Normal Birds - A Review of Avian Anatomy

By F. Dustan Clark, Extension Poultry Veterinarian, Center of Excellence for Poultry Science at the University of Arkansas's Avian Advice - A necropsy is the examination of a bird externally and internally to determine the cause of death. The method for doing a necropsy varies and depends somewhat on the bird involved, the preference of the individual performing the necropsy, the disease(s) suspected, and where the necropsy is being done.
calendar icon 25 July 2005
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Normal Birds - A Review of Avian Anatomy - By F. Dustan Clark, Extension Poultry Veterinarian, Center of Excellence for Poultry Science at the University of Arkansas's Avian Advice - A necropsy is the examination of a bird externally and internally to determine the cause of death. The method for doing a necropsy varies and depends somewhat on the bird involved, the preference of the individual performing the necropsy, the disease(s) suspected, and where the necropsy is being done.
The Author

Dr. Dustan Clark
Extension Poultry Health Veterinarian

Regardless of the method; the most important point to remember is to systematically evaluate each organ and organ system for changes associated with disease. Since only a few diseases cause very specific lesions in the organs; it is very important to be “familiar” with the normal external and internal anatomy. Usually a necropsy starts with a detailed examination of the external anatomy of the bird.

External Anatomy

Feathers and Skin
Feathers cover the majority of the skin and are arranged in feather tracts rather than randomly distributed. The feathers should be clean at the point of attachment to the skin and the edges of the feathers should be smooth with no clear areas present in the barbs.

The skin of a chicken and/or turkey is thin and semi-transparent over most of the body. The muscles, veins, and fat deposits can be observed through the skin in most birds. The muscles appear as dark areas; whereas, fat is yellow. The skin on the face and bottom of the foot is thickened and is normally white or yellow in color. The comb, wattles, and car lobes are usually bright red in color in commercial layers and broiler breeders. It is normal for market and breeder turkeys to develop red or bluish skin on the head and neck. Normal commercial layers and breeder hens may have a reddish yellow skin on the comb, ear lobe, or other facial structures (this is especially true if they are beginning to come into production or are out of production).

The lower legs are covered with scales which are yellow to white in coloration. The thickened skin on the bottom of the foot (footpad) is usually a pale yellow-tan or yellow-white color (the scales of the leg arc similarly colored). Chicks and poults have yellow colored leg scales. Adult broilers and commercial layers can have yellow or white leg scales. Turkey leg scales are white to light tan colored. The leg coloration will change in hens from yellow to white and vice versa as they go into or out of egg production. The skin, leg, and feather coloration of many of the varieties of chickens, ducks, and turkeys kept as backyard, hobby, pet, or exhibition flocks may vary from those listed. The best source for individual breed differences is the book “ The American Standard of Perfection,” which is published by the American Poultry Association or the “American Bantam Standard.”

Ears, Eyes, Nostrils and Beak
The ear in a bird is covered with fine feathers and is a small opening located on the side of the head. The eye should be a bright yellow-orange in color and free of discharges. The eyes should be clear with dark black pupils surrounded by a colored iris. The color of the iris varies with the breed and age of the bird, but in general is steel-grey in chicks and poults. In adult broilers, layers, and broiler breeders the iris is yellow-orange; but brown in adult turkeys. The nostrils are slit like openings on top of the beak and at the base of the beak. They are surrounded by tan-yellow fleshy skin called the cere. The beak is a yellow-horn to white-horn color in the normal bird and has a smooth surface with the end of the beak pointed or blunted in a beak-trimmed bird. Again, colors other than those listed may be normal for many of the varieties of chickens, ducks, and turkeys kept as backyard, hobby, pet, or exhibition flocks. As before, the best source for these breed differences is the “ The American Standard of Perfection” or the “American Bantam Standard” book.

Internal Anatomy

Once the external anatomy has been evaluated the internal anatomy of the bird is examined. The skin should be removed and the bird opened to expose internal organs. The procedure of initially opening the bird to evaluate the internal organs may vary depending on the personal preference of the individual performing the necropsy. However, regardless of the procedure, it is important to evaluate all organs present systematically and thoroughly.

