i86 ANIMAL BIOLOGY. [Part I. 



as a whole, and the manner in which the elaborated products 

 are absorbed. 



Digestion and Absorption. The organic food-stuffs, exclusive, 

 that is, of water and saline matters, are of two kinds, nitrogenous, 

 such as the proteids and gelatins ; and non-nitrogenous, such as the 

 carbo-hydrates, starch and sugar, on the one hand, in which the 

 oxygen and hydrogen are in due proportion to form water, and 

 on the other hand oil and fat, in which there is a relative 

 deficiency of oxygen. These nitrogenous and non-nitrogenous 

 food-stuffs have to be consumed in such proportion as to make 

 good the material lost to the body in nitrogenous excreta, in 

 water, and in carbonic acid gas. 



In the mammalia they are prepared for further digestion by 

 mastication. In the mouth the food is mixed with saliva, which 

 has no action on the fats or proteids, but converts the starch 

 into glucose. Besides having this direct chemical action, the 

 saliva serves to moisten the food and make it more readily 

 swallowed. This process of deglutition is a somewhat com- 

 plicated one, bringing many muscles into play. The morsel of 

 food is pushed back into the pharynx, where it is grasped and 

 pushed onwards, the circular muscles of the part of the tube in 

 front of it relaxing while the muscles behind it are successively 

 contracted. Thus it passes down the oesophagus into the 

 stomach. 



There it is for a while imprisoned by the firm closure of the 

 cardiac and pyloric apertures, which only momentarily open for 

 the admission of food or the emission of its contents. By the 

 action of the strong gastric muscles the food is rolled about and 

 thoroughly mixed with gastric juice, the acidity of which at once 

 stops the further action of ptyalin on starch. Gastric digestion 

 thus succeeds salivary digestion. The initiation of the con- 

 version of starch into glucose ceases, but the proteids are con- 

 verted into peptones, and the fats are, by the breaking down 

 of their proteid framework, set free, and incorporated with 

 the food, though they are not further acted upon. Thus 

 the food is converted into chyme, a fluid of the consistency of 

 pea-soup, which is allowed to pass through the pylorus into the 



