CHEMISTRY OF DIGESTION AND NUTRITION. 319 



absorbed in this organ, a certain amount of water must go with them, but the 

 bulk of the water passes out of the pylorus. In the small intestine absorp- 

 tion of water and of inorganic salts evidently takes place readily,and accord- 

 ing to the experiments of Kohmann and lleideuhaiu, already referred to, the 

 laws governing their absorption are different from what we should expect at 

 first sight if the process were simply one of diffusion. 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 not only 

 dilute solutions, but solutions of nearly the same osmotic pressure as the 

 blood were readily absorbed. Indeed, specimens of the animal's own serum 

 introduced into a loop of the intestine were completely absorbed, although in 

 this case there was practically no difference in composition between the liquid 

 in the intestine and the blood of the animal. In another paper by Heiden- 

 hain 2 he has 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 thev 

 approach the rectum ; the absorption of w r ater 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, unless we assume that it is 

 due entirely to the osmotic pressure of the indiffusible proteids of" the blood 

 as explained on p. 69. 



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

 composition with the character of the food. Upon a diet comp< ^>v<\ 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, especially with vegetables containing a large amount of indigest- 

 ible material. The average weight of the feces in twenty-four hours upon a 

 mixed diet is given as 170 grains, 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 \'wv^ 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 Pfliiger'a Archiv fur die gesammle Physiologic, 18{»4, Bd. '>*'<, S. 579. 



2 Ibid., 1888, Bd. 43, supplement. 



