in size. The first meiotic division produces a secondary oocyte and a very much smaller polar body which may be seen adhering to it. The second meiotic division results in the production of another polar body. The secondary oocyte now becomes the functional egg or ovum, with the haploid number of chromosomes. Meanwhile the polar body may undergo a second meiotic division resulting in a total of 3 polar books which are all haploid. The events in the formation of oocytes are essentially similar to the formation of spermatozoa, but whereas in spermato genesis all 4 products of meiosis develop into functional genetics in oogenesis only one of the 4 bluns a functional egg. The rest are extruded as non-functional polar bodies which eventually degenerate.

The polar bodies are the inevitable result of the fact that meiosis involves two successive divisions, their function is simply to receive half the chromosomes, thereby making the ovum haploid.

Spermatogenesis Oogenesis

Growth Growth

2nd – meiotic division (usually occurs immediately after fertilization

10 – oocyte

1st – meiotic division

1st– polar body

Spermatids differentiation


2nd – meiotic division

1st – meiotic division

Male Reproductive Organs of farm Animals

  1. Testis – (testicles) vary from species to species as far as shape, size and location are concerned, but the essential structure is the same. They are contained in a sac called the scrotum. They lie outside the abdominal cavity of the male animal. Each testis consists of a mass of seminiferious tubules surrounded by a heavy fibrous capsule called the tunica albuginea. A number of fibrous septa or trabeculae pass inward from the tunica albugines to form a framework or stroma, for support of the seminiferous tubules. In all domestic animals except the horse, these trabeculee unite near the centre of the gland to form a fibrous cord, called the mediastinum testis. The cells of Leydig, which secrete the male hormone testosterone, are located in the connective tissue between seminiferious tubules.
  2. Epididymis –The spermatozoa pass from the seminiferoius tubules by way of the vasa efferentia to the head of the testis with the basdeferensa or ductus deferens. The head of the epididymis attaches to the same end of the testis that the blood bessels and nerves enter. The body of the epididymis parallels the long axis of the testis and the fail continues as the ductus deferens, which doubles back along the body of the epidymis to the region of the head, where it enters the spermatid cord. The epididymis serves as a place for spermatozoa to mature prior to the time they are expelled by ejaculation. Spermatozoa are immature when they leave the testicle and must undergo a period of maturation within the epididymis before they are capable of fertilizing ova.
  3. Ductus deferens (Vas deferens) – Is a muscular tube which, at the time of ejaculation, propels the spermatozoa from the epididymis to the ejaculatory duct in the prostatic urethra. The ductus deferens leaves the tail of the epididymis, passes through the inguinal canal as a part of the spermatic cord, and at the internal ungenial ring turns caudal, separating from the vascular and nervous parts of the cord. As the two ductus deferentia approach the urethra, they converge and continue caudal dorsal to the bladder enclosed in a fold of peritoneum, the urogenital fold which is comparable to the broad ligaments of the female.
  4. Spermatic cord – As the testis descends from the region caudal to the kidney, it brings with it the same blood, nerve and lymphatic supply present in the embryo. These structure, that is, the testicular vessels and nerves, make up a large part of the spermatic cord, which connects the testis with the rest of the body. The spermatic cord also includes the ductus deferns, which connects the fail of the epididymis with the prostatic urethra (the portion of the urethra surrounded by the prostate gland). A few smooth muscle fibers scattered throughout the spermatic cord make up the internal cremaster muscle which helps to hold the other structures of the cord together.
  5. Scrotum – The scrotum is a cutaneous (skin) sac that conforms in size, shape and location to the testes it contains. The scrotal skin is thin, pliable, and relatively hairless. A layer of fibroelastic tissue mixed with smooth muscle fibers, called the tunica dartos, is immediately deep to the s kin and in the cold weather the muscle fibers of the dartos contract and help hold the testes against the abdominal wall. The turnica dartos passes on the median plane between the two testes to help form the scrotal septum, whichi divides the scrotum into two lateral compartments, one for each testicle. Between the tunica dartos and the underlying deep fascia is a thin layer of areolar connective tissue or superficial fascia.

There are 3 layers of deep fascia which are difficult to separate by dissection. The are presumably derived from the aponeuroses of the 3 abdominal muscles. The external oblique muscles, the internal abdominal oblique muscle and the trransversus abodminis muscle.

The outer layer of peritoneum covering the testis the tunica vaginalis communis (parietalis) is deep to and blends with, the deep fascia of the scrotum. The scrotal ligament is derived from the gubernaulum. It is a band of connective tissue extending from the tail of the epididymis to the scrotum.

  1. Penis – This is the male organ for copulation. It is divided into 3 general areas –
  • The glands or free extremity
  • The main portion or body and two crura or roots which attach to the ischial arch of the pelvis

The internal structure of the penis is caveruous tissue (erectile tissue) consisting of blood simisoids separated by sheets of connective tissue called septa, which are derived from the tumica albuginea, a heavy fibrous capsule surrounding the penis. The crura (roots) of the penis originate on the caudal surface of the ischial arch one on each side of the symphysis of the pelvis. They converge to form the corpus penis (body of the penis). Ventral to the body is the urethra surrounded by the corpus spongiosum penis (corpus cavernosum urethrae). The corpus spongiosumpenis is a continuation of the erectile tissue of the bulbus penis (urethral bulb) which is located between the roots of the penis. In some animals the corpus spongiosum penis continues forward to become the erectile tissue of the glans penis.

The glans penis is variable from species to species. The horse and sheep have a free portion of the urethra, the urethral process which projects beyond the glans. The bull and ram have a helmet shaped glans called the galeaglandis, and the external urethral opening of the bull opens into a twisted groove.

Female Reproductive System

Reproduction is the female is a complex process that involves the entire body of the animal.e the followings:

  1. Two ovaries
  2. Two uterine (fallopian) tubes
  3. The uterus
  4. The vagina and
  5. The vulva

The ovum (or egg) is expelled from the ovary and is received by the infundibulum and is carried to the ulterine tube, where fertilization normally occur during the passage of the ovum from the ovary to the uterus. The fertilized ovum develops into an embryo within the uterus, and then into a fetus, and finally passes out of the uterus through the vagina and vulva as a new born animal.

Ovaries – The ovaries are the primary (or essential) organs of reproduction in the female, just as the testes are in the male. The ovaries may be considered to be both endocrine and cytogenic (cell producing) in nature; since they produce hormones, which are absorbed directly into the bloodstream, and also ova, which are expelled from the gland.

The ovaries are paired glands consisting of a right ovary located behind the right kidney, and a left ovary located behind the left kidney. Their distances from the kidney vary with species. In most species the ovaries are somewhat almond-shaped structures. When palpated through the wall of the rectum an ovary feels solid because of the large amount of connective tissue that makes up the stroma of the gland.

Normal size of the ovary varies considerably from species to species, even within a species there is some variation. For example, the ovary of a young mare may be less than 0.39cm in diameter when no cysts are present or as large as 1.57cm in diameter with the presence of numerous cysts.

The medulla or central portion of an ovary (zona vasculosa) is the most vascular part, while the majority of the cortex or the outer portion (zona paremchmatosa) consists of dense irregular connective tissue interspersed with parenchymal epithelial cells that have migrated from the surface. The outer layer of cortex is a dense connective tissue capsule called tunica albuginea. The outermost surface consist in the fetus, of a single layer of germinal epithelium, the primary sex cells.

Cords of germinal epithelial cells invade the stroma of the ovary and eventually form isolated clumps of cells known as primary follicles. One large cell in each follicle is an oocyte or ovum surrounded by a single layer of follicular cells. Ova in primary follicles increase in size, and the follicular cells multiply into several layers, forming maturing follicles. A thick membrane, the zona pellucida, appears between the ovum and the inner layer of follicular cells of the maturing follicle. As soon as a fluid-filled cavity, the antrum, appears within the mass of collicular cells, the follicle may be called a Graafian follicle or a vesicular follicle, and the layer of follicular cells is called the stranum granulosum (A double layer of cells from the stroma of the ovary surrounds the stratum granulosum, forming the theca folliculi. (1) the tunica interna or (theca interna) is cells. It is layer of irregular shaped cells resembling epithelial cells. It is believed to be the source of estrogens (the female sex hormones) found in the follicular fluid. (2) the tunica externa (theca externa) is a layer of connective cells that blends on its inner surface with the the cainterna and on it outer surface with the stroma of the ovary).

