THE NEUROMUSCULAR MECHANISM 87 



The muscle-fiber has been said to be the working 

 unit of a muscle. A motor perikaryon, with its long axon 

 extending to the muscle, branching at last and joining 

 a cluster of muscle-fibers, may be looked upon as the 

 neuromuscular unit (Fig. 16). In this combination there 

 is a set of structures not yet described. The reference is 

 to the intermediate elements between the nerve-fibers 

 at their terminations and the protoplasm of the muscle. 

 These mediating structures are called motor end-plates, 

 and they have characters which differentiate them both 

 from nerve and muscle. They must be transmitters of 

 the excitatory process from the nerve-fibers to the muscle- 

 fibers, and the interest felt in them springs from the fact 

 that they conduct with very variable facility under 

 changing conditions. 



The simplest conceivable reflex finding expression 

 through skeletal muscle would be realized in case a single 

 afferent fiber were stimulated and its influence concen- 

 trated upon the clendrites of a single motor neuron. 

 The resulting flutter of a few muscle-fibers buried among 

 thousands of their fellows in a muscular mass could hardly 

 be discerned. The most restricted reflex which we can 

 observe doubtless involves a fairly large number of 

 motor neurons with their dependent muscle-fibers. More- 

 over, it is supposed that adjuster or association neurons 

 almost always intervene in the directing of the impulses. 



In the present chapter it is intended to advance from 

 the discussion of the simple reflex, already familiar in 

 principle, to instances of motor performance which are 

 commonly held to lie outside this class. One thinks 

 immediately of the so-called "voluntary" movements. 

 It has previously been stated that there is no clear de- 

 marcation between the two varieties. The more care- 

 fully one analyzes the conditions surrounding each, 

 the less distinction there seems to be. A reflex move- 

 ment is one occurring promptly and, as a rule, inevitably 

 in response to a change of external conditions. A volun- 

 tary movement is also an adjustment depending upon 



