Plate I - Lateral aspect of chicken

The general form of the domestic chicken is presented in a position for anatomical study. Structurally, birds share with mammals the distinction of being the most highly specialized of vertebrates.

While basically similar to mammals anatomically, evolution has brought about modifications for adaptation to flight. The forelimb is changed to a wing in which the manus has been reduced by the disappearance of digits and fusion of metacarpals.

Reduction of the manus combined with comparative simplicity of movements has removed the necessity of large and strong forearm muscles. The muscles of the pectoral region (breast), on the contrary, are powerful to propel the bird in flight and are large and well developed.

The magnitude of this development may be gathered by the fact that the muscles composing the breast weigh about as much as the all the rest of the muscles of the body together, contributing about one-twelfth of the entire body weight.

Plate Ia - Diagram of avian eye

The eye is composed of the outer, transparent cornea (1) enclosing the anterior chamber (2) and a pigmented diaphragm, the iris (3), which regulates the amount of light entering through the pupil by dilation and contraction.

The yellowish color of the iris is probably due to the presence of fat globules contained within the cells.

The scleral ring (4) and sclera (9) are dense and white while the crystalline lens (5) is a strongly biconvex transparent body suspended immediately behind the iris. The anterior chamber, enclosed in front by the cornea and behind by the iris and lens, communicates with the posterior chamber (6) through the pupil.

The chambers are filled with aqueous humor, a clear fluid about 98% water with a little sodium chloride and traces of albumin and extractives. The retina (7) or nervous tunic of the eyeball consists of nerve cells and fibers directly or indirectly continuous with the optic nerve (11). A remarkable structure known as the pecten (10) projects from the region of the entrance of the optic nerve.

Plate IIa - Brain

The chief structures of the brain are the large cerebrum (32) of the forebrain, the cerebellum (33) of the hindbrain and the optic lobes (34) of the midbrain.

The medulla oblongata of the hindbrain bears a close resemblance to the spinal cord with which it is continuous.

There are 12 pairs of cranial nerves (36), the first of which, the olfactory (37), forms a projection on the anterior aspect of each cerebral hemisphere.

Plates III and IIIa: Pulmonary System and Diagram of Air Sacs

The nasal cavity (38) is short and narrow while the trachea (39) or windpipe is relatively long, ending by branching into the right and left bronchi.

The lungs (40) are relatively small in comparison with the size of the thorax and are closely applied to the vertebrae and ribs.

The axillary air sac (41) is connected with the cranial end of the lung transmitting air to the sternum, sternal ribs, shoulder-girdle and humerus.

The anterior thoracic air sacs (42) stretch from the clavicular to the abdominal sacs and unlike the other air sacs do not communicate with the interior of the bones.

The abdominal air sacs (43) are large and communicate directly with the cavities of the sacrum, pelvis and femur. The heart (44) is seen in part projecting from behind the axillary and thoracic air sacs.

The syrinx (45) or broncho-tracheal larynx is demarcated by a lateral compression where the trachea divides into right and left bronchi. Elastic membranes homologous to those of the mammalian larynx are found in this organ.

A view of the left half of the skeleton shows the skull (12), the thirteen cervical vertebrae (13) of the neck, the seven thoracic vertebrae (14), the fused lumbosacral vertebrae (15), the five or six coccygeal vertebrae (16) of which the last, produced by the union of several vertebrae, is the largest.

The latter, known as the pygostyle (17), forms a foundation for the feathers of the tail. The pelvis (18) is formed by the union of the ileum, ischium and pubis. Of the seven pairs of ribs (19) the first and second and sometimes the seventh do not reach the sternum (20).

The phalanges (21) are rudimentary while the metacarpus (22) is in the form of a single bone produced by the union of three osseous elements corresponding to the first, second and third metacarpal bones of the mammalian limb.

The ulna (23) and radius (24) form the bones of the forearm while the stout and slightly curved humerus (25) has an ovoid head for articulation with the scapula and coracoid (26).

The clavicle (27) is thin, rodlike and slightly bent. The united clavicles form the furcula or wishbone which supports the shoulders preventing them from coming too close together during flight.

