72 THE INDIVIDUAL ORGANISM 



the walls of the intestine. The presence of the bile salts is necessary for 

 the absorption of the fatty acids, with which they apparently unite; the 

 bile salts are thus reabsorbed into the body and are eventually returned 

 to the liver. These salts are necessary also for the absorption of carotene 

 and vitamin D and aid in the absorption of vitamins K and E. The end 

 products of digestion, together with the other substances mentioned 

 above, pass through the walls of the intestinal mucosa and into the 

 capillaries and lymph vessels of the submucosa. Once they enter the 

 blood system, either directly or by way of the lymphatic vessels, the 

 absorbed substances are carried throughout the body, to be taken in and 

 utilized by the individual cells. All but a small portion of the mono- 

 saccharides, however, are taken from the blood in its passage through the 

 liver, where they are stored in the form of glycogen, or animal starch. 



The elimination of undigested wastes. The undigested materials 

 that are left in the small intestine are finally passed into the large intes- 

 tine. 1 The latter is much shorter than the small intestine, although 

 distinctly larger in diameter. The undigested materials pass rather 

 slowly through the large intestine, which "ascends" from its juncture 

 with the small intestine in the lower right side of the abdominal cavity; 

 crosses, at about the level of the navel, from the right to the left side; 

 and "descends" on the left side, passing into the rectum, which ter- 

 minates in the anus. The undigested waste, in its passage through the 

 large intestine, is acted upon by a multitude of bacteria, and the fecal 

 matter voided from the anus consists in considerable part of the bacteria 

 that have multiplied in and considerably modified this undigested mass. 



1 The small and large intestines do not meet end to end; the inner end of the large 

 intestine forms a blind pouch, the caecum, from which projects the slender, fingerlike 

 vermiform appendix. 



