70 BIOLOGICAL ASPECTS OF HUMAN PROBLEMS 



tion of saliva is a direct and unimpeachable function 

 of the cells of the salivary gland. No one has thought 

 it worth while to propose that the saliva is the cause 

 of the glandular activity; nobody has deemed it 

 necessary to suggest a parallelism between glandular 

 action and salivary flow, to enable philosophers to 

 hedge as to the relationship between the saliva and 

 the glandular action. If we stop to consider the 

 nature of nervous stimulation and the nature of 

 saliva, we see that they are very different things ; 

 they have the quality of "disparateness," as the 

 metaphysicians like to say. But it makes nobody 

 uncomfortable to believe that saliva is a result of 

 the activity of salivary cells, and nobody takes the 

 trouble to question the prevailing view on this 

 subject. 



If we apply a drop of acid to the skin of a decapi- 

 tated frog, the animal will be seen to draw up his 

 leg on the same side and to energetically strive to 

 rid his skin of the acid. The act looks like a volun- 

 tary act, but obviously it is not, because the animal 

 is headless. The apparently protective act is reflex 

 in its nature. That is, a sensory stimulus passes 

 from the skin along a sensory nerve into the spinal 

 cord (by the posterior nerve roots) and by more or 

 less devious paths, enters the ganglion cells of the 

 anterior horns of the spinal cord. Here apparently 

 the energy represented by the sensory stimulus 

 suffices to unlock another kind of energy stored in the 

 ganglion cells, and what is called a motor stimulus 



