THE NEUROMUSCULAR MECHANISM 89 



and finally enter into relations with the motor apparatus 

 on the opposite side of the body (Fig. 17). 



The typic relation of skeletal muscles to the nervous 

 system having been indicated, three subjects now sug- 

 gest themselves for discussion in some detail. These are 

 the gradation of responses, coordination, and neuro- 

 muscular fatigue. 



Gradation of Responses. It is a familiar but none the 

 less a wonderful thing that we so successfully adapt our 

 muscular efforts to the purposes which they are designed 

 to serve. If one raises a book from the table one does not 

 fling it upward with useless excess of energy, nor does one 

 waste much time in preliminary contractions which do not 

 suffice to move it. Unless one is deceived in his estimate 

 of the weight to be handled, there is evidence of the most 

 admirable economy in the execution of the movement. 

 Judgment may sometimes be found at fault, as when a 

 bottle of mercury resists so surprisingly the attempt to 

 lift it or, in the opposite fashion, when an empty box is 

 picked up by one who has been handling a series of boxes 

 which were heavy. 



How this grading of contractions to their uses is brought 

 about is not entirely clear. In all probability, a vigorous 

 voluntary effort means a cerebral process which is com- 

 paratively widespread and intense in degree. A small 

 movement, or the sustaining of a trifling weight, may 

 mean a less intense or a more localized process, or it may 

 mean both. If intensity is what counts, we may assume 

 that whether a contraction made by a given muscle 

 is great or small, the number of projection fibers em- 

 ployed to bring it to pass is the same. On the other 

 hand, it is interesting to consider the second possibility, 

 that is, that a contraction will be large if the number of 

 neuromuscular units cooperating to produce it is large, 

 and reduced in proportion to the reduction of the number 

 of these units engaged. Keith Lucas, 1 of Cambridge, 

 England, is the physiologist to whom we owe the second 

 'Journal of Physiology, 1905, xxxiii, 125; 1909, xxxviii, 113. 



