173 Comparative Animal Physiology 



horse, and 7.9 in pig (Waldschmidt-Leitz, cited by Sumner and Sommers.^^") 

 This gastric lipase is not physiologically important; at least it cannot be 

 effective at the pH of the stomach. Pancreatic secretion contains most of the 

 active lipase of vertebrates. In fish it is difficult to separate pancreatic tissue 

 from the intestinal wall, and extracts of the intestinal wall (plaice^^) or of 

 mtestinal slime (carp) yield a Upase which is active m a weakly alkaline 

 medium. In the plaice Pleiironectes fat may be absorbed in the stomach after 

 regurgitation from the intestine.^^' ^^ Pancreatic lipase is more effective in 

 the presence of the emulsifying bile salts. 



Bile salts alone can produce neutral fat droplets 2/x in diameter, but together 

 with monoglycerides they can produce droplets less than 0.5/a in diameter, 

 which can be absorbed directly. ''" In mammals part of the fat is hydrolyzed 

 and the fatty acid absorbed into the mucosa, whence it is carried in the blood 

 to the liver. A large part of the fat (2/3 to 3/4) is absorbed into the lacteals 

 and transported in lymph to the thoracic duct; it was formerly thought that 

 all the fat was digested and soaps formed and absorbed, but it seems more 

 probable that finally emulsified neutral fat is absorbed directly into the lac- 

 teals. •'^" It appears, then, that only a small part of ingested fat is hydrolyzed 

 in the digestive tract, and that bile salts are important for absorption of fat. 

 The neutral fat is then broken down elsewhere in the body. Deposited fat to 

 some extent reflects the type of fat ingested; the melting point of subcutaneous 

 fat in hogs is lower when the pigs are fed oils of low melting point than when 

 they are fed harder fats. Absorption of triglycerides has not been extensively 

 examined except in mammals. 



CORRELATIONS OF DIGESTIVE ENZYMES WITH FOOD HABITS; 



CONCLUSIONS 



Numerous examples on the preceding pages have shown that the pattern 

 of digestive enzymes differs greatly, even among closely related animals. 

 Digestion is adapted to the dietary habits in that a given kind of animal may 

 feed on and digest predominantly protein or carbohydrate or even one sub- 

 stance within these classes. The extent to which digestion is labile for individu- 

 als if one food is missing and another substituted has hardly been considered, 

 yet this must be the way new food habits arise. It is difficult to obtain quanti- 

 tative measures of the amount of activity of a given digestive enzyme in 

 different animals, and in only a few groups, such as insects, ^'^ has an effort 

 been made to compare amounts of enzymes in different species. Each sort of 

 digestive activity has usually been described as strong or weak, present or 

 absent. 



Yonge '^''' ''■'" has pointed out that two trends are evident in the relation 

 of feeding behavior and diet to digestive enzymes: (1) selection exclusively 

 of food that can be digested, and (2) indiscriminate feeding. Animals of the 

 first group, food specialists, are of many sorts. The carnivores, or meat eaters, 

 have strong proteases and weak carbohydrases. Animal groups with strong 

 proteolytic activity but in which carbohydrases are weak or absent are coelen- 

 terates, Turbellaria, Asteroidea, cephalopod molluscs, stomatopod crustaceans, 

 and some insects. '"" Many coelenterates lack amylase; they digest protein 

 in the gastrovascular cavity and may have very weak carbohydrases intracel- 

 lularly, as in Mydra,-- in madre^wre corals which do not digest sugars,'^" 



