The Animal Body. 225 



Absorption of food. In the ways mentioned above, the pro- 

 teins, fats and carbohydrates of the food are gradually digested. 

 Throughout the length of the small intestine absorption proceeds 

 rapidly; water, salts and the products of digestion pass out 

 from the intestine into the circulating lymph and blood. There 

 are two pathways by which absorbed material reaches the blood. 

 In the intestinal wall are numerous projections, called villi. Im- 

 bedded in these structures are the minute branches of two sys- 

 tems of vessels. One set is the lacteals, belonging to the lym- 

 phatic system and the other the capillaries of the blood system. 

 Materials passing into the lacteals reach the thoracic duct and by 

 it, in a roundabout way, are carried into one of the main blood- 

 vessels at the neck. As a general truth it may be stated that the 

 fats are largely absorbed through this channel, and it is impor- 

 tant to observe that when they reach the lacteals they are again 

 in the form of neutral fats. 



Materials absorbed by the capillaries of the blood system are 

 carried directly to the liver through the portal vein, and there 

 subjected to the action of that organ before they enter the gen- 

 eral circulation. Most salts and the carbohydrates and proteins 

 follow this course. In the liver the soluble sugars are converted 

 into glycogen, the animal starch, and as such temporarily stored. 

 The amount of sugar in the blood is a constant but small quan- 

 tity and as this is required in the tissue, the glycogen is recon- 

 verted back into soluble sugar to maintain the supply in the 

 blood. 



The fragments of protein digestion, the proteoses, peptones 

 and amino-acids, are not found as such in the blood or at least 

 only in traces. Either in passing through the intestinal wall, 

 or after reaching the liver, they are reconstructed into complex 

 proteins before being cast loose into the circulatory system. 

 These reconstructed proteins are the serum albumin, serum glo- 

 bulin and haemoglobin of the blood, which serve as sources of 

 protein for the various body tissues. The processes of absorption 



