56 THE NEW PHYSIOLOGY. 



response evidently depends on the existing state of 

 excitation of the whole retina. It also depends on the 

 normal nutrition of the retina and brain. If the blood 

 is abnormal in composition, the ordinary response is 

 interfered with ; and we are as yet only at the beginnings 

 of knowledge with regard to the minute changes in blood 

 composition and other conditions of environment which 

 are sufficient to affect the response very materially. 



It is the same with every physiological response. The 

 further we investigate the more evident does it become 

 that each physiological response depends on a vast 

 number of conditions in the environment of the re- 

 sponding tissue. On superficial investigation we do not 

 realise this : for we can often get exactly the same 

 response, time after time, with the same stimulus. To 

 the attainment of this result all that is necessary is to 

 see that the conditions are " normal." It is only after 

 more thorough investigation that we find that " normal 

 conditions " imply something which is both extremely 

 definite and endlessly complex. We then begin to 

 realise that the maintenance of normal conditions is 

 from the physical and chemical standpoint a phenomenon 

 before which our wonder can never cease. 



Physiological investigation of causes seems, thus, to 

 lead us up to a tangled maze of causal conditions. 

 He who looks for definite ' causal chains " in physio- 

 logical phenomena finds in place of them a network 

 of apparently infinite complexity. The physiologists 

 who led the revolt of last century against vitalism did 

 not see this network. To them it seemed that there 

 were probably simple physical and chemical explanations 

 of the various physical and chemical changes associated 



