80 



THE HUMAN MECHANISM 



branch of the afferent neurone is obviously a mechanism to 

 combine the action of the efferent neurones in this way. The 

 diagram also gives a clew, at least, to the explanation of 

 another element of coordination: when two or more muscles 

 work together to accomplish a given act, one of the muscles 

 usually works harder than another; not only must they work 

 together, but the amount of force exerted by each must be 

 adjusted to the needs of the movement as a whole. This 



adjustment is most 

 probably effected by 

 differences in the con- 

 nection of the , syn- 

 apses with their cells ; 

 thus those muscles 

 which contract most 

 forcibly are innervated 

 by neurones whose den- 

 drites and main cell 

 body come into more 

 intimate contact with 

 the synapses of the 

 afferent neurone ; or 

 the number of fibrils 

 of the synapse may 

 be greater in their case 

 than in the others. These, however, are only possibilities ; the 

 whole subject requires further elucidation. 



11. Definition of reflex action. An action such as we have 

 just been studying is known as a reflex 1 action. By this we 

 mean an action called forth by the more or less direct action of 

 afferent upon efferent neurones and without the intervention of 



1 The word literally suggests the idea of reflection from the afferent to 

 the efferent neurones, as light is reflected from a surface ; but the student 

 has already learned enough to understand that efferent impulses are something 

 more than mere mechanical reflections, or rebounds, of afferent impulses. 



FIG. 46. Diagram of the nervous mechanism 



represented in Fig. 45, with the addition of 



the neuron 6 (see sect. 12) 



