v INTERNAL EESTITUTIVE SECRETIONS 277 



of sodium chloride solution in a loop of the gut, and solutions of 

 sodium sulphate and a number of other inorganic neutral salts. 

 Among the most important results of his work may be noted the 

 fact that isotonic solutions of the various salts are absorbed at 

 different rates by the intestinal wall, independent of the molecular 

 weight of their ions, and the different degree of their electrolytic 

 dissociation. This shows that the wall of the intestine does not 

 behave like a more or less permeable homogeneous membrane, but 

 like a membrane with specific properties. 



0. Cohnheim's experiments are a continuation of those of 

 Heidenhain upon the changes effected in intestinal absorption by 

 the action of poisons which alter or destroy the vitality of the 

 intestinal epithelium. With this object he compared the absorp- 

 tion in a loop of intestine of sugar solutions at equal concentra- 

 tion, before and after the addition of various poisons (sodium 

 or potassium fluoride, potassium arseniate, etc.) in such small 

 quantities as not to injure the animal. According to the degree 

 of intoxication of the intestinal epithelium, he obtained a decrease 

 or complete cessation of absorption. He noted further that the 

 poisoned mucous membrane gave the same results as in the dead 

 animal, i.e. the intestinal wall lost its property of being traversed 

 in one direction only. The sugar solution introduced into the 

 loop was found under these conditions to contain a considerable 

 quantity of sodium chloride from the blood and lymph vessels. 

 Normal absorption is thus proved not to be the effect of any merely 

 physical process, since it is so profoundly altered by a few 

 milligrammes of a given substance. Granting this to be the 

 effect of poisoning, it follows that absorption must have a living, 

 organised substrate, since this alone is capable of intoxication. 

 Cohnheini justly distinguishes two factors in intestinal absorption : 

 the impenetrability of the wall of the gut to the circulating fluids, 

 and its capacity for absorbing solutions from the intestine. Both 

 these factors, and each independently of the other, can be altered 

 or suppressed by intoxication. 



If the absorption of crystalloids (represented by salts, sugars, 

 and amino-acids) is not governed by the laws of diffusion and 

 osmosis, but depends on the specific physiological activity of the 

 mucous membrane, and particularly of the epithelial cells which 

 cover it, we may assume a fortiori that the absorption of colloids 

 (represented by proteins, proteoses, and peptones) which provide 

 the essentials of nutrition, must equally be independent of these 

 physical laws, whether they are changed or left unchanged by the 

 digestive processes. 



Before leaving the subject of the absorption of salts and 

 sugars, it must be added that they suffer no chemical change 

 through the metabolic activity of the epithelial cells while travers- 

 ing the wall of the intestine which, as we shall see, is not the 



