80 THE XKUVOUS SYSTEM AND ITS CONSERVATION 



precise voluntary control have an exceptionally lil)cral 

 apportionmenl <>f motor nerve-fibers. Such is the case 

 with the little muscles which rotate the eyeball, executing 

 movements over which we have the most complete com- 

 mand. Doubtless the small muscles which act upon the 

 vocal cords have, as already noted, a relatively large 

 number of nerve-fibers by which their eminently skilled 

 action is determined. 



Each motor fiber proceeds from a perikaryon which may 

 be in the lower sections of the brain if the destination is a 

 muscle of the head, but which will be in the spinal gray 

 matter if the muscle is in the trunk or limbs (see page 33. 



motor 

 plates 

 larger. 



16. A "neuromuscular unit" as defined in the text. The 

 perikaryon is united through its branched fiber and end- 

 with five nnisele-fibers. The typie number would be much 



Kvery perikaryon so situated is evidently in a position 

 to bring into action a moderate number of muscle-fibers 

 if it is itself stimulated. A center for a mu>de, to be 

 worthy of the name, should consist of many perikarya so 

 enchained as to act simultaneously upon the contractile 

 units of the muscle, leaving none of them unst imulated. 

 A fact to lie borne in mind i< that the filters of skeletal 

 miiM'le cannot communicate the contractile process from 

 one to another. They are insulated units and their 

 Coordination must be secured, if at all, by the manner 

 in which the motor perikarya of the gray matter are 

 assembled. 