The first organs that come into view when the skin of a chicken or turkey is removed for necropsy are the muscles, sternal bursa, and bone (keel). The breast muscles are a greywhite in color normal poultry. The point of the keel is white and the edge of the bone is straight. The sternal bursa is a white sac-like structure that is located on the sternum and contains a small amount of clear fluid. If the leg muscles are observed they are a darker grey-white color and the sciatic nerve (located between the leg muscles) is a glistening white with cross striations.

Thoracic (Chest) and Abdominal Anatomy

After the sternum and breast muscles are removed the internal organs are evaluated, The heart is a triangular shaped organ (the base of which is toward the head of the bird) that is surrounded by a clear sac (pericardial sac). The heart is greywhite in color and has a band of yellow fat near the base. Internally, the heart is the same color with clear membranous valves between heart chambers. The left ventricle (lower left chamber) of the heart is thicker than the right ventricle. The heart is almost completely surrounded by the lobes of the liver.

The liver is the largest internal organ, is firm, and has prominent sharply defined edges. The color of the liver varies with diet. Baby chicks and poults tend to have a liver that is yellow in color due to yolk absorption. Adult birds can have a yellow-tan liver if on a high fat diet and the organ may be soft. The adult bird usually has a dark red to red brown colored liver.

The avian gallbladder is attached to the liver lobe and can be easily examined by moving the liver to one side. This sacklike structure is greenish-black in color due to the bile present in it.

The trachea and syrinx (voice box) are visible at the base of the heart. These structures are white with the trachea a round tube like structure that divides into smaller left and right bronchi. The syrinx is a flattened area of the trachea that is at the end of the trachea before it dividing into bronchi. The bronchi are identical to the trachea in color and shape but are of a smaller diameter. However, a better examination of the trachea is done in the neck of the bird. The aorta is also visible at the base of the heart and is the artery that connects to the heart’s left ventricle.

This tubular structure is thick walled and pink white to red-white in color. The aorta and smaller connecting arteries are better examined after the organs in the thorax and abdomen are removed. The fat pad that covers the organs must be cut or torn to reveal the gizzard (ventriculus) and the stomach (proventriculus) to the bird’s right side. The spleen is readily visible at the junction of the stomach and gizzard after they are exposed. This lymphoid organ is oval or elliptical in shape and dark red to purple in coloration. The spleen in an adult bird is approximately one inch long. The air sacs on the left are also readily visible after the stomach and gizzard are set aside. These clear membranes are attached to the lungs and increase the respiratory capacity of the bird. Female birds that are in production may have yellow fat deposits on the air sacs. The air sacs on the bird’s right side should also be examined; it is usually necessary to move the liver, stomach, and gizzard to the bird’s left side to examine them adequately.

Avian lungs are closely adhered to the ribs and are an orange-red or pink-red color. The lungs can be removed for a close examination of the ribs. The ribs, as with all avian bones, are smooth thin walled and white. Immediately below the lungs are the kidneys, adrenal glands, and gonadal tissues (testes or orvaries). The kidneys are firmly embedded in depressions in the bone (synsacrum) and have three distinct lobes (cranial, middle, and caudal). The bird has two kidneys, a left and right, and these organs are dark red to dark brown with a fine reticulated patient visible. A small, white tube (the ureter) connects each kidney to the cloaca. The adrenal glands are small tan triangular shaped glands located at the section of the kidney near the lung. Gonadal tissue is also located near the kidney. The male has two testes, one on either side of the midline.

These organs are bean shaped or elliptical shaped and tan. Two small white coiled tubules connect the testes to the cloaca. In the female only the left ovary and oviduct are generally present near the left kidney. In an immature female the ovary is roughly triangular in shape or shaped like an inverted L. It is white to light yellow in color and may have a granular or gritty appearing surface. The developed oviduct is a large grey-white tubular organ that has distinct longitudinal structures. The oviduct connects the ovary to the cloaca and adds egg components such as albumen, shell membranes and shells as it transports the follicle (yolk) to the surface.

