SOME MATTERS OF HYGIENE 405 



position not impaired for service by the presence of 

 poisons either derived from the intestine or from the 

 metabolism. 



Efficient muscles must also be distinguished by efficient 

 innervation. The best possible end-plate transmission 

 may be assumed. Another feature may be a more general 

 employment of the units than is secured by the untrained 

 subject. It may be that a muscle which seems less 

 strong than we should anticipate in view of its size is 

 one in which there are many idle fibers. Finally, the 

 working capacity of any. set of muscles must depend 

 upon the organization of the central nervous system and 

 the manner of using it. 



Let us emphasize the idea just advanced. It means 

 that efficiency depends upon coordination, for coordina- 

 tion is secured through the interrelations of the neurons 

 in the cord and the brain. The attainment of skill and 

 ease comes with the establishment of these associations. 

 The cerebellum as well as the cerebrum must be in- 

 volved. Only very lately has it been appreciated that 

 the afferent as well as the efferent mechanism must be 

 credited with a share in pushing motor resources to their 

 limit. The matter is too involved to be presented in full 

 but a paragraph may be given to it. 



We have only to add one more step to a familiar series. 

 If we try to make a list of the factors on which the 

 effective power of a muscle depends we shall parallel the 

 enumeration above. We shall think first of the muscle's 

 own size and nature. Then we shall recognize the limita- 

 tion imposed by the end-plates and then the dependence 

 of these for stimulation upon the motor centers of the 

 cord. These are played upon by impulses from at 

 least two sources : those that come from the receptors, as 

 in the production of simple reflexes, and those which 

 come from the cerebral motor cortex. We might well 

 mention the impulses from the cerebellum also. 



Now a spinal center may be supposed to transmit im- 

 pulses with a maximum effect when it is beset from as 

 many angles as possible, when all the available means of 



