COMPARATIVE DIGESTIBILITY OF FOOD-STUFFS. 433 



the different degrees of perfection with which mastication is accom- 

 plished, to differences of construction in the alimentary canal, different 

 constitutions, and the different degrees of concentration of the digestive 

 juices. Thus, the herbivora, through the high development of their 

 molar teeth, the roomy and long digestive eanal, and large amounts of 

 amylolytic ferments in their digestive secretions, are especially suited for 

 digesting carbol^drates. Their gastric juice is comparatively poor in 

 acid, and, as a consequence, prevents their living on a highly albuminous 

 diet ; while the highly alkaline nature of their intestinal contents facili- 

 tates putrefactive changes in albuminoids, the characteristic results of 

 such fermentations being met with in large amounts in their urine. In 

 the carnivora, on the other hand, we find a short and less capacious 

 intestinal canal, with a relatively voluminous stomach and an active acid 

 gastric secretion, thus fitting them for the digestion of albumen, while 

 the conditions for the digestion of carboh} r drates are less favorable. 



Omnivora naturally occupy a mean between these two classes. It 

 is to be remembered, however, that as long as animals are fed on their 

 mothers' milk there is no difference in the digestive act in the carnivora 

 or in the herbivora. 



Not all the nutritive substances which are contained in the food are 

 actually digested, but a considerable proportion, under the most favorable 

 circumstances, is apt to remain undigested and pass unchanged into the 

 faeces. The cause of this is frequently to be found in the fact that the 

 food is taken in such quantities that the amount of digestive secretion 

 poured out by the alimentary tract is insufficient to act upon it. There 

 appears to be, again, a limit of absorbability even of the amount of food 

 digested. This limit, of course, varies in each group of, animals, and the 

 residue of food, even though it may be digested, remains unabsorbed in 

 the alimentary canal to undergo breaking down into various decompo- 

 sition products, such as leucin, tyrosin, etc. Again, another cause for 

 indigestibilit}' of food is to be found in the fact that in many cases, 

 especially in the food of the herbivora, the nutritive principles are con- 

 tained in resisting envelopes which are impermeable to the digestive 

 secretions, and which require mechanical comminution in mastication 

 before being accessible to the act of digestion. Imperfect mastication, 

 therefore, from whatever cause, will reduce the digestibility of food. By 

 this term, digestibility of food, is meant the amount of any food-stuff 

 which through digestion is rendered capable of absorption and does 

 actually enter the blood, in proportion to the amount which remains 

 undigested or which is not so absorbed. This quantity, which may be 

 termed the co-efficient of digestion, varies according to the composition 

 of food and to the mode of digestion of different classes of animals. We 

 will, therefore, allude to these sources of variation in turn : 



