NERVE ELEMENTS AND THEIR FUNCTIONS. 83 



is a reflex act and the chain or series of nerve cells concerned is 

 known as the reflex chain or reflex arc. In this simplest case 

 the reflex chain has but two links. Probably only the simplest 

 movements, such as a jerk or twitch, are produced by so simple 

 a chain. 



Usually the afferent impulse instead of being handed over 

 directly to an excitatory cell, is spread more widely through the 

 spinal cord by means of a larger number of branhcesof the incoming 

 neurite and by means of central cells (Fig. 41, B, D). The arrange- 

 ment of these cells will be treated later (Chapter XIV) but it may 



tract cell 



FIG. 41. Diagrams intended to show several forms of reflex chains in the nervous 

 system of vertebrates. A, somatic sensory and motor reflex; B, reflex by way of 

 tract cells; C, visceral sensory and motor reflex; D, a diagram of the spinal cord 

 and nerve roots from the side, showing the same elements as in A, B and C. d. c. 

 t., direct cerebellar tract. 



be said that at least one set of cells usually intervenes between 

 the receptive and the excitatory cells. The function of these 

 cells is to spread the impulse so that it will affect a 

 larger number of excitatory cells and so provoke a more 

 ample or more intense movement in response. The extent 

 to which the muscles of the body may be called into movement 

 by such a reflex mechanism depends chiefly upon the strength 

 and the duration of the stimulus. A larger number of links may 

 be introduced into the reflex chain and the excitatory impulses 