Located near these organs and near the midline is the descending aorta. This thick walled artery is a continuation of the aorta as it leaves the heart. It is from this major artery that numerous smaller arteries arise to supply blood to the internal organs. The aorta is pink-white to red-white in color.

The digestive tract should be examined next. The stomach (proventriculus) is a spindle shaped organ that has the gizzard (ventriculus) attached to it. The stomach is grey in color and internally the lining in glistening grey-white with small papillae (gland openings) present. The gizzard is a round dark brown to dark red organ attached to the gizzard. Internally, the gizzard (ventriculus) has a koilin lining which is yellow to yellow-green in color.

The duodenum is the first section of the small intestines. It is loop shaped and surrounds the pancreas. The pancreas is a white-tan fleshy organ. The duodenum, like all of the small intestines is a tan-grey to white-grey tube which has a fine textured lining similar to the surface of a towel. The jejunum and ilium are the next two sections of the small intestines. Two sack-like structures are attached to the small intestines at the junction of the large intestines and ileum. These structures are the cecae which are thin walled with small thick areas in the wall (cecal tonsils) at the points of attachment to the small intestines. Cecal contents are dark green or dark brown. The large intestine (which is very short) lies between the ileum to the opening to the surface called the cloaca. The cloaca is similar in color to the small intestines but is of larger diameter. Feces in the large intestine and cloaca is generally drier and green to brown feces in color. The ileum contains a more liquid feces of similar color. White pasty urates are often present in the cloaca. The bursa of Fabricius is a round tanwhite lymphoid organ which is organ located behind (dorsal) the cloaca.

Most blood vessels are examined along with the organs such as checking the large vessels coming to or leaving the heart when the base of the heart and syrinx are examined. Blood vessels vary in size depending on the organ supplied. Arteries are thicker walled than veins, and are a pink white to red white in color. Veins are thin walled, tend to flatten out when touched and are a dark blue in color due to the blood in them.

Neck Region

The mouth and neck of the bird should also be examined. A cut is made at the corner of the mouth and extended down the neck, thus exposing the structures for closer examination. In the mouth of the bird the tongue can be examined. This triangular shaped organ is dull grey-white and has a few bumps (papillae) on the surface. Directly behind the back of the tongue (and connected to it) is the glottis. The glottis is the opening of the trachea. It is white in color and has two folds (left and right) which come together to close the opening when the bird swallows. The oropharynx is the region at the back of the mouth and is a glistening grey-white color. Located on the roof of the mouth is the cleft opening called the choana. This structure should be clean with a small amount of clear mucous usually present in the cleft. The choana is also grey-white in color and numerous conical papillae are around the cleft.

The esophagus should be opening and examined. It too is grey-white in color and has a smooth surface. There is an organ at the base of the esophagus called the crop. The crop is a pouch of the esophagus and as such is the color and texture of the esophagus. The trachea is also present in the neck. This white tubular structure has rings of cartilage visible from the outside. Inside the trachea is a small amount of clear mucous and the lining is a glistening clear white.

The remaining most obvious organ in the neck is the thymus. This organ is multi-lobed and tan in color. Often yellow fat is intermixed with the lobes. This organ 15 located near the base of the neck and crop.

The beak should be removed to expose the nasal cavity. There are scroll like structures in the nasal cavity which are a tan-white in color. There is also a small amount of clear mucous on these scrolls.

Another area to examine is the breast musculature. The superficial breast muscle should be cut into to check the deep breast muscle (supracoracoideus muscle). This deep muscle is the same color as the superficial breast muscle. The joints of the leg are also cut into and examined. All joints in the leg should contain a clear fluid. The cartilage in the leg joints can also be examined at this time. Cartilage is a bright white to grey- white in color and has a smooth surface. The ends of the leg bones are usually cut to examine the bone marrow, and check for cartilage plugs. If a cartilage plug is present in the end of the tibiotarsal bone it appears as a triangular shaped plug that is white to grey-white in color. Bone marrow is red in color and soft in texture. The structures and organs discussed are those examined on a routine field necropsy. Naturally, any area that looks “abnormal” is more closely examined.

Source: Avian Advice - Winter 2005 - Volume 7, Number 1

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