Some of the membrane granulosa cells form a mound surrounding the ovum. The mound is called the cumulus oophorus (germ hill or discus proligerus). Its inner most layer – the corona radiata consists of cylindrical follicular cells arranged in a radial manner over the entire surface of the zona pellucida. Cells of the corona radiate send processes through the zona pllucida to the vitelline membrane (the cell emberane of the ovum and presumably supply yolk material to the egg. The vesicular (ie Graafian) follicle, as continue to increase in size, where in some species it can be palpated or observed as a cyst-like bulge.

Monotocous animals (ie animals not bearing litters such as the horse and cow, normally have only one offspring pergestation. At each heat period, one follicle usually develops more rapidly than the others so that when it ruptures, only one ovum or egg is released, and the rest of the follicles then regress or reduces and form atretic follicles.

Polytocous animals such as carnivores and swine which normally produce two or more offspring pergestation, usually have several follicles rupture at approximately the same time. The ova may all come from one ovary, or some may come from each ovary. The immediate cause of rupture of the follicle at ovulation is not known yet. In most species it appears to occur as a slow oozing process that may result from death of cells in the follicular wall.

Immediately following the ovulation, the follicular cavity fills with a variable amount of blood and lymph forming a structure called the corpus hearmorrhagicum. It is relatively larger in swine than in sheep and cattle (ruminants). The corpus heamorrhagicum is gradually reabsorbed and replaced by a corpus luteum.

Granulosa (follicular) cells multiply rapidly to form the major part of the corpus luteum, but some cells are derived from the theca interna. The corpus inteum has a yellow colour in the mare, cow and carnivores, but is grayish white or flesh –coloured in the ewe and sow. The corpus inteum decreases in size and eventually leaves a whitish scar the corpus albicans, as a ruminant on the surface of the ovary.

Uterine tube – The uterine tubes are (also called oviducts or fallopian tubes) are paired, convoluted tubes that conduct the ova from each ovary to the respective horn of the uterus and also serve as the usual site for fertilization of ova by spermatozoa. The portion of the uterine tube adjacent to the ovary is expanded to form a funnel-like structure called the infundibulum. The fring-like margin of the unfundibulum is called the fimbra. The fimbra appears to take an active part in ovulation at least to the extent of partially or completely endosing the ovary and directing the ovum into the abdominal opening of the uterine tube. The lining of the uterine tube is a highly folded mucous membrane that is covered mainly with simple columnar ciliated epithelium. During heat and before parturition, the nonaliated cells become actively secretory. The rest of the wall of the uterine tube includes a connective tissue submucosa, an inner circular smooth muscle layer, and superficially, a layer of connective itssue covered with peritoneum. Both the cilia and muscles function in the movement of ova and possibly in the movement spermatozoa.

Uterus – The uterus of the domestic mammal consists of a corpus (body), a cervix (neck), and two horns of corpus. The relative proportions of each vary considerably with the species, as do the shape and arrangement of the horns. The corpus (body) of the uterus is largest in the mare, less extensive in the cow and sheep and small in the pig and dog. Superficially, the body of the uterus of the cow appears relatively larger than it actually is because the caudal parts of the horns are bound together by the intercommual ligament.

Vagina – is that portion of the birth canal that is located with in the pelvis between uterus cranially and the vulva caudally. The vagina also serves as a sheath for acceptance of the penis of the male during copulation (ie the act of breeding or service). The mucous membrane of the vagina is glandless, stratified squamous epitheuum.

Vulva – The vulva (pudendum fenininum) is the external portion of the genitalia of the female that extends from the vagina to the exterior. The junction of the vagina and vulva is marked by the external urethral orifice or a ridge called hymen which can interfere with copulation if complete enough. The cliton’s is located ventral to the vulva and has the same embryonic origin as the penis in the male. It consists of two roots (1) a boy (2) an a glans. It is made up of erectile tissue covered with stratified squamous epitheluum and is well supplied with sensory nerve endings.


Definition – Embryology which is also called developmental anatomy is the study of the early prenatal (before birth) development of an animal. It begins with the fertilization of the ovum by a spermatozoan to form a zygote which in turn became a morula, a blastula, a agastrula and than an embryo. Strictly speaking, the period of the embryo terminates when the various organs and organ systems are formed. The embryo then became a fetus that more or less resembles and adults of the same species.

In cattle, the embryo became a fetus approximaltey at the end of the second month of gestation (conception). The fetus became a new born animal (neonate) at parturition (ie birth).

The ovum contains a large amount of nutritional material (ie yolk) that provides energy for the early stages of cell division. The ovum and spermatozoan each contribute one-half (1/2) of the chromosomes to the newly formed zygote.

When the mammalian morula reaches the uterus, it became a blastula (or blastocyst) consisting of many cells; each termed a blastomere. The blastula is a hollow ball consisting of layer of cells, the trophoblast which surrounds the blasts code, a cavity into which the inner cell mass protrudes (from the trophoblast). The inner cell mass eventually forms the body of the embryo. In this process 3 germ layers are produced.

  1. The Ectoderm – (ie the outer skin) develops from the outer cells of the inner cell mass and is continuous with the trophoblast.
  2. The Endoderm –(inner skin) grows into the blastocele just deep to the trophoblast to form the archentcron or primitivegut.
  3. The Mesoderm – (middle skin) grows between the ectoderm and endoderm and splits into two layers forming a cavity – the celon between the two layers.

The outer layer of the mesoderm and the adjacent ectoderm make up or form somatopleure (muscle portion) which forms part of the body wall and also enters into the formation of the fetal membranes. The inner layer of the mesoderm and endoderm form the splachnopleure (visceral protion) which forms the wall of the gut.

The dorsal surface of the embryomic disc (ie the forerunner of the embryo) develops an elongated thickening called the primitive streak, and a rod-shaped mass, the notochord nlesoderinal cells on each side of the notochord form the segmentally arranged somites – which in turn develop into vertebrae and muscles. Other areas of mesoderm produce urogenital organs and blood vascular organs.

The ectoderm above the notochord forms a groove that blums the neural tube which eventually forms the central nervous system. The epidermis of the skin is also derived from the ectoderm.

As these changes are occurring, the trophoblast blums elongated and attached to the lining of the uterus, where it absorbs nutrients from the uterine glands. The actual fetal membranes develop later.

Differentiation of the relatively indifferent cells of each of the 3 germ layers to form specialized tissue cells is called histogenesis. Much is known about when and where various tissues and organs develop, but little is known about why these changes take place. But it has been established that the DNA (Deoxy ribonucleic acid) of the nucleus in the form of chromosomes contains the genetic information that influences development of ill the various parts of the animal.

In general, the ectoderm forms the outer epithelium and nervous system, the endoderm forms the lungs and gut epithelium and its derivatives and the mesoderm forms muscles connective tissues blood and most of the urogenital system (ie organs that are associated with the urinary system and reproductive system).

Diagram showing the cleavage and development of the embryo in a mammal



Pregnancy refers to the condition of a female while young are dev loping within her uterus. This interval, the gestation period, extends from fertilization of the ovum to the birth of the offspring. It includes fertilization, or union of the ovum and sperm; nidation, or implantation of the embryo in the uterine wall; placentation, or the development of fetal membranes; continued growth of the fetus.

Normal gestation periods vary greatly from species to species, and there is considerable variation between individuals within each species. Average gestation periods are: mare 336 days, about months; cow, 282 days a little over 9 months; we, 150 days, about 5 months; sow, 114 days or 3 months, 3 weeks, and 3 days; and bitch, 63 days, about 2 months. If the young are carried throughout a normal gestation period, it is a full-term pregnancy. Abnormally early termination of pregnancy is called abortion, pr premature birth. In domestic animals, premature birth is nearly always fatal to the fetus.

