FIFTH LECTURE. 



THE PHYSIOLOGY OF EXCRETION. 



ARNOLD GRAF. 



Assimilation, respiration, and excretion are essentially 

 cellular processes, that is, they take place within the cell body, 

 and are manifestations of the minute organization of the cell. 



Food is, for instance, taken up by the animal, and, after a 

 preliminary mechanical preparation by chewing, transferred into 

 the intestine, where certain substances which are secreted by 

 special elements dissolve the food and thus prepare it for its 

 further fate. We often call the processes taking place in the 

 intestinal tract assimilation. This is logically an inadequate 

 term, because these processes are only the forerunners of true 

 assimilation ; they fulfil only the task of making food digestible. 

 True assimilation takes place within the tissue cells, to which 

 the liquefied and chemically transformed food is carried by 

 special elements of the blood. The tissue cells use the food 

 for the regeneration of their protoplasm, which during the life 

 processes of the cells has become partly used up. 



The term respiration is widely used to denote the action of 

 breathing, the mere inhalation of air into the lung. This 

 mere pumping of air into the ramifications of the lung is not 

 respiration, although we generally call it thus, but only the 

 preparation to this end. Respiration takes place within the 

 blood corpuscles in the higher animals, in the blood plasm in 

 the lower ones, and is a purely cellular process.^ If we call the 

 pumping of air into the lungs or the solution of food inside 



1 The intracellular respiration providing the oxygen for the cellular activity 

 coincides, in all probability, with certain phases of metabolism, by which oxygen 

 is set free within the cell. 



