132 The Vertebrate Organ Systems 



doves, the crop secretes a special substance, pigeon milk, for the nourish- 

 ment of the nestlings. 



The Stomach. — Among the vertebrates, the stomach varies in 

 shape from a slight enlargement of the digestive tract to a well-defined 

 pouchlike structure. 



The stomach is important not only as an organ of digestion but 

 also as one of temporary storage. This latter function is very impor- 

 tant in animals that must ingest a large amount of food and escape 

 before enemies arrive. The food can then be digested after the animal 

 has found a secure retreat. Even man utilizes his stomach as a tem- 

 porary storage organ. He eats three times a day and digests his 

 food while he works or sleeps. 



Man has a very generalized type of stomach (Fig. 41,5), one 

 that is quite similar to that of most other mammals. It has a capacity 

 of about two and one-half quarts. The outer border is known as the 

 greater curvature due to its having a much greater length than does 

 the inner border, the lesser curvature. The boundary between the stom- 

 ach and esophagus is marked by the cardiac valve, a sphincter muscle. 

 The region of the stomach just below the entrance of the esophagus 

 is known as the cardiac region. The entire expanded middle portion 

 of the stomach is the fundus. Near the junction with the small intes- 

 tine, the stomach narrows to the pyloric portion. The entrance to 

 the small intestine is guarded by the important sphincter muscle, the 

 pyloric valve. Externally it is difficult to distinguish the bound- 

 aries of these various regions ; internally, however, the epithelial 

 lining varies. 



While the structure and form of the human stomach are typical, 

 some vertebrates have modifications. One of the more unusual of these 

 is that found among the ruminants (cows, sheep, deer) (Fig. 41, A). 

 In these forms, the stomach has four distinct chambers : the rumen. 

 the reticulum, the omasum, and the ahomasum. Among the wild ru- 

 minants, this modification is of great survival value. The animals feed 

 entirely on grasses and other vegetation which take a long time to 

 digest. These animals are often large and require a large intake of 

 food. With this chambered stomach, the animal can consume a large 

 amount of food and then retire to a safe spot to chew it. 



When food is taken in, it first passes to the rumen for temporary 

 storage. At a later time, the unmodified food passes into the reticu- 

 lum. The wall of the reticulum has many pits which give it a reticu- 