Penetration of the ovum by the spermatozoon stimulates formation of the second polar body (see Chapter 26. p. 400) and also contributes one-half of the chromosomes of the new individual. Physical or chemical stimuli can also stimulate division of the ovum. The mechanism of sperm penetration is still in doubt, as is the reason why only one sperm usually penetrates the ovum. However, occasionally several spermatozoa are found in the perivitelline Space (between vitelline membrane and zona pellucida), and sometimes more than one is found inside the ovum. Penetration by more than one sperm is abnormal.

Time of fertilization appears to be important. Sperm must remain in the female reproductive tract, the uterus, or the uterine tube, for a certain period in order to fertilize ova effectively. This is called capacitation of the spermatozoa. Capacitation involves a partial breakdown of the outer acrosome and plasma membranes, so that acrosomal enzymes can be released. The enzymes, in turn, can penetrate the zona pellucida. Capacitation also activates the metabolic activity of the sperm cell by increasing the cell‘s rate of glycolysis and increasing its oxidative metabolism. Capacitation begins in the uterus and is complete in the oviduct.

Both ciliary action and muscular contractions are involved in movement of the fertilized ova through the uterine tubes into the uterus.

Polytocous animals, those giving birth to several offspring at one time, have a definite spacing of the blastocysts (developing embryos) in the uterus. It has been suggested that the implantation of one blastocyst in some way produces a surrounding refractory area in the endometrium that inhibits further implantation in the immediate vicinity. There is some evidence that the embryos near the uterine tubes are slightly more advanced in development than those near the cervix. Blastocysts of the rabbit are evenly distributed in the uterus by seven days after mating. Uterine contractions probably are involved in movement of the blastocysts, as there is no evidence that they move in any but a passive manner.


The fimbria of the oviduct are in close association with the ovary, and upon ovulation, the ovum enters the infundibulum of the uterine tube. The ovum is then transported down the oviduct into the uterus by the combined action of the cilia on the mucosal surface of the epithelial cells, and by contractions occurring in the muscular walls of the uterine tube. The contractions, in turn, are influenced by (1) the ratio of the hormones, estrogen and progesterone (high estrogen levels increase contractions). (2) the level of prostaglandins present, and (3) the degree of stimulation of the oviduct by the sympathetic division of the autonomic nervous system. The ova remain viable in the oviduct for about 12 hours. During this time fertilization can occur, and it normally takes place in the ampulla of the oviduct.

Following ovulation, pregnancy cannot occur normally without penetration of the ovum by a Spermatozoon, within the time period that the ovum remains viable. Ova in unmated animals gradually lose their covering of cells and begin to disintegrate as they pass down the uterine tube. Ova that stay in the uterine tubes too long before exposure to spermatozoa may have lowered fertility and possibly result in a high percentage of abnormal embryos and birth defects.

In the mare and the bitch, the ova enter the oviduct as primary oocytes and then mature in the oviduct. On the other hand, in the cow, ewe, and sow, the ovum has already formed the first polar body and is in metaphase of the second maturational division at the time of ovulation. The polar body may be found in the perivitelline space between the ovum and the zona pellucida until it disintegrates. The second division occurs after fertilization by a spermatozoon.

Immediately following ovulation, the ovum within the vitelline membrane (cell membrane of ovum) is surrounded by a heavy mucopolysaceharide membrane, the zona pellucida, and by a variable amount of granulosa cells that make up the corona radiata outside the zona pellucida. The zona pellucida is believed to be a product of the innermost layer of granulosa cells (corona radiata), which was a part of the cumulus oophorous of the follicle. Microvilli from the vitelline membrane of the ovum penetrate into the zona pellucida, as do processes from the follicular cells. The zona peliucida is believed to be semipermeable membrane that helps protect the ovum. In some instances a coat of mucus is applied to the ovum outside the zona pellucida as it travels down the oviduct. This may aid attachment of the ovum to the uterine wall later.

There is a great variation in the amount of cumulus cells surrounding the zona pellucida in different animals. A well-defined corona radiata has been described in the dog and some other animals, but is absent in the cow, sheep, pig, and horse.

Following capacitation and contact with the ovum, the spermatozoon can penetrate the zona pellucida. The cumulus surrounding the ovum has already broken down shortly after ovulation in the cow, mare, ewe, and sow. Penetration by the sperm is accomplished by the acrosomal enzymes, acrosin, which is a trypsin-like enzyme, and hyaluronidase, which breaks down the hyaluronic acid-protein matrix of the zona membrane.

The spermatozoon, upon penetration, then attaches to the vitelline membrane. This stimulates the second meiosis of the ovum to proceed, and results in the formation of the second polar body, at which time the ovum chromosomes form the pronucleus (Fig. 27-1). The head of the sperm then enlarges, becoming the male pronucleus. The two pronuclei then come together and fuse membranes, forming one cell, with the chromosomal genetic DNA material of both male and female now combined in it. The new cell is ready for cleavage and formation of the blastocyst.

Only one sperm usually succeeds in penetrating the ovum. After that, others are prevented from entering by changes in the tuna pellucida that resist polyspermia. The osmosis that follows inward, tends to force the zona away from the ovum, which makes it more difficult for another sperm to enter. More sperm will attach to the outer zona surface, and some sperm are occasionally found in the perivitellme space (between the vitelline membrane and zona pellucida), but they usually cannot penetrate into the ovum. It is not yet known specifically what makes the membrane impenetrable once one spermatozoon has entered, but it is obviously beneficial to reproduction, because the extra genetic input would otherwise destroy the new zygote.

Fig. 27-1. Different regions of a single pig ovum, B hours post coitum. (x 450). A shows a nonfertilizing sperm in the zona pellucida; B shows the penetrating sperm which has entered the vitellus (note its swollen head): C shows the nuclear apparatus of the ovum, with the recently formed nucleus of the second polar body to the left, and the presumptive female pronucleus (arrowed) to the right. (Hafez, Reproduction in Farm Animals, Lea Febiger.)


Following fertilization, the new zygote is transported down the oviduct into the uterus, as the blastocyst is being formed. In the cow and ewe, the new embryo reaches the uterus four days postestrus. Entry occurs slightly sooner in the sow, and slightly later in the bitch. The rate of transport varies with the levels of endocrine secretions, particularly the estrogen-progesterone ratio. In the cow, it takes about eight days to reach the blastocyst stage, whereas it is only about six days in the sow, ewe, and goat.

Implantation (nidation) is the process whereby the new embryo becomes established at a developmental site on the endomemtirum in the uterus, where it will then develop and become a fetus. Until implantation occurs, the cell divisions and growth are provided with necessary nutrients by the ovum yolk and by secretions from the uterus. Upon implantation, the nutrients are supplied henceforth through the placenta.

The zona pellucida is shed by the blastocyst prior to implantation, and the uterine endometrium proliferates and becomes more vascular and secretory in preparation for nourishing and accepting the embryo. Both physical and chemical factors are involved in this stimulation of the uterus to accept the embryo. The connective tissue beneath the endometrium appears to respond first to the presence of the blastocyst, even before the endometrium does.

After fertilization, implantation occurs in the sow about 11 days, in the bitch about 15 days, in the ewe about 16 days, in the cow about 35 days, and in the mare about 55 days.

The relationship between the endometrium and the developing fetal membranes is complex and varies from one species to another. Some of these differences are discussed on page 421.

Failures of Reproduction

Reproductive failures in domestic animals from fertilization failure and embryonic death at approximately 50% of the potential production of bred animals. Results reported with a group of normal sows were: loss due to fertilization failure and tubal obstruction, 2.3%; loss from no apparent reason. 9.5%; embryonic death, 32.4%; and young born at term (no loss), 55.8%. A group of repeat breeding cows, those that did not conceive on the first service, showed a much poorer survival percentage: ovulation failure and tubal obstruction. 6.0%; no apparent reason. 39.3%; embryonic death. 32.5%: and normal embryos at 34 days, 22.2%. In cattle, chances of successful conception decrease rapidly with each rebreeding. Possible causes of embryonic death include inherited lethal factors, infections, nutritional deficiencies, disturbance of endocrine functions, and defects in the egg or sperm before fertilization.


