142 DIGESTION 



rapidly absorbed. Further, absorption of water is attended 

 with increased oxygen consumption. Nevertheless, the 

 process must be influenced in one direction or the other 

 by the osmotic conditions. Hypertonic saline is usually 

 absorbed only after a prehminary dilution, due doubtless 

 to osmosis, while the absorption of hypotonic solutions is 

 facihtated by the higher osmotic pressure in the epithehal 

 cells. It may be mentioned, however, that absorption 

 of hypertonic solutions may occur without prehminary 

 dilution. 



As to the form in which the three classes of foods are 

 absorbed, this question is best deferred, since it has an 

 important bearing upon the metabolic history of these 

 substances. Suffice it to say at present that carbohydrates 

 are absorbed only after hydrolysis to monosaccharides, 

 proteins chiefly, if not entirely, after they have been broken 

 up into amino -acids, and fats only after saponification 

 into glycerine and soaps. After absorption, carbohydrates 

 and proteins enter the blood direct, fats chiefly indirectly 

 by the lacteals and thoracic duct. 



THE LARGE INTESTINE 



In different animals the large intestine varies in size 

 relatively to the whole of the gut, according to the nature 

 of the food which is habitually taken. Its large size in 

 certain herbivora is associated with the extensive bacterial 

 decomposition which takes place within it, and by means 

 of which the cellulose of the food is converted into a form 

 which is readily absorbed. But in man and carnivora 

 this process does not occur, cellulose not being absorbed. 



The digesting mass, as it passes through the ileoceecal 

 valve, is as fluid as it was when it entered the small intes- 

 tine. It enters the large intestine to a great extent deprived 

 of nutriment. It consists of waste products, undissolved 

 substances, bacteria and the digestive juices. In the 

 large intestine this fluid mass becomes concentrated by 



