THEODOR SCHWANN 257 



distinction between the cell-contents and the external cytoblastema, we 

 must ascribe to the cell-membrane not only the power in general of 

 chemically altering the substances which it is either in contact with, 

 or has imbibed, but also of so separating them that certain substances 

 appear on its inner, and others on its outer surface. The secretion of 

 substances already present in the blood, as, for instance, of urea, by 

 the cells with which the urinary tubes are lined, cannot be explained 

 without such a faculty of the cells. There is, however, nothing so 

 very hazardous in it, since it is a fact that different substances are 

 separated in the decompositions produced by the galvanic pile. It 

 might perhaps be conjectured from this peculiarity of the metabolic 

 phenomena in the cells, that a particular position of the axes of the 

 atoms composing the cell-membrane is essential for the production of 

 these appearances. 



Chemical changes occur, however, not only in the cytoblastema and 

 the cell-contents, but also in the solid parts of which the cells are com- 

 posed, particularly the cell-membrane. Without wishing to assert that 

 there is any intimate connexion between the metabolic power of the 

 cells and galvanism, I may yet, for the sake of making the representa- 

 tion of the process more clear, remark that the chemical changes pro- 

 duced by a galvanic pile are accompanied by corresponding changes in 

 the pile itself. 



The more obscure the cause of the metabolic phenomena in the cells 

 is, the more accurately we must mark the circumstances and phe- 

 nomena under which they occur. One condition to them is a certain 

 temperature, which has a maximum and a minimum. The phenomena 

 are not produced in a temperature below 0° or above 80° R. ; boiling 

 heat destroys this faculty of the cells permanently; but the most fa- 

 vorable temperature is one between 10° and 32° R. Heat is evolved 

 by the process itself. 



Oxygen, or carbonic acid, in a gaseous form or lightly confined, is 

 essentially necessary to the metabolic phenomena of the cells. The 

 oxygen disappears and carbonic acid is formed, or vice versa, carbonic 

 acid disappears, and oxygen is formed. The universality of respira- 

 tion is based entirely upon this fundamental condition to the metabolic 

 phenomena of the cells. It is so important that, as we shall see fur- 

 ther on, even the principal varieties of form in organized bodies are 

 occasioned by this peculiarity of the metabolic process in the cells. 