As the embryo increases in size, the process of diffusion, which nourishes the zygote, becomes inadequate to maintain life and continued growth. The extra-embryonic membranes, or placenta, develop as a means of meeting this increasing need for more nutrition. This process is known as placentation.

The placenta consists of an arrangement of membranes such that nutrition from the dam can reach the fetus, and in turn, waste products from the fetus can be excreted by the dam. In domestic animals, the terms fetal membranes and placenta are used interchangeably, although technically the fetal membranes are known as the fetal placenta. In some species a portion of the endometrium is shed at parturition. This is called the maternal placenta, or decidua. The fetal placenta includes the chorion, allantois, amnion, and vestigial yolk sac (Fig. 27-2).

The chorion, the outermost membrane, is in contact with the maternal uterus. The amnion is the innermost membrane, closest to the fetus. The allantoic sac, a space formed by two layers of allamois, located between the amnion and chorion, is sometimes called the first water bag, it is continuous with the anterior extremity of the bladder by way of the urachus, which passes through the umbilical cord. The outer layer of allantois is fused to the chorion by connective tissue, and the inner layer of allantois is fused to the amnion. The amniotic sac, which immediately surrounds the fetus, is sometimes called the second water bag.

Fig. 27-2. Fetus of horse within the placenta. The chorion and allantois make up the choriollantois, often called the chorion. (After Witschi, Development of vertebrates, W.B. Saunders Co).

Fig. 27-3. Types of placentas. (by permission of Professor E.C. Amoroso, F.R.S) a Epitheliochorial-diffuse, from a sow; b, epitheliochroial-cotyledonary, from a cow; C, hemochorial-zonary, from a cat. (Nalbandov, reproductive physiology, courtesy of W.H. Freeman and Co.)

The terms “‘first” and “second” water bags refer to fetal membranes at the time of parturition, when the allantoic sac is expelled first and the amniotic sac, second.

Branches of the umbilical arteries and veins are located in the connective tissue between the allantois and chorion. These vessels are an important part of the fetal circulation. The umbilical arteries and their branches carry unoxygenated blood from the fetus to the placenta, and tributaries of the umbilical vein carry oxygenated blood from the placenta to the fetus.

As a general principle, blood from the fetus never mixes with blood from the dam. However, the two circulations are close enough at the junction of chorion and endometrium so that oxygen and nutrients pass from the maternal blood to the blood of fetus, and waste products pass from the fetal blood into the blood stream of the dam. The exact relationship of the blood vessels of the fetus to the blood vessels of the dam depends on the species and the type of placenta involved. (Figures 27-3 and 27-4 show various types of placentas.)

Fig. 27-4. Epitheliochorial placenta of cow (left), ewe (middle) and mare (right), villi from chorioallantois (black) invade crypts in maternal uterine epithelium (stipled). The apposition of maternal and fetal tissues is diffuse (mare) or localized as placentomes (cow and ewe), each placentome is composed of fetal cotyledon and maternal caruncle. (Adapted from Mossman, 1937) (From Hafez, E.S.E. Reproduction in farm animals. Ed. 3. Philadelphia, Lea & Febiger, 1974)

In some rodents, the blood vessels of the fetus enter pools of maternal blood, so that only the endothelium of the fetal blood vessels separates the two circulations. This type of placental arrangement is called hemoendothelial. (See Table 27-l.)

Thehemochorial type of placenta is found in man and certain lower types of rodents. Here not only the fetal vessels but also the chorion of the fetal placenta is invaginated into pools of maternal blood. Both hemochorial and hemoendothelial placentas are usually attached to the uterus in a diskshaped area only. Hence the term discoidal is used to describe their general area of at tachment. Carnivorous animals such as the cat and dog have the chorion of the fetal placenta in contact with the endothelium of the blood vessels of the dam. This type is known as the endotheliochorial and is attached in a girdle-like band, so the attachment is known as zonary.

Both the discoidal and zonary types of placental attachment are deciduate in nuture, since a portion of the maternal en dometrium, or maternal placenta, is shed at the time of parturition.

Table 27-1: Classification of mammalian placenta based on tissue layers separating maternal and fetal blood

Placental Microscopic structure
Type Maternal tissue Fetal tissue Cross form Species
Epithellochorial C:\Users\u\Desktop\Pictures\PQdXQQWQe4FMP4BVAbEBBE3Y.jpg Diffuse Pig, horse
C:\Users\u\Desktop\Pictures\PQdXQQWQe4FMP4BVAbEBBE3Y.jpg Cotyledonary or multiplex Sheep, goat, cow
Endotheliochorial C:\Users\u\Desktop\Pictures\PQdXQQWQe4FMP4BVAbEBBE3Y.jpg Zonary Dog, cat
Hemochorial C:\Users\u\Desktop\Pictures\PQdXQQWQe4FMP4BVAbEBBE3Y.jpg C:\Users\u\Desktop\Pictures\PQdXQQWQe4FMP4BVAbEBBE3Y.jpg Discoid Man, mouse, rat guinea pig, rabbit
Hemoendothelial Discoid Rabbit

*A syncytium is present between trophoblast and maternal connective tissue as observed under the electron microscope. The placentas of sheep, goats and cows are therefore classified as epitheliochorial rather than syndesmochorial (Bjorkman, 1965, J. Anat. 99, p. 283).

(Adapted from Amoroso, 1952, In Parkes, A.S. (ed). Marshall’s physiology of Reproduction. New York, Longman) (from Hafex, E.S.E Reproduction in farm animals. Ed. 3, Philadelphia, Lea & Febiger, 1974)

The rest of the domestic animals have indeciduate placentas, in which little or no maternal tissue is lost at parturition.The placental attachment of ruminants is known as the epitheliochorial type, and the area of attachment is cotyledonary in nature. In this type, the chorion of the fetus is in direct contact with the epithelium of the uterus of the dam. This relationship, however, is localized in mushroom-like areas of attachment known as caruncleses. These caruncles project inward form the surface of the uterus approximately ½ inch and vary in size from ½ to 4 or more inches in diameter. The term cotyledon is used correctly in reference to the portion of the fetal placenta that attaches to the caruncle. Placentome refers to the combination of maternal caruncle and fetal cotyledon.

The size of caruncles increases as pregnancy progresses, and the caruncles are larger in the gravid (pregnant) horn than in the nongravid horn. The epithelial surface of the caruncle is covered with crypts into which the villi of the fetal placenta project. The area between the caruneles is completely devoid of any attachment between the fetal placenta and the maternal uterus. The shape of the caruneles in the sheep is slightly different from those of the cow. In the sheep, the elevations contain a rather large central depression, which is the only portion of the caruncle to contain crypts for the attachment of the chorionic villi.

The placental attachment in both the horse and pig is diffuse, or villous, in nature. Chorionic villi, which cover much of the fetal placenta, project into crypts scattered over the entire endometrium of the uterus. The histologic arrangement is epitheliochorial, in which the chorion of the fetal placenta is in contact with the epithelium of the endometrium of the uterus.

Pregnancy Diagnosis

The knowledge of whether a breeding female is pregnant or not is important to a livestock breeder. There are criteria that may help to determine whether a female is pregnant and how long she has been pregnant. These criteria include absence of estrus (heat), change of contour of the abdominal wall, palpation of the internal genitalia through the rectum, ballottement (palpation) of fetus through the abdominal wall, x-ray pictures, and biologic tests.

Absence of Estrus. If accurate records of estrus periods and breeding dates are kept, the earliest indication of pregnancy in most animals is the failure to come in season at the time of the next expected heat period. Such an absence of estrus, however, is not absolute proof of pregnancy. A nonpregnant animal may not show estrus because of failure of the corpus luteum to regress normally (retained corpus luteum) or bemuse of disease or other abnormality of the genitalia.

Another possible reason for an animal to miss one or two heat periods following breeding is conception followed by early abortion. If such is the case, the female may then show a fairly normal season following the abortion.

Although there are exceptions, as noted, the fact that a female misses one or more estrous periods following breeding is good evidence that she has conceived.

