594 STRUCTURE OF VERTEBRATES 



together make the small intestine, and colon and rectum the large 

 intestine. The gullet is generall}^ a mere passage, down which 

 food passes quickly, and only of much size in long-necked tetra- 

 pods. In some birds, particularly seed eaters, its lower part is 

 expanded into a thin-w^alled storing crop, and in some carnivorous 

 species a limited amount of digestion, bacterial or autolytic, 

 goes on in it. Its wall in pigeons breaks down to form the ' pigeon's 

 milk ' on which the young are fed. The first three chambers 

 (rumen, reticulum, psalterium, Fig. 385) of the complex 

 ' stomach ' of ruminants are formed from the gullet. 



The stomach was probably originally a storing organ, and 

 storage is probably still a large part of its function in elasmo- 

 branchs, but it usually also carries out preliminary proteolytic 

 digestion, as in mammals. In cyclostomes, lung fishes and man}^ 

 teleosts, where large prey is not taken and storage is not needed, 

 there is no stomach. In many birds there is a division of the 

 stomach into an anterior glandular part, the proventriculus, 

 and a posterior muscular part, the gizzard, so that mechanical 

 and chemical breakdown of the food are largely separated. 

 Gizzard and crop are often found well developed in the same 

 species. 



Marking the distinction between stomach and intestine there 

 is usually a sphincter, the pylorus, which regulates the passage 

 of food, another indication of the storage function of the stomach. 

 The division of the small intestine into duodenum and ileum 

 cannot be made out in many lower vertebrates, or indeed in 

 some mammals, and primitively at least its whole length produces 

 enzymes which digest the- three main classes of foodstuff, and 

 absorbs the products of the breakdown. Much enzyme produc- 

 tion has, however, been transferred to the pancreas, a diffuse 

 glandular outgrowth from its anterior end. To facilitate absorption 

 the internal surface is increased, especially in herbivores. In the 

 dogfish and other primitive fish there is a spiral valve, which 

 in effect divides up the lumen of the intestine into a long helical 

 tube, and this was probably the primitive condition. In teleosts 

 and tetrapods the same result is achieved by an external coiling 

 of the intestine, with increase of its length, which reaches a 

 maximum in herbivorous mammals, w^here it may be twenty 

 times the length of the body. The surface is also increased in 

 mammals, and to a lesser extent in other groups, by internal 

 finger-like projections or villi. 



