SOURCES OF INFORMATION 23 



only known organic relationships were those effected by means 

 of the nervous system and, until very recently, these alone were 

 considered worthy of attention. It was only after a long time 

 and as the result of infinite labour, that the existence of the 

 chemical correlationships was recognized, and the part which 

 they play in the animal economy estimated at its proper value. 



To-day, the doctrine of the internal secretions plays an 

 important part in almost every department of physiology and 

 pathology, and is employed in the solution of some of the greatest 

 problems that biology affords. Nothing is more characteristic 

 of this change of view than Schiefferdecker's hypothesis of the 

 part performed by specific internal secretion in the functions of 

 the nervous system. " Internal secretion," he says, " determines 

 the effect which the products of metabolism, excreted by the 

 nerve cells during the simple processes of nutrition, will exercise 

 upon other nerve cells or upon the cells of the end organ, such 

 activity being called ' trophic.' It also determines the effect 

 which the products of metabolism excreted in the course of 

 specific activity will produce, and this effect is known as ' irrita- 

 tion ' or ' stimulus.' ' 



This view of the origin of nervous activity can hardly be 

 accepted as final, but the mere proposition of such a hypothesis 

 illustrates the magnitude of the difference between " then " and 

 " now." The older physiologists thought that each organic 

 interactivity was due to nervous intervention ; to-day we believe 

 that even the nervous correlationships themselves are effected by 

 means of chemical agents. 