Change of Contour of Abdomen. As pregnancy advances in any female, a definite dropping of the abdominal wall occurs, as well as a widening of the abdomen. This increase in size of the abdomen is commonly called “bellying down.” It is due not only to the increase in size of the fetus, but also to increase in fetal fluids and enlargement of the uterus.

Palption per Rectum und Ballottement. A skilled veterinarian can diagnose pregnancy with a high degree of accuracy by rectal palpation in cattle and horses. The diagnosis of pregnancy and estimation of stage of pregnancy are based on a knowledge of the rate of development of the fetus and changes in the genitalia and associated structures of the dam (Fig. 27-5).

In the cow, the presence of a corpus luteum in the ovary and a slight enlargement of one horn of the uterus as compared to the other is suggestive of early pregnancy. At about three months, fetal membranes may be felt slipping between the fingers and very small caruncles are palpable in the uterine wall. The middle uterine artery on the pregnant side will be slightly larger than on the nonpregnant side, and a “buzzing” or ‘fremitus” becomes noticeable in the artery. Both the fetus and caruncles are definitely palpable in the cow by four months of pregnancy.

At 5 to 7 months of pregnancy, the uterus drops over the brim of the pelvis and stretches the cervix taut. Ovaries and fetus become difficult to palpate, but the caruncles on the uterus are definite and large. From this stage onward the calf can usually lie bumped in the right flank. This technique, known as ballottement, is accomplished by gently forcing the list into the lower right hank of the cow in a reciprocating manner so that the fetus rocks from side to side. Then the fist is forced into the flank and held until the calf bumps into the fist. The calf is the only solid object that can be palpated through the right flank of a pregnant cow. From 8 to 9 months, the fetus can again be palpated through the rectum and various parts of the fetus may be identified.

Non pregnant uterus in the cow Uretrus of a cow 60 to 70 days pregnant

1, Cervix, 2, body of uterus, 3, horn of uterus, 4, oviduct, 5, ovaries, 7, corpus luteum, 8, intercornual ligament, 9, rectum.

Nonpregnant uterus in the mare Uterus of a mare 60days pregnant

1, uterine horns, 2, body of uterus, 3, ovary, 4, ovarian ventricle, 5 rectum.

Fig. 27-5. A Nonparent uterus in the cow. B.. Uterus of a cow 60 to 70 pregnant. 1, cervix, 2 body of uterus, 3 horn of uterus, 4, oviduct, 5 ovaries, 7 corpus luteum, 8 intercornual ligament, 9, rectum. C., Nonpregnant uterus in the mare. D. Uterus of a mare 60days pregnant. 1, Uterine horns; 2, body of uterus, 3 ovary, 4, ovarian ventricle, 5, rectum (from Roberts, Veterinary Obsterics and genital disease, courtesy of Edward Brothers).

In the mare, pregnancy diagnosis by rectal palpation is more difficult than in the cow. The bulge of the amniutie sac surrounding the fetus is the earliest diagnostic feature. It increases in size approximately as shown in Table 27-2.

Most pregnant bitches can be successfully palpated through the abdominal wall at three to four weeks of pregnancy. One hand with the fingers extended may be placed on each side of the dorsal flank region of the standing bitch. The hands are brought as close together as possible and gently moved ventrad till the gravid uterus is felt.

Use of X-ray studies. Diagnosis of pregnancy with x-ray film is of limited value in domestic animals. Horses, calttel, and sheep are to large for satisfactory abdominal x-ray produce. In dogs and cat, x-ray pcitures may be used effectively to determine pregnancy after the fetal bones have begun to calcify.

Biologic tests. The discovery of pituitary-like gonadotrophins, hormones that stimulate the overaies and the testes and are screted by the placenta during pregnancy, has led to a means of diagnosing pregnancy in several species. In mares, at about 50 to 84 days of pregnancy, gonadotrophic substance is found in the blood. The test is conducted by using 10ml of blood serum collected from a mare between 50 84 days after breeding. The serum is injected into the ear vein of a mature. Nonpregnant female rabbit that has been isolated from all male rabbits for at least 30 days. A positive test showing that the mare is pregnant is indicated by dark red fullicles, corpora hemorrhagica, in the ovaries of the rabbit 48 hours after the injection. The ovaries of the rabbit may be examined during a surgical exploratory operation, and the rabbit may then be saved for future use, or the rabbit may be butchered and the ovaries examined at that time.

Although placental gonadotrophins probably are secreted by all domestic animals during pregnancy, the quantities are insufficient-except in the mare-to produce a reaction in the ovaries of test animals.

In the human, other tests for pregnancy depend upon the fact that the placenta also produces appreciable quantities of estrogenic hormones and chorionic gonadotrophin, the metabolites or end-products of which are excreted in the urine. The estrogen output by the placenta is highest in the latter part of pregnancy, when progesterone levels fall off.

When urine containing the estrogens is injected into ovariectomised rats, there is cornification of the test animal’s vaginal epithelium. A smear of the vaginal wall is made and the cellular changes observed with the aid of a microscope. However, newer simpler methods of pregnancy testing are now used.

Table 27-2: Pregnancy diagnosis by rectal palpation in mare

Days of pregnancy Size of bulge (in inches)

30 2 diameter by 3 length

45 3 diameter by 4 ½ length

60 5 to 6 length

90 5-6 diameter by 8-9 length

100-150 Festus palpable


Parturition, or labor, which is the act of giving birth to young, marks the termination of pregnancy. It is customary to divide the act of parturition into three stages. The first stage consists of uterine contractions that gradually force the water bags against the uterine side of the cervix, causing it to dilate, this stage lasts 2 to 6 hours in the cow and ewe, 1 to 4 hours in the mare, and 2 to 12 hours in the sow and bitc

In the second stage, actual delivery of the fetus occurs. Passage of parts of the fetus through the cervix into the vagina along with rupture of one or both water bags reflexly initiates actual straining or contraction of the abdominal muscles. The combination of uterine contraction and abdominal contraction forces the fetus through the birth canal.

The third stage of parturition consists of delivery of the placenta, which normally follows the fetus almost immediately. Several factors appear to be involved complexly in the initiation of parturition, particularly changes in hormone levels, as measured in the maternal blood plasma. In the cow, progesterone levels decline rapidly in the last 48 hours prior to delivery. At the same time, the estrogen levels are rising, but just prior to parturition, they decline rapidly. The corticosteroid levels follow the estrogen pattern, first rising and then falling abruptly just before parturition. Prolactin levels essentially do the same, whereas LH levels remain relatively unaltered.

In the case of the mare, progesterone levels increase during the last 30 days of pregnancy and then abruptly decline the day after delivery. Meanwhile, estrogen levels slowly decrease in the last 30 days, and then abruptly drop at the time of delivery.

The ewe shows a slow decrease in the blood level of progesterone, as does the cow, but the decrease begins several days before it occurs in the cow, and the level does not fall as much as in the cow. Estrogen levels and corticosteroids increase greatly during the last 24 hours, whereas the prolactin level rises on the last day of pregnancy.

Plasma levels in the sow show an abrupt decrease in progesterone concentration in the last 48 hours, preceded by a gradual, progressive decrease 2 to 3 days earlier. Meanwhile, estrogen levels increase for a week before farrowing, and corticosteroids increase during the last 2 days.

Both estrogen and progesterone plasma levels in the bitch decrease two to four days before delivery. What causes the changes in hormone levels and starts the uterine contractions is still uncertain. However, evidence to-date implicates an increase in the amount of ACTH being released from the hypophysis of the fetus. That, in turn, increases secretion of adrenal gland steroids. Increasing the corticosteroid levels in the fetus causes an increase in the release of a prostaglandin (PGF2α) from the maternal uterine wall. The prostaglandin may then start the myometrial contractions, after the progesterone levels have decreased and the estrogen levels have increased.

The prostaglandin may also stimulate the release of oxytocin. It is well known that the oxytocin secreted mm the posterior pituitary gland causes uterine muscle to contract. In fact, extract of the posterior pituitary is used extensively stimulate contractions of the fatigued uterus during prolonged labor. Estrogen levels also increase, which stimulates uterine contraction, whereas progesterone levels fall off.

