THE STOMACH

Species variation in the gastrointestinal tract is most marked where the stomach is concerned. The stomach of fish- and flesh-eating species (raptors, hawks, ospreys, vultures, and owls) is primarily a storage organ appro­priate for the chemical digestion of a soft diet.

The stomach of these birds is divided by a constric­tion (isthmus) into a predominantly glandular proven­triculus and a predominantly muscular ventriculus (gizzard) placed one behind the other close to the median plane. The proventriculus is ventrally in contact with the left lobe of the liver. The larger, more caudal gizzard also touches this but has more extensive contact with the sternum and the lower part of the left lateral abdominal wall; it is exposed when the sternum and abdominal muscles are removed during necropsy (Figure 37-16).

The proventriculus is spindle-shaped and about 4 cm long. Its whitish mucosa, lined with a mucus-secreting, columnar epithelium, is clearly demarcated from the more reddish lining of the esophagus (Figure 37-18 and Figure 37-19). It presents numerous macroscopic eleva­tions (papillae) through which pass the collecting ducts from a thick bed of glands, very visible on the cut surface of the wall. The papillae are so prominent that they may be mistaken for parasitic lesions. There are two kinds of epithelial cells in the glands: oxynticopep- tic cells that produce both hydrochloric acid and pep­sinogen and cells that produce mucus.

The isthmus is the transition from the glandular stomach to the muscular gizzard. It has no glands in its thinner, less rigid wall.

The ventriculus or gizzard is lens-shaped in herbi­vores, poultry, and waterfowl and is positioned with its convex surfaces facing more or less to right and left. Its

Figure 37-17 Stomach, ventral surface (A) and opened ventrally (B). 1, Esophagus; 2, proventriculus; 3, papillae; 4, deep proventricular glands, visible on cut surface; 5, lumen of gizzard; 6, caudal blind sac; 7, cranial blind sac; 8, pyloric orifice; 9, cranioventral muscle mass; 10, duodenum.

Figure 37-18 Stomach of chicken. 1, Esophagus; 2, spleen;

3, proventriculus; 4, gizzard with aponeurosis (4'); 5, duodenum.

interior is elongated, enlarged by cranial and caudal blind sacs, of which the former connects with the pro­ventriculus. The duodenum arises on the right surface, adjacent to the cranial blind sac. The bulk of the organ consists of two thick masses of muscle that insert on glistening tendinous centers, one on each surface. Thinner muscles cover the blind sacs. The mucous mem­brane is thin but very tough; bounded by a cuboidal epithelium, it largely consists of tubular glands, whose secretion solidifies on the surface. It forms, catalyzed by the low pH due to the hydrochloric acid from the pro­ventriculus, a hard cuticle of koilin (a carbohydrate- protein complex). The koilin, a rough plicated layer, is replenished from the glands below as it is worn on the

Figure 37-19 Opened stomach. Note grit inside gizzard (right).

surface. It obtains a yellow-green color from the bile refluxed from the duodenum. In herbivorous and omnivorous birds, powerful contractions of the gizzard crush the food, assisted by ingested grit, which must be provided in the diet. Being radiodense, the grit identifies the gizzard in radiographs. The gizzard is the site of protein digestion.

In granivores, psittacine species, and songbirds the gizzard is less muscular because these birds dehusk and crumble their seeds before swallowing. These birds do not always require grit.

Muscular activity moves food back and forth between the proventriculus and gizzard during digestion; the location of the pylorus then enables some of the food that does not require grinding to escape into the duo­denum, bypassing the gizzard.