Animal anatomy, histology, pathological anatomy

Autopsy of bird corpses
By origin and anatomical structure, birds are closer to reptiles than to the class of mammals, and in this they differ sharply from higher vertebrates.
Anatomical features of birds.The skin is dry, tender and thin. This is due to the fact that their epidermal layer at the base of the skin is poorly developed. The subcutaneous tissue is loose, which creates greater mobility of the skin. There are no sweat or sebaceous glands. There is only one gland - the coccygeal gland (supra-tail, tail), which is a cluster of modified sebaceous glands. The coccygeal gland is located under the skin above the last caudal vertebrae. It consists of two oval or round lobes measuring up to 14 mm in waterfowl and 4 mm in chickens. Its excretory duct is located under the feather cover and protrudes in the form of a papilla covered with tassel-shaped feathers.
Feathers are derivatives of skin. There are contour and down feathers.
Derivatives of the skin also include comb, corals, earrings, earlobes, spurs, claws and beak.
Skeleton. The skeletal bones of birds are usually thin, but very strong, dull white and contain a large amount of minerals. In young birds they are filled with bone marrow, in adults - with air. In birds, unlike animals of other species, the vertebrae are less mobile, since many of them are fused. The number of vertebrae in chickens is 38–41, in ducks 43–46, in geese 46–49. The pelvic part of the spine consists of 13–14 vertebrae fused together. The tail part includes 5–7 movable vertebrae; they serve as a support for the tail feathers.
The muscles of birds, compared to the muscles of other vertebrates, are denser, with relatively thin fibers. Their tendons are long and often undergo ossification. Bird muscles are conventionally divided into white and red. White (pale) muscles are poorly exercised (muscles of the chest bone), red muscles are exercised (muscles of the limbs).
Heart. There is no atrioventricular valve - it is replaced by a triangular-shaped muscular plate, the free edge of which protrudes into the cavity of the ventricle. Red blood cells are large, oval in shape, and have a nucleus.
Lymphoid organs and lymphoid tissue. In addition to the main lymphoid organs (spleen, bursa of Fabricius, thymus), lymphoid tissue in the form of accumulations of various sizes is present in the submucosal layer of the trachea, lungs, kidneys, ureters, testes, thyroid gland, pituitary gland, liver and other organs.
Chickens do not have lymph nodes similar to those of mammals. True, some researchers show that chickens have lymph nodes in the neck area, grouped in the back and middle sections of the neck. Each group includes 3–4 or 3–5 single nodes. In young chickens these nodes are yellowish in color; in older chickens they are brownish-gray or gray. However, they are difficult to detect.
other researchers believe that chickens and pigeons do not have lymph nodes that would interrupt the course of the lymphatic vessels.
Lymph nodes, or two pairs of lymphoid structures called lymph nodes, are found only in geese and ducks. They are located in the cervical and sacral parts of the body, in pairs on each side and can reach a length of 1.5 cm.
Fabrician bag. This is an unpaired lymphoid organ, unique to the class of birds. It is located on the dorsal wall of the terminal part of the cloaca in the form of a semi-oval diverticulum. This is a cavity organ that communicates with the cloaca cavity through a small hole located on the dorsal wall of the cloaca 4–6 mm from the anus. The growth and development of the pouch is closely related to the sexual maturation of birds. It is well developed in day-old chicks, its weight increases 21 times by the age of one month. It reaches its maximum development by 2 months of age (deviations are possible), then stabilization occurs and after 4 months of age it undergoes reverse development; by 7 months of age its traces remain. There is a direct connection between the growth of the bursa and the development of the testes.
The thymus in birds consists of 6–7 pairs (in chickens) of lobes, arranged in two rows: one row on the neck, the other flies to the trachea. The thymus is most developed in young birds.
The lymphoid tissue of the cecum (intestinal “tonsilla”) is visible to the naked eye in the form of slight thickenings located at a distance of 0.5 cm from the branching point of the cecum.
Respiratory system. In addition to the lungs, birds have nine air sacs; the tubular bones (skeleton) are also filled with air.
In birds, there are two larynxes - upper and lower. The upper larynx is involved in the breathing process, the lower – as a vocal apparatus (has vocal cords). Birds do not have thyroid cartilage or epiglottis.
The lungs are small in size and not divided into lobes. Their minimal structural unit is the parabronchial complex.
Digestive organs (Fig. 316). Birds have no teeth, no palate, no cheeks, no lips, and the oral cavity has no border with the pharynx. The esophagus passes into the goiter. Birds have two stomachs - glandular and muscular. The mucous membrane of the latter is covered with cuticle. There are always pebbles in it, with the help of which the food is crushed. In the middle of the jejunum there is a small process with a blind end - this is Meckel's diverticulum - a remnant of the connection between the embryonic yolk sac and the intestine. Some birds do not have caecums (or they are poorly developed). The rectum is short and leads directly into the cloaca. The cloaca is the last expanded section of the intestine, into which the excretory ducts of the genitourinary organs of males and females open.



Fig. 316. Digestive organs of birds.
1– esophagus; 2– goiter; 3–glandular stomach; 4– muscular stomach; 5– duodenum;
6 – liver; 7– pancreas; 8 – small intestine; 9–large intestine; 10– cecum;
11– cloaca.

The liver consists of two lobes: right and left. On the right lobe (from the adjacent duodenum) lies the gallbladder. Some birds do not have a gallbladder (pigeons, guinea fowl, some types of ostriches, etc.).

The kidneys are deeply squeezed between the transverse processes of the sacrum. Birds do not have a bladder or renal pelvis. Urine contains large amounts of uric acid and therefore resembles a pasty white mass in appearance.
Male genitals. The testes are located in the abdominal cavity at the anterior edge of the kidneys.
Female genitals (Fig. 317). The left ovary and left oviduct are usually functional.



Fig. 317. Chicken reproductive organs.
1– ovary; 2– follicle; 3– oviduct funnel; 4– oviduct; 5 – protein part; 6– isthmus;
7 – bird uterus; 8– egg; 9 – rectum; 10 – vagina; 11– cloaca.