Signs of Approaching Parturition

As well as the obvious enlargement of the abdomen, the mammary glands enlarge and begin to secrete a milky material within a few days of parturition. There may be some edema (swelling) of the ventral abdominal wall about the same time as the mammary gland secretion begins, particularly in the first pregnancy. The vulva swells and usually discharges a thick mucus. Other signs include relaxation of the abdominal wall with sinking of the flanks, dropping of the belly, and sinking of the rump on both side: of the tail head.

As the time of parturition becomes imminent, the animal becomes restless, usually seeks seclusion, lies down and gets up frequently, attempts to urinate often, and then begins actual labor. The bitch and sow usually try to build a nest before starting labor.

Normal Presentation

The calf is normally presented front fee first with the head extended and the nose between the front feet (Fig. 2L6). The dorsum of the calf is in contact with the sacrum of the dam. This position, called anterior presentation, takes advantage of the natural curvature of the birth canal of the dam and the curvature of the fetus. A posterior presentation with the hind feet first, hocks up, occurs frequently enough in cattle to be considered normal.

Fig. 27-6. Position of the calf in the uterus after it has been oriented for normal delivery. (from physiology of reproduction and artificial insemination of cattle by G.W. Salisbury and N.L. VanDemark.

Contractions of the uterus force the fetal placenta (water bags) against the cervix of the uterus. This constant pressure causes the cervix to dilate gradually so the fetus can pass through into the pelvis of the dam. When the water bags break, the uterus contracts more strongly upon the fetus. About the same time, the abdominal muscles begin to contract forcefully to expel the fetus through the birth canal.

The contraction of abdominal muscles called straining, is a reflex response to stimuli from the presence of parts of the fetus within the vagina and vulva of the dam. Straining is readily evoked by an operator inserting his hand and arm into the vulva and vagina of a cow when attempting to deliver a calf, unless the cow has received an appropriate local anesthetic. The neural stimulus also feeds back to the hypothalamus causing increased oxytocin secretion.

The uterus of the sheep and goat is similar to that of the cow, so nearly everything said about pregnancy in the cow applies to them except gestation period Sand the fact that multiple births are much more common than in cattle. The legs of a colt are relatively longer than those of a calf, and the colt is carried to a larger extent in the body of the uterus, while a calf is carried almost entirely in one horn of the uterus. Presentation of the foal is essentially the same as that of a calf.

With pigs and dogs, the young are carried it both horns of the uterus and may be presented either anteriorly or posteriorly with equal facility.

Usually the placenta or afterbirth is delivered a short time following birth of the young, but it may accompany the fetus or, rarely, precede it. The placenta is considered to be retained pathologically if an abnormally long period of time elapses between birth of the young and delivery of the placenta.

Normally, the placenta of the cow and ewe should be delivered within 24 hours following parturition. Since the mare is susceptible to metritis, infection of the uterus, any retention of the placenta over two or three hours is a cause for concern. In the pig and dog, each placenta normally is still attached to the fetus and may completely surround it at birth. Immediate removal of the placenta from the nostrils of the newborn is essential for life and is usually done by the dam. Manual removal of retained placenta from the cow is a common method of treatment.

While this operation, commonly called “cleaning.” is relatively simple for a skilled person, it may be dangerous to the cow and also to the operator if proper precautions are not observed. Cows infected with Brucella abortus (Bang’s disease) often show a high incidence of retained placenta. Treatment of a retained placenta with stilbestrol, a synthetic female sex hormone, is sometimes used with varying degrees of success.

Retained placenta in species other than the cow may be more serious and often endangers the life of the animal. Early treatment of these cases requires not only removal of the placenta, but local treatment of the infected uterus and systemic treatment of the dam as well.

Dystocia-Difficult Birth

Normal parturition with no complications is by far the most common situation in domestic animals. However, there are occasions when the dam has difficulty giving birth to young and may need some assistance. From the onset of actual labor, a cow should calve within a maximum of eight, hour or intervention likely will be necessary. The ewe should complete lambing within mare does not foal within one to three hours after starting labor, a veterinarian should be called. Pigs and dogs should average one offspring at least every hour, or else intervention may be necessary.

Improper presentation is a common cause of obstetric trouble. Other causes include disparity of size of dam and fetus (too large a calf or too small a birth canal), or some pathologic condition of dam or fetus.

Abnormal Presentations. Any deviation from the anterior presentation or posterior presentation as described is considered to be abnormal and usually requires correction before the fetus can be delivered. Figure 27-7 shows some of the many abnormal presentations that may be encountered.

Correction of any of these abnormal presentations requires returning the calf to an anterior or posterior presentation. In most instances this involves repelling the fetus into the uterus away from the pelvic inlet in order to have room for manipulation of the calf. An epidural anesthetic administered by a veterinarian will stop all straining by the cow and make the operation much easier on both the cow and the operator. A detailed description of abnormal presentations and their correction may be found in Diseases of Cattle, Atkinson et al. (1942) and in most standard textbooks of veterinary obstetrics.

Other Causes Dystocia. Excessive size of the fetus in relation to the size of the birth canal of the dam presents a difficult problem. Even though the presentation may be normal, excessive traction in delivering the P newborn will likely be damaging to both the fetus and the dam.

Treatment of Dystocia. Cesarean section (surgical removal of the calf) is the safest treatment for most types of dystocia, and is safe for both fetus and dam if the operation is performed by a skilled veterinarian before complications occur. The other alternative is embryotomy (cutting the fetus into pieces which are small enough to remove through the birth canal). This procedure may save the life of the dam.

Pathologic conditions of the fetus that cause difficulty in parturition include hydrocephalus (water on the brain), ankylosed (fused) joints, shortened tendons, Siamese twins, and monstrosities such as calves with two heads or extra appendages.

Fig. 27-7. Abnormal presentation of the calf for delivery. (Redrawn from DISEASE OF Cattle, U.S.D.A Special report, 1942) (From physiology of reproduction and artificial insemination of cattle by G.W. Salisbury and N.L. VanDemark. Copyright by W.H. Freeman and Company 1961). A, Anterior presentation, one foreleg retained; B. anterior presentation, forelegs bent at kneel C, anterior presentation, forelegs crossed over neck; D. anterior presentation, downward deviation of head; E, anterior presentation, upward deviation of head; F. anterior presentation with back down; G, anterior presentation, with hind feet in pelvis; H. croup and thigh presentation, I, croup and hock personation; J, posterior presentation, the fetus on its back, k, all feet pretend, L, dorsolumbar presentation.

Pathologic conditions in the dam that can interfere with parturition usually involve the birth canal. Such factors as fracture of the pelvis, tumors of the genitalia, and excess fat in the pelvis decrease the size of the birth canal, thereby interfering with passage of even a normal-sized fetus. In addition, torsion, or twisting, of the uterus, rupture of the uterus, or rupture of the prebuic tendon, which is the insertion of the rectus abdomins muscle, will seriously impair or prevent normal parturition. Again, the best treatment for the preceding conditions is cesarean section, with embryotomy a second choice.



The skin is composed of two layers, the epidermis and the dermis. It is usually creamy white and thin. Unlike mammalian skin it has few sebaceous and no sweat glands, but it does have a uropygeal or oil gland not present in mammals. This in poultry is small, about 5 mm m diameter, but is much larger in water birds. It lies on the dorsal surface of the tail and has two lobes producing a yellow ceruminous material, which is used in preening the feathers. Other accessory structures of the skin are the slaws, beaks, wattles, combs and ear lobes. The fold of skin filling in the angle between the humerus and radius, i.e. between the body and the wing, is called the patagium.

In most areas the skin is covered with feathers, which arise from feather follicles that project into the dermis. Feathers are composed entirely of a horny substance called keratin. They fall into three main categories:

(1) Contour feathers.

(2) Downy feathers.

(3) Filoplumes, which are hair-like.

Contour feathers are arranged in rows within areas called pterylae. These areas are separated by non-feathered areas called apteria. The largest contour feathers are the flight feathers of the wings and the feathers of the tail.

