CHEMISTRY OF DIGESTION AND NUTRITION. 259 



bulk of the water passes out of the pylorus. In the small intestine absorption 

 of water and of inorganic salts evidently takes place readily, and, according to 

 the experiments of Rohmann and Heidenhain already referred to, the laws 

 governing their absorption are different from what we should expect if the 

 process were simply one of osmosis. The differences as regards the absorption 

 of salts are especially emphasized by the experiments of Heidenhain. 1 Making 

 use of an interesting method, for which reference must be made to the original 

 paper, Heidenhain has shown that if dilute solutions of NaCl (0.3 to 0.5 per 

 cent.) are introduced into an isolated loop of the small intestine, absorption of 

 both water and salts takes place readily, in spite of the fact that in this case 

 the blood is the more concentrated solution and has therefore the greater 

 osmotic pressure. Moreover, specimens of the animal's own blood-serum intro- 

 duced into an intestinal loop are also completely absorbed, although in this case 

 there is practically no difference in composition, as regards water and salts, 

 between the blood of the animal and the serum introduced into the intestine. 

 In another paper by Heidenhain 2 he proved that the absorption of water in 

 the small intestine, when ordinary amounts are ingested, takes place entirely 

 through the blood-vessels of the villus, and not through the lacteals ; when 

 larger quantities of water are swallowed, a small part may be absorbed through 

 the lacteals, as shown by the increased lymph-flow, but by far the larger 

 quantity is taken up directly by the blood. 



In the large intestine the contents become progressively more solid as they 

 approach the rectum ; the absorption of water is such that the stream is 

 mainly from the intestinal contents to the blood, giving us a phenomenon 

 somewhat similar to the absorption of water by the roots of a plant. This 

 process is difficult to understand upon the supposition that it is caused by 

 osmosis, using that term in its ordinary sense. We must suppose an active 

 attraction of a peculiar character for water on the part of some substance in 

 the epithelial cells of the wall of the large intestine. 



Composition of the Feces. The feces differ widely in amount and in 

 composition with the character of the food. Upon a diet composed exclu- 

 sively of meats they are small in amount and dark in color ; with an ordinary 

 mixed diet the amount is increased, and it is largest with an exclusively vege- 

 table diet. The average weight of the feces in twenty-four hours upon a 

 mixed diet is given as 170 grams, while with a vegetable diet it may amount 

 to as much as 400 or 500 grams. The quantitative composition, therefore, will 

 vary greatly with the diet. Qualitatively, we find in the feces the following 

 things : (1) Indigestible material, such as ligaments of meat or cellulose from 

 vegetables. (2) Undigested material, such as fragments of meat, starch, or fats 

 which have in some way escaped digestion. Naturally, the quantity of this 

 material present is slight under normal conditions. Some fats, however, are 

 almost always found in feces, either as neutral fats or as fatty acids, and to 

 a small extent as calcium or magnesium soaps. The quantity of fat found is 



1 Pfluger's Archiv fur die gesammte Physiologie, 1894, vol. 56, p. 579. 



2 Ibid., vol. 43, 1888, supplement. 



