598 SPECIAL PHYSIOLOGY. 



the ordinary temperature of a warm-blooded animal, and inversely, that the 

 gastric juice of the warm-blooded animal acts slowly, or not at all, at the 

 temperature of the fish ; and, moreover, that the solvent powers of the gastric 

 juice of a Mammal are not lost, until it has been heated to 120 ; whilst in 

 the case of the fish, they are lost at 80 (Brinton). If this be confirmed, it 

 shows a remarkable modification of the properties of the same secretion, in 

 different animals having particular conditions of existence. It would be in- 

 teresting to note whether post-mortem digestion of the gastric cavity of the 

 non-vertebrated animals ever takes place. 



From its peculiar color, the bile can be easily recognized. It may thus be 

 detected in those Annulose and Annuloid animals, even in the minute Ro- 

 tifera, in which the liver is not massive, as it is in the Yertebrata, Mollusca, 

 and higher Molluscoida, being represented only by hepatic tubuli ; colored 

 secretions are also detected in the gland cells of still lower animals. The 

 office of the bile must be similar in all animals in which it is found. 



The peculiar property of the, pancreatic juice, that of emulsifying and decom- 

 posing fat, has been shown by Bernard to exist not only in Mammalia, but 

 also in Birds, Reptiles, Amphibia, and Fishes, as, for example, in the goose, 

 turtle, frog, salamander, and ray. Moreover, he asserts that, when the pure 

 saliva, in any animal, is incapable of converting starch into sugar, the pan- 

 creatic fluid possesses this property. 



The action of the intestinal juice upon food has been shown to be, in the 

 higher animals, as in Man, supplementary to that of the other secretions. In 

 the lower animal forms, in which the intestinal tubuli come to represent im- 

 portant glands, an analogous blending of function may prevail. 



The organic food of all animals, whether derived from other animals or from 

 plants, consists of similar proximate chemical substances ; and the solution of 

 these is probably accomplished, in all cases, by processes of a similar nature. 

 But our knowledge of the specific chemical differences and modes of action of 

 the digestive fluids in different animals, is yet imperfect. In the Mammalia, 

 and perhaps in all Vertebrata, the composition and action of the fluids resem- 

 ble those observed in man. The gastric juice of the herbivorous sheep and 

 calf dissolves animal food, as readily as that of the omnivorous pig and car- 

 nivorous dog. But many modifications, yet undiscriminated by the chemist, 

 doubtless exist, as illustrated by the ascertained varieties in the acids of the 

 gastric juice and the bile, in certain Mammalia and Birds (pp. 531, 540). Still 

 more important differences in composition and power may exist, especially in 

 the lower tribes, in some or all of the digestive secretions, adapting them to 

 the solution of substances ordinarily indigestible. Thus cellulose, lignin, 

 and even resinoid bodies from the vegetable kingdom, and yellow elastic tis- 

 sue, cartilage, and the horny, chitinous, and coriaceous integuments from 

 the animal kingdom, are eaten and probably partially digested by certain in- 

 sects and other creatures, though usually those substances resist digestion. 



In the complex alimentary canal and glandular appendages of the Verte- 

 brata, Mollusca, and Annulosa, in the more simple digestive "system of a poly- 

 zoon, or of a rotiferous animalcule, and even in the digestive cavity of the 

 hydra, with its single external opening, its communication with the cavity of 

 the body, and its want of distinct glandular appendages, the digestive secre- 

 tions are always produced by glandular epithelial cells, whence they are 

 discharged into the alimentary canal, at suitable points, to act upon the food. 

 In the absence of massive or tubular glands, or of clusters of special cells, it is 

 even possible that adjacent cells, nearly or quite similar under the microscope, 

 may perform the office of different glands. Digestion, in the hydra, may be as 

 complex a process, regard being had to the chemical composition of its food, 

 as in the highest Vertebrata. So long, indeed, as we recognize in any animal 

 a distinct digestive cavity, we may reasonably infer the occurrence of a diges- 

 tive process, rendering different nutrient substances soluble and absorbable. 

 The starchy, albuminoid, fatty, and the less digestible substances used as food, 

 must require their peculiar transmuting, liquefying, or emulsifying solvents, 

 produced in infinitesimal quantity, but acting with characteristic power. It 

 is observable, however, that of these fluids, the peptic and emulsifying agents 

 are the most essential, specific, and universal : for in the cold-blooded aquatic 



