334 THE GROWTH OF THE BRAIN. 



Statistics of this sort are very important, and there 

 should be more of them. At the same time they must 

 be interpreted with caution. The monotony of exis- 

 tence and the general environment of the button-makers 

 is probably conducive to an early decline. Further, this 

 decline may be directly dependent on changes in the 

 spinal cord rather than the cerebrum. Finally the 

 muscles as well as the central system are involved, and 

 there is no collateral proof that other capabilities begin 

 their decline at the same time or continue it at a similar 

 rate. The false inference to be here avoided is the 

 same that was pointed out in connection with the 

 diminution of brain-weight due to age. The conclu- 

 sions are applicable to other artisans under like con- 

 ditions, but not necessarily to those living under quite 

 different conditions. Yet there is abundant evidence 

 that the capacity for very high-grade manual work de- 

 manding concentrated attention and judgment is limited 

 to a decade or two at the prime of life, and the history 

 of champion athletes points to like limitations. 



In this connection the fact that there is a multiple 

 innervation of many parts of the body is perhaps signi- 

 ficant. In its simplest form this means, for example, that 

 a given muscle can be contracted in nearly the same way 

 under the influence of different sets of nerve elements. 

 Expertness translated into physiological terms must 

 mean the use of the same elements in the same way to 

 produce a given result. The mechanical precision would 

 be much increased by this, but the strain would always 

 fall on the same elements, which in the case of a manual 

 art are almost certain to be over-exercised. This estab- 

 lishment of a preferred pathway tends to the suppression 

 of activity in the other possible pathways, and to the 

 abolition of that fringe of connections which is normally 

 present in every pathway, but the existence of which, 



