254 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



makes rapid progress. The passage of the food along the small intestine, 

 although rapid compared with its passage through the large intestine, requires 

 a number of hours for its completion. According to the observations made 

 upon a patient with a fistula at the end of the small intestine/ food begins to 

 pass into the large intestine in from two to five and a quarter hours after it 

 has been eaten, and it requires from nine to twenty-three hours before the last 

 portions reach the end of the small intestine ; this estimate includes, of course, 

 the time in the stomach. During this progress it has been converted for the 

 most part into a condition suitable for absorption, and the mucous membrane 

 with which it is in contact is one peculiarly adapted for absorption, since its 

 epithelial surface is greatly increased in extent by the vast number of villi 

 as well as by the numerous large folds known as the " valvulse conniventes." 

 In addition to these considerations, however, we have abundant experimental 

 proof that absorption takes place actively in the small intestine. The absorp- 

 tion of fats can be demonstrated microscopically, as will be described presently. 

 Experiments made by Rohmann 2 and others with isolated loops of intestine 

 have shown that sugars and peptones are absorbed readily and in much more 

 dilute solutions than in the stomach. Moreover, in the case just referred to, 

 of an intestinal fistula at the end of the small intestine, a determination of 

 the proteid present in the discharge from the fistula, after a test-meal contain- 

 ing a known amount of proteid, showed that about 85 per cent, had disappeared 

 that is, had been absorbed before reaching the large intestine. With refer- 

 ence to water and salts, it has been shown that they also are readily absorbed ; 

 some very interesting experiments demonstrating this fact have been reported 

 recently by Heidenhain in a paper which is referred to briefly on page 95. 

 It must be remembered, however, that under normal conditions the absorption 

 of water and salts is more or less compensated by the secretion formed along 

 the length of the intestine, so that when the contents reach the ileo-ca3cal valve 

 they are still of a fluid consistency similar to that of the chyme as it left the 

 stomach to enter the intestine. A consideration of the mechanism of the 

 absorption of fats, sugars, peptones, and water will be taken up presently, 

 after a few words have been said of absorption in the large intestine. 



Absorption in the Large Intestine. There can be no doubt that absorp- 

 tion forms an important part of the function of the large intestine. The 

 contents pass through it with great slowness, the average duration being given 

 usually as twelve hours, and while they enter through the ileo-csecal valve in a 

 thin fluid condition, they leave the rectum in the form of nearly solid feces. 

 This fact alone demonstrates the extent of the absorption of water. As for 

 the sugar and peptones, examination of the intestinal contents as they entered 

 the large intestine in the case of fistula cited in the preceding paragraph 

 showed that there may still be present an important percentage of proteid 

 (14 per cent.) and a variable amount of sugars and fats more than is 



1 Macfadyen, Nencki, and Sieber : Archivfur experimentette Pathologic u. Pharmakologie, 1891, 

 vol. 28, p. 311. 



2 Pfluger's Archivfur die gesammte Physiologic, 1887, vol. 41, p. 411. 