A typical contour feather (Fig. 23.1) consists of a central shaft or rachis; a lower part, the calamus or quill; and an upper part or vane. At the end of the quill is a small opening, the inferior umbilicus. During growth it connects with a papilla of the dermis. The quill is rounded and almost transparent, and contains a series of scales. At the junction of the quill and axis is another small opening, the superior umbilicus, from which arises a Small additional feather, the afterfeather. The axis of the vane, the rachis, has a central groove, and is solid, tapering and flexible.

The vane consists of filaments or barbs arising from each side of the rachis at 45° and in turn smaller filaments or barbules project distally and proximally also at about 45° (Fig. 23.2). The result is that the barbules cross each other at an angle of about 90°. Hooklets from each set of barbules engage with recesses in corresponding barbuies (Fig. 23.3). This interlocking gives firmness to the vane. The contour feathers can be raised or depressed by muscles attached to the feather follicles.

C:\Users\u\Desktop\bluetooth\b6fKgEW1PP7K1RPPFfKWN2bT.jpg Feathers are extremely light in spite of their complex structure and size.

Fig 23.1: Contour feather

Skeletal system

The skeleton (Fig. 23.4) of the fowl differs in certain aspects from that of mammals. The bones are light and some have pneumatic cavities, which may be in direct communication with the respiratory system by way of the air sacs, e.g. humerus and coracoid.

The skull bones are fused early in development and hence the sutures are not apparent. The orbits are large. The two premaxillae that form the bony basis of the upper beak are fused as are the dentary parts of the mandibles that form the bony basis of the lower beak. There is only one occipital condyle, which allows for extensive rotation of the head. The whole bony skull is extremely light.

C:\Users\u\Desktop\bluetooth\930Yhahh4gUCea7rtXfaWtVR.jpg The vertebral column consists of the cervical, thoracic, lumbar, sacral, caudal and coccygeal vertebrae. (NB Authorities differ in the differentiation of cervical and thoracic vertebrae.)

Fig. 23.3 Magnified diagram of interlocking barbules.

(1) The cervical vertebrae number 13 or 14 in the fowl compared with seven in mammals. They form the basis of the very long flexible neck. Except for the atlas, which is very small and ring-like, they have long bodies. The last cervical is often [used with the 1st thoracic vertebra. The last two cervical articulate with the rust two ribs.

(2) There are four thoracic vertebrae. The first three are fused together with the last cervical. The 4th thoracic is movable.

(3) The four lumbar, live sacral and six caudal vertebrae are all fused solidly together to form the synsacrum. They are also fused with the pelvis: bones forming the bony pelvis.

(4) There are six coccygeal vertebrae and these are small except the last-the pygostyle, which is flattened laterally.

The vertebral column of the fowl is therefore unlike that of mammals in that the neck portion is extremely flexible while the rest of it is rigid, except at the 4th thoracic vertebra, because of the extensive fusion.

There are seven pairs of ribs that articulate with the last two cervical, the four thoracic and the 1st lumbar vertebrae. The first two pairs do not reach the sternum. The other ribs consist of two segments, the vertebral and sternal segments. The sternal segments articulate with the sternum. With the exception of the 1st and 7th pairs, they all have small flat uncinate processes that project backwards over the outer surfaces of the next ribs.


Fig. 23.4 Skeleton of fowl.

The sternum (Fig. 23.5) articulates in front with the coracoids and with the five sternal ribs. The pectoral girdle on each side is composed of the scapula, the coracoid and the clavicle. The fused clavicles are commonly called the ‘wishbone’. The bones fit together like a tripod, the coracoids articulating with the sternum at the rostrum. The top of the tripod is the fused ends of the clavicles, which articulate with the humeri.

The wing is composed of the humerus, radius and ulna, carpus, metacarpus and the digits.

Fig. 23.5 Stemum of fowl.

The leg consists of:

(1) the femur, which articulates with the pelvis at the acetabulum

(2) the patella, which is small

(3) the tibia ‘drumstick’

(4) the fibula, which is slender and pointed with a flattened head and extends three-quarters the length of the tibia

(5) the metatarsus, on the medial aspect of which, in the male, is a bony projection that bears the spur

(6) the digits, of which there are four; the 1st digit, which is the only one that projects backwards, has two or three phalanges, the 2nd digit three phalanges, the 3rd four phalanges and the 4th five phatanges. The last phalange on each digit is the core of the nail or claw.

The pelvic bones (Fig. 23.6) on each side are the ilium, ischium and pubis. They are fused together and with the synsacrum, i. e. the last thoracic vertebra, the lumbar, sacral and caudal vertebrae form a fused solid complex structure.

The muscular system

This varies in certain aspects from the muscular system of mammals. The diaphragm is rudimentary and therefore does not divide the body cavity into thoracic and abdominal cavities. It also plays no part in respiration.

The pectoral or breast muscles are very much enlarged. They are lighter in colour (white muscle) than the muscles of the legs. The pectoral muscles are made up of two types of muscle fibres, large and small. The large type, which predominates in the pectoral muscles, is light in colour. The small type is dark. The pectoral muscles make up half the weight of the total musculature. Their function is to depress the wings in flying. The supracoracoid muscles, which raise the wings, are much smaller and are covered by the pectorals.

Fig. 23.8 Pelvic bones of fowl

The ambiens muscle, which does not occur in mammals, is used in perching.-It lies on the medial aspect of the thigh, originates close to the acetabulum and joins the flexor muscles of the digits.

The digestive system

The digestive system (Fig. 23.7) consists of the mouth, tongue, oesophagus, crop, proventriculus, gizzard, duodenum, ileum, caeca, rectum, cloaca and vent. For descriptive purposes liver and spleen are included.

1. The mouth (Fig. 23. 8) has no teeth and therefore there 1s no mastication of food. The grinding up of food takes place in the gizzard.The1’e are no lips or cheeks as in mammals. These are replaced by the beak, which is composed of

dense and horny skin covering the mandibles and premaxillae. It is used for picking up and sometimes tearing food. The mouth and tongue have numerous horny papillae directed backwards. The mucous membrane contains numerous glands, which produce mucus. There are no salivary glands. The palate has a long narrow median slit opening into the nasal passages.

2. The oesophagus extends from the pharyngeal area at the back of the mouth to the proventriculus. About one-third of the way down the oesophagus and just before entering the body cavity is the crop, used for the temporary storage of food (Fig. 23.9). The oesophageal mucous membrane has numerous glandular cells. These are absent in the crop. The remaining two-thirds of the oesophagiis is the thoracic portion and this is in the body cavity. It enters the proventriculus.

C:\Users\u\Desktop\bluetooth\EhWCA342HdWRhX5VfS244W5Q.jpg 3. The proventriculus the first part of the stomach. It 15 tubular 1n shape and smaller than the gizzard. Its mucous membrane has numerous glands producing gastric juice. Before joining the gizzard the proventriculus narrows.

Fig. 23.7 Viscera of fowl

4. The gizzard is the second part of the stomach. It is very muscular and often contains grit or small stones, which assist in the grinding up of the food. The interior has a pale, ridged, thick and horny lining. Issuing from the gizzard is the first part of the small intestine, the duodenum.

5. The duodenum is a U-shaped loop, between the legs of which is situated the pale-coloured pancreas. The second part of the small intestine is the ileum.

6. The ileum consists of coils or loops suspended from the dorsal wall of the body cavity by the membraneous mesentery. The ileum joints the large intestine, which is very similar in diameter Id the small intestine. Half way along the ileum 1s a small diverticulum – the vitelline diverticulum -which may become impacted and greatly enlarged. Where the ileum joins the large intestine the cam branch off.

Fig. 23.8 Mouth of fowl.

7. The caeca or caecal tubes (Fig. 23.10) are two blind-ended sacs that project backwards. They are easily distinguished as their contents are dark in colour.

At the beginning of the caéca, are two small swellings in the mucosa, the so-called ‘caecal tonsils’.

8. The rectum, the next part of the large intestine, is short and terminates at the cloaca.

9. The cloaca (Fig. 23.11) has three parts: ‘

(1) The coprodeum (faecal chamber), which receives the intestinal waste from the rectum.

