388 RESPIRATION 



edge there is thus nothing new in principle in the fact, character- 

 istic of physiological reactions, that any particular reaction is de- 

 pendent upon the whole life of an organism. Nevertheless it is just 

 here that we strike the dividing line between the physical sciences 

 and biology. 



A physiological reaction, when we examine it closely, is always 

 found to depend on a vast number of conditions of structure and 

 environment. It is true that under "normal conditions" the same 

 stimulus will produce the same reaction again and again; but 

 when we inquire what normal conditions represent we find some- 

 thing which is indefinitely complex from the physical and chemi- 

 cal standpoint. We have only to alter slightly the diffusion pres- 

 sure of one or other of the many substances, only partially known, 

 in the blood plasma, in order to obtain a quite different reaction. 

 For instance a given fall in the diffusion pressure of oxygen fails 

 to excite the respiratory center if the hydrogen ion concentration 

 of the blood is very slightly below normal; and if the calcium 

 ion concentration were a little above or below normal there would 

 doubtless also be an abnormal result. The presence of a trace of 

 ether or morphia, or probably of numerous other substances, 

 affects the center in a similar manner. The excitability of a tissue 

 to any given physical or chemical stimulus may thus vary in- 

 definitely under slightly different conditions. 



If we attempt to investigate physiological phenomena from the 

 standpoint merely of physics or chemistry, we are thus at once 

 landed in confusion. In investigating ordinary physical or chemi- 

 cal phenomena, we can examine one by one the parts or units 

 we are dealing with and ascertain their properties, so that from 

 the empirical knowledge thus gained we can predict what will 

 result when they act on one another. In other words we can give 

 physical and chemical explanations of their mutual action. But 

 when we attempt to do this as regards the actions on one another 

 of the parts of an organism, or of the organism and its environ- 

 ment, we are met by the difficulty that we cannot ascertain the 

 structures and properties of any of the separate parts, since their 

 structures and properties actually depend on the existing physio- 

 logical relations of the parts and environment to one another. The 

 relativity of the phenomena confronts us at every turn in the 

 attempt to reach physical and chemical explanations of physio- 

 logical reactions. 



Up to a certain point we can, it is true, understand living organ- 

 isms mechanically. We can, for instance, weigh and measure them 