The femur (28) is stout and cylindrical while the tibia (29) with its attached, poorly developed fibula is much longer than the femur.

The adult metatarsus (30) is represented by one long bone composed of second, third and fourth metatarsal bones in union. The phalanges (31) form the four digits.

Plate IVa - Diagram of heart and great vessels

Detailed structure of the heart and associated blood vessels. The right ventricle (47) pumps blood to the lungs by means of the pulmonary artery (52) while the heavily walled left ventricle (48) forces blood to the body through the aorta (50).

The right atrium (49) receives blood from the caudal vena cava and two cranial venae cavae (51) and transmits it through a strong muscular plate (analogous to the tricuspid valve of mammals) into the right ventricle. The left atrium (53) is a common opening through which the pulmonary veins pour blood after it has passed through the lungs.

Plate IV - Pulmonary System

The gross appearance of the respiratory system from the medial aspect.

Plates V and Va: Gastrointestinal tract and Diagram of structures about the gizzard

The esophagus or gullet (54) has at the thoracic inlet a marked dilation, the crop (55) which serves to store food after rapid ingestion.

The esophagus terminates at the glandular stomach or proventriculus (56) which is relatively small but richly supplied with glands and lymphoid tissues. The muscular stomach or gizzard (57) immediately succeeds the glandular stomach from which it is separated merely by a constriction.

The small intestine (60) begins at the exit from the gizzard and is of relatively uniform calibre throughout. Of the three parts, duodenum, jejunum and ileum, only the first (58) can be distinguished.

The elongated loop of the duodenum, consisting of descending and ascending limbs, extends as far posteriorly as the entrance to the pelvis. Between the limbs of the duodenum lies the pancreas (59). The ceca (61) are blind tubes arising at the junction of the small and large intestine (62). The large intestine is very short, ending in the cloaca. Some anatomists describe the large intestine as consisting of two parts, the colon and rectum. The cloaca (63) is the common opening of the digestive and urogenital tracts. The large, dark-brown liver (64) is divided into two lobes, the right being the larger. The relatively large gallbladder lies on the right posterior part of the visceral surface of the liver. The spleen (65) is a reddish-brown rounded structure situated near the junction of the proventriculus and gizzard and is associated with the circulatory system.

Plate VI - Gastrointestinal tract (right half) and VIaDiagram of cloaca

The terminal portion of the gastrointestinal tract viewed from the right side shows the junction of small and large intestine with entrance of the oviduct (67) and bursa of Fabricius (66). The latter is found best developed in chickens about 4 mo. of age and has usually disappeared at 1 yr.

Plates VII Genitourinary system and VIIaDiagram of oviduct

The ovary (68) with ova (69) in various stages of development is an unpaired structure in the hen (while there are two ovaries in the embryo the right does not develop).

The left ovary lies in the dorsal part of the abdominal cavity opposite the last two ribs. The oviduct (67) varies in appearance according to its functional state. Its anterior part corresponding to the Fallopian tube of the mammal has a slit-like opening leading to the expanded infundibulum (70) succeeded by the more narrow ampulla and isthmus.

Behind this the oviduct enlarges to form a wide, thick-walled tube (73) which may be regarded as the homolog of the mammalian uterus. This is succeeded by the narrow vagina (74) which opens into the cloaca at the left ureteral opening.

The kidneys (75) lie along each side of the vertebral column from the vertebral end of the sixth rib to the iliac fossa. Each kidney consists of three or four lobes.

Plate VIII - Musculature

The cervical (76), pectoral limb (77), pectoral girdle (78) and pelvic limb (79) muscles of birds are highly modified to meet the needs of function in locomotion.

Plate VIIIa - Diagram of nephron

Microscopic appearance of the functional renal corpuscle. The glomerular tuft (80) enclosed by Bowman’s capsule (81).

The proximal convoluted tubule (82) leaves the capsule by a short neck and is connected to the distal convoluted tubule (83) by the loop of Henle (84) terminating in the collecting tubule (85).