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THE POPULAR SCIENCE MONTHLY. 



is the gastric juice, the presence of which determines the location in 

 the food-tract of the stomach proper. This is because the fluid is pro- 

 duced by the lining of the stomach, and not by a distinct organ. That 

 even the microscopic animals have some digestive fluid, like gastric 

 juice, is regarded as proved by the fact, already noticed, that solid 

 food is dissolved by them without mechanical aid. This fluid is well 

 shown in the radiated animals. Its active principle is a ferment called 

 pepsin, which acts only in the presence of an acid. The acidity of the 

 fluid is given by free hydrochloric acid. Gastric juice dissolves only 

 nitrogenous substances, as meat, albumen, and gelatine, having little 

 or no effect on oil or starch. 



Next to the gastric juice in importance, if we may judge by its 

 early appearance in the animal kingdom, is the bile. This alkaline 

 fluid is found in all animals having a distinct digestive cavity. The 

 earliest biliary organs are minute cells upon the stomach-lining, as in 

 the anemone. A higher form is found in the small tubes surrounding 



Fig. 11. Water-celes op Camel's Stomach. 



the intestines of the insect, from which there are slow gradations to 

 the superior liver of the higher mollusks and fishes. In the articulates, 

 mollusks, and all higher animals, the bile is poured into the intestine 

 and so separated from the gastric juice. The action of the bile is not 

 fully known ; but it appears to dissolve fats slightly, and helps to 

 subdivide them into minute particles which are " diffused through the 

 liquid like atoms of butter in milk." It probably aids also in the pro- 

 cess of absorption. 



The pancreatic juice, another alkaline fluid, is found below the ver- 

 tebrates only in the higher mollusks. As a gland the pancreas is rudi- 

 mentary in the cephalopods, but appears better developed in the fishes, 

 and proportionally largest in birds. The function of this fluid is a 

 general one, as it acts on nearly all aliments, and seems to be the prin- 

 cipal means of digesting oils and starch, or carbonaceous foods. It is 

 poured into the small intestine near the stomach. 



By the mucous lining of the intestines there is produced an alkaline 



