The digestive tract and associated accessory organs together constitute the digestive system. The basic structure of digestive tract is not similar in all vertebrates. Stomach is a very specialized portion of the digestive tract. The sac-like muscular enlargement of digestive tract between oesophagus and intestine is called stomach. It acts as temporary storage place of undigested food particles and makes the food particles ready for digestion.
There is practically no stomach in cyclostomes and some fishes. The first part of stomach, next to the oesophagus and placed closed to heart and so called as cardiac region. The sac-like middle part of the stomach is called fundus. The terminal end of the stomach connecting to intestine is called pyloric region which has a pyloric valve.
In elasmobranchs the stomach is ‘J’ shaped. In frog the cardiac end is wide, there is no fundus, pyloric end is short and narrow. In lizards and snakes the stomach is spindle- shaped. In crocodiles and birds the stomach has two parts, a proventriculus with gastric glands, and a highly muscular gizzard which represents the pyloric region and has a hard cornified lining for grinding food. In mammals the stomach lies transversely and has a large fundus, in many ruminants the stomach has four chambers, a rumen, reticulum, omasum and an abomasum. It is claimed that the first three chambers are modifications of the oesophagus, and abomasums constitutes the cardiac, fundic, and pyloric parts of the stomach, but it has been shown embryologically that all four chambers are modified regions of the stomach. In camels there is no omasum, the rumen and reticulum have pouch-like water cells which were once believed to store water, but they are probably digestive.
Histologically the stomach has the typical parts of the alimentary canal, but it has two peculiarities, the muscularis mucosa is made of an outer longitudinal layer and an inner circular layer of muscles. The epithelial lining is thick with several types of glandular cells forming gastric glands of three types called cardiac, fundic, and pyloric gastric glands. The cardiac and pyloric glands secrete only mucus from their surface cells. Fundic glands (or cardiac glands in some) have three kinds of cells, mucous neck cells produce mucus, oxyntic cells produce hydrochloric acid, they may be present in the cardiac region also, zymogen cells or peptic cells secrete pepsin. In most animals the zymogen cells also secrete two proenzymes called propepsin and prorenin which are converted by hydrochloric acid into pepsin and rennin respectively. The secretions of all stomach cells form a mixture called gastric jouice.
Structure of stomach in various vertebrates
Stomach is straight in lower vertebrates (Cyclostomes, Belone, Salamander, etc.)
- In Proteus, Necturus, Snakes it is long, spindle shaped.
- In turtles, tortoise stomach forms curved tubes.
- “J” shaped or “U” shaped stomach is found in elasmobranches, seal, man etc.
- In Polypterus, stomach appears like a blind pouch due to fusion of cardiac and pyloric parts.
- In crocodiles and some birds stomach is consisting of anterior proventriculus with gastric glands and posterior gizzard or ventriculus.
- In ruminants (cow) stomach has four chambers or compartments. Of these, the first three chambers (rumen, reticulum, omasum) function as reservoirs of food and the last (abomasums) is true stomach and functions in digestion.
- In monotremes true stomach is absent.