THE SOURCES OF ENERGY 61 



things, are also insoluble, and they must be converted into 

 dextrose. Digestion consists, therefore, of the solution of the 

 proteids and fats, and the conversion of the starches and other 

 carbohydrates into dextrose. 



As soon as food enters the mouth digestion begins. The 

 salivary enzyme, ptyalin, acts on the starchy substances, con- 

 verting them into sugar, and in the stomach the pepsin of the 

 gastric juice acts upon the proteids. The fats are not touched at 

 all until the food enters the duodenum, and then the main diges- 

 tive operations begin. The trypsin of the pancreatic juice and 

 the erepsin of the intestinal juice act energetically on the proteids, 

 carbohydrates, and fats, and the bile secreted by the liver aids 

 in a way that we cannot explain here. The result of the action 

 of all these enzymes is that the proteids are split up into their 

 constituent amino- acids, the starches are converted into dex- 

 trose, and the fats and oils are split up into fatty acids and 

 glycerine. Now all these substances are soluble in water (or at 

 least in the liquid bathing the internal walls of the alimentary 

 canal), and so they can soak through into the blood. These 

 substances — amino- acids, fatty acids, dextrose, and glycerine — 

 are what the animal organism wants — they are its proximate 

 food principles. 



Absorption and Distribution. — They must get into the blood- 

 stream and be carried to the muscles, glands, and other tissues 

 of the body. How ? That leads us to consider the organs of 

 circulation. Everywhere in the body there are minute blood- 

 vessels, called capillaries, forming a close network, and along with 

 these there is a separate system of vessels called the lymfhatics. 

 We do not consider the latter here, except to say that ultimately 

 they communicate with the bloodvessels. 



We regard the bloodvessels (from our point of view of the 

 distribution of nutritive matter) as beginning in the walls of the 

 alimentary canal. There the capillaries form a very close 

 network just within the internal lining (the mucous membrane), 

 and the liquids in the alimentary canal are separated from that 

 in the capillaries by the mucous membrane, some very loose 

 connective tissue, and the very delicate walls of the capillaries 

 themselves. So it is easy for the digested, soluble food substance 

 to soak through the mucous membrane, the loose submucous 

 layer, and the walls of the capillaries. Now the " soaking " 

 through is not the same process as the soaking of a few drops of 



