278 FATE OF DIGESTED FOOD 



become light-coloured, much more putrescent, and contain a large 

 amount of fat. Bile has a slight solvent power upon fats, as is 

 evidenced by the well-known use of ox-gall for removing greasy stains 

 from textile fabrics. It also possesses, in a well-marked degree, the 

 property .of aiding the pancreatic and intestinal juices in bringing 

 about the emulsification of fats ; this is probably by virtue of its 

 alkalinity and the power possessed by bile acids and their salts of dis- 

 solving lime soap and cholesterol. Bile is said to be possessed of anti- 

 . septic properties and to regulate the putrefaction which occurs in the 

 intestines. It is also a laxative. 



Bile is to a great extent reabsorbed in the intestines and only a 

 portion is expelled in the faeces. 



The Faeces of an animal contain the undigested portions of its 

 food, together with the products of their decomposition under the in- 

 fluence of the enzymes and bacteria present in the alimentary canal, 

 and some portions of the digestive fluids themselves ; mucus and 

 epithelial cells from the walls of the intestines are also present. 



Small quantities of fat, cholesterol, and calcium and magnesium 

 salts of fatty acids also occur, while the specific odour is due to the 

 presence of skatole and indole or their derivatives, though sometimes 

 sulphuretted hydrogen, ammonia, amines and other volatile substances 

 are present. A dark-brown reduction product of bilirubin known as 

 hydro-bilvrubin, C 32 H 40 N 4 O r , is said to be often present. 



Absorption of Digested Food. Water and inorganic compounds, 

 e.g., common salt, are absorbed without change. Little or no absorption 

 takes place in the mouth or gullet. It commences in the stomach, but 

 is mainly accomplished in the intestines. The absorption is not merely 

 a physical process of diffusion ; the living cells through which it occurs 

 exercise a selective action and in many cases produce important chemi- 

 cal changes in the substance being absorbed. 



Absorption of carbohydrates. These are probably entirely absorbed 

 as glucose, the necessary changes being produced in the food by the 

 enzymes in the saliva, pancreatic juice and intestine as already de- 

 scribed. A portion at once enters the blood stream and is conveyed to 

 the tissues, while another portion is probably stored as a reserve in the 

 liver, in the form of glycogen. 



Absorption of fats. A small portion of the fat in the food is saponi- 

 fied ; but, as already stated, the greater portion is merely emulsified. 

 The lacteals of the intestine are the absorbents for fat, and exactly how 

 the minute globules pass through the walls of the intestine is not under- 

 stood. When fatty acids are fed to an animal the contents of the 

 lacteals contain fat, so that glycerine must have come from some other 

 portion of the food and fat must be synthesised. 



Absorption of proteids. Soluble proteids are often absorbed un- 

 changed ; in general, however, the proteids are converted into peptones 

 and albumoses by the action of either pepsin or trypsin. The extra- 

 ordinary fact is that in the blood no peptones can be found ; indeed, if 

 they be injected into the blood stream, poisonous effects are at once 



