IX.] THE PROCESS OF DIGESTION 171 



which these foods undergo in the body in order to 

 become available for the nutrition of the animal. They 

 have to reach the blood, this being the circulating 

 medium which conveys them to every part of the body, 

 and in consequence they must either be soluble or 

 become converted into such a state of solubility as will 

 permit them to pass through the walls of the stomach 

 and intestines. These organs form the alimentary 

 canal, from which there is no direct opening into any 

 other part of the body. The process of digestion takes 

 place as the food is passing along this alimentary canal ; 

 whatever is not absorbed by its walls forms the 

 undigested part of the food, which is excreted as the 

 faeces. The solution of each of the constituents of food 

 is effected by one or more of a series of enzymes which 

 are secreted by the animal ; these enzymes being exactly 

 comparable in their action and nature to the diastase, 

 etc., which have previously been described under plants. 

 Considering first the fat, its digestion is not accom- 

 plished until the food has left the stomach ; at this stage 

 the food materials, which had previously been rendered 

 acid by the gastric juice, are brought into an alkaline 

 condition by mixture with the bile and the pancreatic 

 juice. Amongst other enzymes the pancreatic juice, 

 which is also mixed with the food at this stage, gives 

 rise to a lipase or fat-splitting ferment, which resolves 

 the fats into their component fatty acids, and glycerin. 

 These later bodies are capable of passing through the 

 walls of the intestine, and by means of the blood and of 

 the lymphatic system they are distributed about the 

 body. It is found that fat stored up by an animal, or 

 excreted as milk, does to some extent partake of the 

 nature of the fats contained in its food, showing that 

 the fatty acids (the basal material of the fats) are not 

 entirely broken down before the fat is reconstructed in 



