Feeding and Digestion 157 



hormones inHuence normal gastric secretion in amphibians, although histamine 

 and mammalian gastrin are effective when injected into frogs. •''^' '''' 



In elasmobranch Hsh, as in amphibians, the vagus nerve does not affect gas- 

 tric secretion.''- ^'■' In tasting skates and dogfish there is a continuous secre- 

 tion of small amounts of gastric juice; this secretion is inhibited by adrenalin 

 or by sympathetic nerve impulses but is not altered by stimulating or cutting 

 the vagus. If the spinal cord is pithed, removing sympathetic inhibition, a 

 copious "paralytic" secretion appears, possibly due to vasodilatation. " In 

 perfusion experiments both acetylcholine and histamine increase gastric 

 secretion in elasmobranchs. i"'' Gastric secretion is under nervous control of 

 the vagus in mammals and birds; in amphibians the sympaihetics stimulate, 

 and in elasmobranchs the sympathetics inhibit gastric secretion. No fixed 

 pattern exists among vertebrates, and a search for gastric hormones in lower 

 vertebrates is desirable. Secretin has been obtained from the small intestine 

 of birds, turtles, frogs, salamanders, ''-^ and teleosts and elasmobranchs. ^^ 



In the earthworm Liivihriciis, stimulation of segmental nerves elicits the 

 secretion by the anterior intestine of a protease which clots milk.'-'^ In no 

 other invertebrate animal is there evidence for either nervous or hormonal 

 secretion of digestive fluids. However, histological studies have demonstrated 

 waves of secretory activity in the buccal glands and digestive diverticula of 

 snails, "'' and in the digestive diverticula of the crayfish. ''" In the snail 

 Helix the cytological changes include elaboration of granules that may be 

 stored or may form vacuoles which are liberated into the lumen of the gland. 

 In starved snails the buccal glands show a small amount of asynchronous 

 activity; the digestive diverticula show rhythms in which all the cells work 

 synchronously. When the snails are fed, both glands become rhythmic and 

 show a faster sequence. Feeding is the stimulus to the buccal gland, and food 

 in the crop stimulates the digestive diverticula. In Astacus the rhythm of the 

 digestive diverticula is paced by formation of new gland cells by mitotic divi- 

 sion. The rhythm is slow during starvation but is accelerated markedly when 

 food enters the stomach. In several coelenterates, food in the coelenteron or 

 in contact with mesenteric filaments stimulates digestive secretion. "'° Food, 

 then, is the ultimate stimulus for secretion of digestive Huids in all animals. 

 The manner in which the stimulus is transmitted is knov\n well only for 

 mammals. 



DIGESTION OF PROTEINS 



Proteins during digestion are broken down to their component amino acids 

 which may then be absorbed and built into specific new proteins. Protein 

 digestion is essentially a series of hydroKses of peptide linkages: 



Ri O ; H R. R, O R. 



I II I I I II I 



-C-C N-C hHOH >■ C-C-OH +H,N-C- 



I I I I I 



H I H H H 



Classification of Proteases. The classification of proteolytic enzymes has 

 changed radically during the past fifteen years with better understanding of 

 the composition of proteins. In the following account, some features both of 



