THE ABSORPTION OP FOOD. 81 



proteins as amino bodies, into the blood, whilst fats are absorbed 

 into the lymphatic vessels, as fatty acids and glycerine. These 

 substances are absorbed in solution, which would lead us to ex- 

 pect that, because of the water absorbed along with them, the 

 contents of the small intestine would be more solid at its lower 

 end than at its upper end ; but this is not the case, for the diges- 

 tive juices which have been secreted make up for the loss of 

 water. It is in the large intestine that the water is finally ab- 

 sorbed. 



Attempts have been made to explain the process of absorption 

 in terms of the known laws of filtration, osmosis, surface tension, 

 and imbibition, but little further progress has been made than to 

 establish the fact that although these processes may play a role, 

 they do not explain the whole thing, for if blood serum be placed 

 in an isolated loop of intestine, it will become entirely absorbed 

 even although identical in all the above properties with the 

 blood of the animal. That osmosis does have some influence, 

 however, is evidenced by the well-known effect of a strong 

 saline solution in the intestine; it attracts water from the blood, * 

 thus diluting the intestinal contents and stimulating peristaltic 

 contractions. It is in this way that saline cathartics act. 



Regarding the absorption of fats, it is now definitely known 

 that these first of all split into fatty acid and glycerine by the 

 action of the lipase of pancreatic juice. The fatty acid then 

 unites with alkali to form a soap, or with bile salts to form a sol- 

 uble compound. In either case, the dissolved fatty acid passes 

 into the intestinal epithelium, into which is also absorbed the 

 glycerine, the two re-uniting after their absorption so as to form 

 neutral fat again, which then passes into the central lacteal of 

 the villus, whence it is transported by the abdominal lymphatics 

 to the thoracic duct, which discharges it into the subclavian vein 

 on the left side of the root of the neck. 



Hunger sensations coincide with stomach contractions which 

 differ from those occurring during digestion. Thirst is due to 

 dryness of the throat. It is temporarily relieved by moistening 

 this, but unless liquid is swallowed permanent thirst develops 

 because the tissues become dry. 



