252 DIGESTION AND ASSIMILATION OF THE FOODSTUFFS 



It is a rather noteworthy fact that the Rate of Absorption of digestion- 

 products from the intestine does not appear, in so far as it has been 

 quantitatively investigated, to follow the logarithmic rule, as we 

 should expect if the rate of absorption depended solely upon the 

 concentration, i. e., osmotic pressure, of the substance undergoing 

 absorption. On the contrary, for the absorption of glucose at all 

 events, a square-root rule seems to hold good, i. e., the quantity 

 absorbed in a given time is proportional to the square-root of the 

 concentration of the material which is being absorbed. It is, however, 

 perfectly evident, even apart from these measurements, that the proc- 

 ess of absorption cannot, be purely a question of the diffusion of sub- 

 stances into and through the wall of the intestine in simple propor- 

 tion to their osmotic pressures, for otherwise no Selective Absorption 

 would be possible. We have seen that certain amino-acids are 

 absorbed preferentially, others being absorbed with relative slowness 

 even when they are present in excess. This implies that besides the 

 forces of osmotic pressure, phenomena of solubility in the absorbing 

 tissue-elements or of chemical affinity therewith play an important or 

 decisive part in determining the relative rates of absorption and the 

 types of material absorbed. 



From the Large Intestine, as we have seen, the products of bacterial 

 decomposition of foodstuffs may be absorbed, sometimes with physio- 

 logically undesirable results. A considerable Absorption of Water occurs 

 here also. During digestion and absorption in the stomach and 

 small intestine the contents of the alimentary canal retain a watery 

 consistency which is favorable to the rapidity of hydrolysis, and to 

 the thorough admixture of the digestive secretions with the foodstuffs 

 and the absorption of the products of digestion. In the large intes- 

 tine, however, a large proportion of this water is absorbed, so that 

 the water-content of the feces is normally considerably less than 

 that of the contents of the small intestine. In case the feces are 

 expelled with undue rapidity, however, and before the absorption of 

 water is complete, as when a cathartic is administered, then the 

 feces have a watery consistency and thirst is engendered through 

 insufficient absorption of the water which has been partaken, and 

 which has also been furnished to the intestinal contents by the various 

 digestive fluids. 



Not only water and products of bacterial action may be absorbed 

 from the large intestine, however, but also foodstuffs if they chance to 

 find entry therein without previous absorption. Thus, it is not an 

 uncommon procedure in medical practice to furnish nutrition to 

 very weak individuals or to persons who are unable to swallow, by 

 Rectal Feeding, or the introduction of enemas containing fully hydro- 

 lyzed foodstuffs, such as glucose. The substances thus administered 

 are found to be absorbed, and to be normally utilized for the mainten- 

 ance of the tissues, and the provision of energy. 



