580 LECTURE XXV. 



disintegrated; and from the resulting products the intestines, and per- 

 haps the liver as well, forms substances suitable for the body. The 

 intestine assumes by this function a characteristic position. It takes 

 care that the tissue-cells always receive the same nutriment, and makes 

 them absolutely independent of the nature of the food which is eaten. 

 We cannot be far wrong in ascribing to the intestine in this sense an 

 important part in the maintenance of the individuality of each species of 

 animals. 



A second mechanism for regulating the composition of the blood is 

 found in the lymph. The exchange of material between the blood and 

 tissue-cells takes place, as we have seen, through the lymph. The lymph 

 receives substances from the blood which the cells of the tissues require, 

 and on the other hand it gives up to the blood the waste-products which 

 it receives from the tissue-cells. It is able to retain these last-mentioned 

 substances for quite a time, only gradually giving them up to the blood, 

 for further elimination from the body. 



The chief organs for the elimination of the metabolic end-products, 

 and of the substances which the organism cannot utilize, are the kidneys. 

 They guarantee the maintenance, as far as possible, of the blood-uni- 

 formity. Under normal conditions they are perfectly adequate for 

 this purpose. If it happens that the kidneys, for some reason or other, 

 are not able to remove all of the foreign substances from the blood, 

 then other organs attempt to act as their substitute. Most of all the 

 different glands contained in the organism may be active in this sense, 

 and even under normal conditions there is no doubt that small amounts 

 of the waste-products are eliminated in this way. This is best shown 

 by introducing into the body substances which are foreign to it. Thus, 

 if we introduce potassium iodide into the intestines, some of it will soon 

 appear in the saliva and in the sweat. If morphine is injected subcu- 

 taneously, a part of it is eliminated in the stomach. Urea is likewise 

 found in sweat, particularly when the kidneys are not adequate to the 

 demands placed upon them. The intestines also form an important organ 

 for elimination, and normally. We have already seen that the alkaline 

 earths and heavy metals are unquestionably largely eliminated directly in 

 the intestines, and in fact chiefly through the rectum. The animal organism, 

 furthermore, is able to combine many of these foreign substances together, 

 whereby the blood and the tissues are prevented from being flooded with 

 them. Such substances may then be eliminated gradually in the course of 

 several weeks. We have also repeatedly called attention to the ability 

 of the tissue-cells, and especially of the liver, to make many substances 

 harmless by oxidizing or reducing them, and in some cases conjugating 

 them with certain substances, such as glycocoll, glucuronic acid, sulphuric 

 acid, or urea. 



