INTRODUCTION 19 



the probable sequence in which the three orders of elements 

 have acquired their position in relation to the nervous 

 system of standard type. According to his hypothesis 

 there may have been a certain lowly condition possibly 

 realized in sponges in which contractility was displayed 

 only in response to distinct and direct stimulation brought 

 to bear from the outside world upon tissues having the 

 power of movement. No separate and definitely nervous 

 tissue would be requisite to apply the stimuli, but if such 

 a transmitter should be evolved it would be evident that 

 an efferent element had come into existence. Parker thus 

 conjectures that the efferent is the most primitive feature 

 of the three. 



In the course of development the submergence of the 

 contractile tissues below the exposed surface of the body 

 would make it necessary that a receptor should be added. 

 Its function would be to maintain relations between the 

 exterior and the hidden effector. One receptor joined with 

 one effector would form a sufficient means for bringing 

 about one, and only one, reaction. The arrangement 

 does not admit of any flexibility. It is the simplest ex- 

 ample of what is called a reflex arc. With the addition of 

 intermediate links between the receptor and the effector 

 departments the reaction becomes less and less predictable. 

 This is equivalent to saying that it becomes more and 

 more like the actions of the higher animals subject to 

 reinforcement, suppression, or modification under the 

 influence of changing circumstances. Many illustrations 

 will be given later which will serve to clear up this matter. 



Excitation and Inhibition. The common conception 

 of the function of nerves is that they arouse activity. 

 This is essentially true of those which stretch out from 

 the central axis to the skeletal muscles, and which call 

 forth the evident movements of the limbs, the play of 

 facial expression, etc. The muscles involved in such move- 

 ments are relaxed and paralyzed when their nervous con- 

 nections are interrupted. It is because this fact is so 

 prominent that one may easily fall into the error of sup- 