(2) The urodeum (urogenital chamber) receives the urates from the kidneys via the ureters, the eggs in the female via the oviduct and the sperm in the male via the deferent ducts.

Fig. 23.9 Crop and stomach of fowl

(3) The proctodeum which connects with the bursa of Fabricius (cloacal bursa).

10. The vent is the terminal opening of the digestive tract.

The bursa of Fabricius or the cloacal bursa lies dorsal to the cloaca. It is a small spherical glandular sac, which is at its largest in fowls of 3-4 months 61d. It is part of the lymphatic system and its function is the production of antibodies. It opens into the proctodeum of the cloaca.

Fig. 23.10. Caeca of fowl

Fig. 23.11 Cloaca.

The liver in the fowl is normally of a light chocolate brown colour. This, however, varies greatly, e.g, in very fat hens the liver is very fatty and is yellow. It has two lobes, which are connected by a narrow isthmus. The right lobe is slightly larger than the left. The gall bladder lies on the visceral surface of the right lobe. The left lobe has an independent bile duct conveying bile direct to the intestine and not via the gall bladder.

The spleen lies close to the junction of the proventriculus and gizzard. It is small and spherical and because of its bright reddish-brown colour is easily distinguished.


The external respiratory system in birds consists of the nostrils, nasal cavities, pharynx, larynx, trachea, lungs and air sacs.

The nostril, one on each side of the base of the beak, are rounded or oval openings. They are small and lead into the nasal cavities.

The nasal cavities connect with the mouth through an opening in the palate.

The pharynx is at the back of the mouth and is not clearly defined.

The larynx is heart-shaped. It has no epiglottis or thyroid cartilage and no vocal cords.

The trachea has over 100 complete cartilaginous rings. Just before the terminal end of the trachea and just before it bifurcates into two bronchi is the syrinx or lower larynx or voice organ. It appears as a lateral compession of the trachea, inside which are two thin membranes. These are comparable to the vocal cords of mammals. The trachea bifurcates into the right and left bronchi, one going to each lung.

The lungs are bright red and positioned’ 1n deep recesses formed by the 1st to 5th ribs and are therefore difficult to remove from the body cavity. The dorsal surfaces are deeply grooved by the 2nd. 3rd, 4th and 5th ribs. The ventral surfaces are smooth. Each lung has a primary bronchus, which passes through the lung and leads directly into the large abdominal air sac. From the primary bronchus, secondary and tertiary bronchi spread throughout the lung. The bronchi communicate with the thin-walled air sees and some of the pneumatic bones.

The air sacs are thin walled, transparent and glistening. With the exception of the clavicular air sac they are bilateral and paired.

(1) The cervical air sacs lie dorsal to the oesophagus and extend from the 2nd cervical to the 2nd thoracic vertebrae.

(2) The clavicular air sac lice in the anterior part of the body cavity and has diverticula that enter the pneumatic foramena of the humeri.

(3) The anterior and posterior thoracic air sacs lie caudally and ventrally to the lungs. .

(4) The abdominal air sacs are the largest and cover the viscera from the duodenum backwards.

All the air sacs pass the air back to the trachea so that air only flows one way through the lungs and air sacs.

The circulatory system

The circulatory system in birds follows roughly the same pattern as in mammals. The red blood corpuscles are nucleated, unlike those of mammals.

The lymphatic system

The lymphatic system in fowls also follows the same general pattern as in mammals. However, there are no lymph nodes but the bursa of Fabricius is present, which has no counterpart in mammals.

The endocrine system

This is fairly similar to that of mammals.


The nervous system corresponds to the mammalian system. The spinal cord extends to the 3rd coccygeal vertebra. Derived from the spinal cord are the spinal nerves, which on each side number’ 15 cervical, ‘7 thoracic, 14 lumb-sacral and few coccygeal. Lying in the mid-line of the carcass just under the vertebrae are the brachial and lumbo-sacral plexuses. These are networks of nerves.

The branchial plexus consists of the last three cervical and the first two thoracic nerves. The lumbo-sacral plexus consist of the 1st lumbar and the first four sacral nerves. Thisplexus can be exposed by removing the kidneys.

The autonomic nervous system is that part that regulates the activities of the viscera, including the heart. It is composed of the sympathetic and parasympathetic nerves, which have opposing effects. The sympathetic system consist of two chains of interconnected ganglia with connections to the spinal nerves.

The eye

The eye is relatively much larger than in mammals and has a much larger accurate field of vision. A structure within the eye that does not occur in mammals is the pectin. It is a black vertical comb or fan that overlies the optic nerve. The lower eyelid is better developed than the upper. The third eyelid or nictitating membrane is extensive and capable of covering the complete anterior surface of the eye.

The ear

The external opening is small and covered with coarse feathers. There is no external auricle or ear flap.

The urogenital system

The kidneys (fig. 23.12) are elongated, about 5cm in length, three-lobed, brown in coloru and soft in consistency. They lie in deep recesses in the pelvic bones formed by the last thoracic ribes and the 1st lumbar ribs to the 6th caudal vertebra. They almost fill the renal fossae. In front they are in contact with the lungs. The ureters emerge from the medial aspects of the middle lobes and enter the middle part of the cloaca – the urodeum. There is no bladder in the fowl. The product of the kidneys is not fluid like mammalian urine but is a semi-solid substance composed of urates. It shows up white in the droppings. As the consistency of the kidneys is soft it is difficult to remove them from the bony recesses without tearing.

The testes are ovoid, cream-coloured bodies. They lie just anterior to the kidneys and vary greatly in size according to age. During the breeding season they are greatly enlarged. The deferent ducts are convoluted and run parallel to and medial to the ureters. They enter the urodeal cavity of the cloaca. The straight terminal ends of the deferent ducts are muscular and act as ejaculatory ducts. There is no penis.


Fig 23.12 Kidneys of fowl.

The ovary (fig. 23.13) lies ventral to the left kidney. Only the left ovary is functional. The right one regresses soon after hatching. The ovary consists of a mass of spherical ova, which vary greatly in size during the laying season. The smallest ova are white in colour but as they increase in size they gradually become yellow. Mature ova measure 2-3cm in diameter.

The oviduct consists of five parts, the division of which are not discernible to the naked eye:

  1. The infundibulum is about 10cm in length and collects the ova from the ovary. The ova take about 18mintues to pass through this part.
  2. The magnum is the largest part of the oviduct and measures about 35cm in length. In this part the albume or ‘white of egg’ is secreted. Passage through the magnum takes 3hours.
  3. The isthmus is about 10cm long. It produces the shell membranes. Passage through the isthmus takes 1 ¼ hours.
  4. The uterus or shell gland is about 10cm long. It produces the hard shell. Passage through the uterus takes 20hours.
  5. The vagina is about 8cm long. It conveys the fully formed egg into the urodeal part of the cloaca to be expelled through the proctodeum and vent.

Fig. 23.13 Ovary of fowl

Passage through the oviduct takes 24hours and eggs may be laid either first. If the shell is pigmented, the pigment is laid down during the last five in the oviduct. The right oviduct is vestigial and is often represented by a transparent mentary tube.

The egg

The egg shell, although strong and rigid, is porous. Part of its strength is due curved shape. The outer shell is composed of three layers, a middle calcareous layer, and cuticle and an inner mammillary layer. Inside the outer shell is the shell brane, which consists of two layers. These two layers are separated at the larger end of the egg to form the air chamber. Contained in the shell membrane is the albumen or ‘white of egg’. The albumen is composed of three layers: a thick layer in the middle surrounded by an outer and inner layer of thin watery albumen. At each end of the egg there is a denser twisted cord of albumen, the chalaza. The chalazae are attached to the yolk and keep it in place.

In the albumen is the yellow yolk of the egg. This consists of two types of cells, smaller white yolk cells and larger yellow yolk cells. The colour of the yolk depends upon the amount of xanthophyll in the diet. The yolk is surrounded by the thin vitelline membrane.

Fertilized fowl eggs hatch 21 days after laying, turkey and duck eggs in 28days and geeze in 30-35 days. Unfertilized eggs of course have no embryos.



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