DIGESTION 



373 



and strong jaws, are common. In those birds which eat hard grains there is a 

 powerful muscular organ, the gizzard, whose cavity contains small stones swallowed 

 by the bird and acting as mill-stones grinding the food before it is passed on to 

 the stomach for digestion. 



In animals which chew the cud, Ruminants, the grass, etc., after it has been 

 roughly chewed in its first gathering, passes into a large receptacle, where it 

 undergoes a softening process by the action of bacteria. It is then brought back 

 into the mouth again, bit by bit, and thoroughly masticated before being again 

 swallowed and passed into the digestive stomach. 



In some animals, as the crayfish, there is a masticating apparatus in the 

 stomach, which is followed by a kind of filter, so that food is not allowed to pass 

 into the intestine, into which the chief digestive gland opens, until it has been 

 adequately subdivided. 



The main purpose served by the saliva appears also to be a mechanical one, 

 since in many animals it contains no enzymes. It enables dry food to be readily 

 masticated and swallowed. In animals taking food containing starch, we find that 

 the saliva contains an amylase, sometimes called "ptyalin," which converts starch 

 into maltose and, under favourable conditions, by further hydrolysis to glucose. 

 We have already seen how carbohydrate food, by some chemical mechanism, causes 

 an increase in the amylase content of the saliva in man (Lovatt Evans). 



Having commenced with carbohydrate, we will follow its progress further, and 

 afterwards that of proteins and fats. 



Although ptyalin is a powerful enzyme, it has too little time to do much work 

 while the food remains in the mouth. It is inactive in so strongly acid a solution 

 as the gastric juice, but it has been shown by Griitzner (1905) that the food in the 

 stomach does not at once come into contact with the acid secretion, especially that 

 food which is latest swallowed, which lies for some time in the middle and is pro- 

 tected by that first swallowed. So that, if the food be mixed with blue litmus, the 

 centre of the mass in the stomach remains blue for some time and amyloclastic 

 action can proceed. In any case, starch that escapes the action of ptyalin meets 

 with the pancreatic juice in the intestine, and the amylase contained therein effects 

 complete hydrolysis. 



As regards other carbohydrates, cane-sugar, maltose, and lactose are hydrolysed 

 by appropriate enzymes formed by the glandular epithelium of the small intestine. 



The manner of dealing with cellulose is of some interest. An enzyme which 

 acts on cellulose is found in seeds barley, for example and has also been observed 

 in the alimentary canal of the mealworm and in the secretion of the " liver " of 

 the snail (Biedermann, 1911, p. 980), which forms hexoses and pentoses from 

 various celluloses. It attacks also mannanes, galactanes, etc., the so-called 

 " reserve " or storage celluloses. But in the higher animals the assistance of 

 bacteria, chiefly in the large intestine, is required. In the ruminants, bacterial 

 action also takes place in the paunch, before the second chewing process. The 

 large caecum present in those animals which take cellulose in quantities, such as 

 the rabbit, horse, or sheep, will be remembered and the mechanism of the digestion 

 of cellulose must be effective, since it is said that sheep will get fat on blotting 

 paper. Now the difficulty is that bacteria carry the process of destruction too 

 far, producing hydrogen, methane, carbon dioxide, and lower fatty acids. 

 Pringsheim (1912), however, lias succeeded in showing that a bi-hexose, together 

 with the product of its further hydrolysis, glucose, is formed as an intermediate 

 product. These sugars were obtained by stopping the fermentation at its height 

 by the addition of toluene. It is uncertain whether the antiseptic acts by 

 destroying the particular organisms responsible for the production of hydrogen, 

 methane, etc., or whether it sets free the cellulose-hydrolysing enzyme from an 

 intracellular condition. It may possibly merely put a sudden stop to all further 

 change, so that a certain amount of intermediate products are, as it were, 

 caught on the way. In any case, the " cello-biose " and glucose were isolated 

 from cultures with filter paper of de-nitrifying, methane-producing, or better, 

 thermophile bacteria. The sugars had been produced from the paper. It is 

 interesting that cello-biose is also hydrolysed by emulsin. There seems to be 



