What a Chicken’s Body and Internal Biological System Looks Like

Structure of a chicken, part one and two

by Tokushi Tanaka, State and Area Poultry Specialist

Structurally, birds are among the most highly specialized vertebrates. Their structure includes modifications or adaptations for flight*. The basic structural systems of the chicken are as follows:


The differences between the sexes can be seen in the appearance of the feathers in the neck, saddle, and the tail sections. There are some secondary sexual differences characteristics of birds, with some exceptions.

The feathers help protect the bird from physical injury and keep the body warm. The wing feathers are, of course, necessary for flight.


The muscular system of the bird is characterized by the special development of the large muscles of the breast. The greater part of the breast muscles appears to be on the body itself because of the extensive attachment to the sternum. These muscles weigh about as much as all the rest of the muscles together.

Respiratory System

The respiratory system of birds is quite different from that of mammals. The lungs are firmly attached to the thoracic wall, and the active part of respiration is exhaling. In mammals, the more vigorous part of breathing is inhaling.

Connected to the lungs are four pairs of air sacs located on both sides of the body. These sacs are found in the region from the neck to the abdomen. A single median sac is located in the cavity of the thorax. Besides opening into the lungs, the sacs are directly connected to the cavities of most of the bones of the body.


The skeleton of the bird is compact, lightweight, and very strong. Many bones are hollow; many are fused together, forming very strong structures to which the large muscles used in flight are attached.

Digestive System

The digestive system of the fowl is relatively short, a characteristic feature of meat-eating animals. Fowls do not have any teeth; instead, they have the horny mandibles that form the beak. Food is thoroughly pulverized in the gizzard, which corresponds to the chewing in the herbivore or non-meat-eating animals.

At the junction of the intestine and the rectum are two blind pouches called ceca. These are usually 4 to 6 inches long and more or less completely filled with fecal matter. Their function is not fully understood, though they seem to help in the digestion of fiber.

Urinary System

In the fowl, the urine is discharged into the cloaca and excreted along with the feces. No liquid urine is voided. The white, pasty material appearing in the droppings of the birds is largely uric acid, whereas the nitrogen in the urine of mammals is mainly in the form of urea.

Reproductive system

The male fowl has two testes situated high in the abdominal cavity, along the back, near the forward end of the kidneys. These never descends into an external scrotum, as in the case of other farm animals. They are more or less ellipsoid in shape and are light yellow, frequently having a reddish cast caused by the numerous blood vessels on the surface.

The testis consists of a large number of very slender ducts. The sperms are given off the lining of these ducts, called the somniferous tubules. The all lead eventually to the vas deferens, a tube which conducts the sperm outside the body.

Each vas deferens open into a small papilla. Together these serve as the intomittent organ. They are located on the dorsal wall of the cloaca. The so-called rudimentary copulatory organ of the fowl has no connection with the vasa deferentria and is located on the median ventral portion of one of the transverse folds of the cloaca. It is this rudimentary organ, or male process, that is used in identifying the sex of baby chicks by cloacal examination.

The female usually has only one functional gonad. During the early stage of embryo development, there are two gonads, but only the left one finally develops. The right gonad, if present, is usually a nonfunctional rudiment.

The ovary is situated to the left of the median line of the body behind the lungs and at the forward end of the kidney. It is attached to the dorsal wall of the body cavity. In the inactive condition the ovary appears as a small, whitish mass of irregular shape. In the active condition it appears as a yellowing cluster of spheres of varying sizes. Each sphere is enclosed in a follicle. These spheres are ova, or reproductive cells of the females, and are commonly referred to as “yolks”*.

The oviducts is a large coiled tube occupying a large part of the left half of the abdominal cavity in the laying hen. It is suspended from the dorsal body wall. The oviduct consists of six parts: (1) mouth, (2) funnel or infundibulm, (3) magnum, (4) isthmus, (5) uterus, and (6) vagina, which leads to the cloaca. At the anterior end of the oviduct is its mouth which is spread out beneath the ovary to receive the ova or yolks when they are ready to leave to ovary. The posterior end of the oviduct connects with the cloaca, from which the completed egg is expelled. to which the large muscles used in flight are attached.

Additional offline book resources:

1. Feeding Poultry: The Classic Guide to Poultry Nutrition for Chickens, Turkeys, Ducks, Geese, Gamebirds, and Pigeons
2. Storey’s Guide to Raising Chickens
3. University of California: Feeding Chickens

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