Johnston, Parts of the N.enrone. 617 



are necessary and each must be capable of a graded series of con- 

 tractions and of entering into combination with one or another 

 set of muscles to perform this or that particular act. With the 

 specialization of muscles has gone an increase in the number of 

 fibers in the ventral roots. In the frog (Birge 1882, Hardesty 

 1899) the average number in each ventral root is from six to ten 

 or more times as great as in the lamprey. In man the total 

 number in the ventral rootsof one side is 203,700 (Ingbert 1904), 

 or an average for each nerve of 6570. If the last four nerves are 

 left out of account for the sake of a fairer comparison, the number 

 in each of the remaining nerves is 7290. The number is largest 

 in those nerves which supply the largest volume of muscle. After 

 allowance is made for sympathetic fibers in the ventral roots, 

 there is obviously in the higher forms as compared with the 

 lamprey a much larger number of motor fibers and they have a 

 much smaller diameter relative to the body weight. On the other 

 hand, the number of motor fibers in man is much less in propor- 

 tion to the body weight than in the lamprey and the individual 

 muscle fibers are many times larger. I know of no enumeration 

 or estimate of the number of muscle fibers supplied by one nerve 

 fiber in man or any of the higher animals, although the necessary 

 preliminary observations for the study of this in the frog have been 

 made by Miss Dunn (1900, 1902). 



The question of interest on which all this bears is, does speciali- 

 zation of muscles entail a decrease in the number of muscle fibers 

 supplied by each nerve fiber or only a definite distribution of 

 motor fibers to each special muscle with provision for functional 

 isolation in the central nervous system ? We know at least that 

 the grading of strength andextent of muscular contraction depends 

 upon bringing into play a larger or smaller number of the fibers 

 of the muscle. It is desirable that the ratio between the number 

 of motor nerve fibers and of muscle fibers supplied should be 

 determined in several animals (frog, mammals, man), and the 

 determination should be made also for difi^erent muscles engaged 

 in the several forms of muscular activity. We should expect that 

 in the muscles of the fingers a smaller number of muscle fibers 

 would be supplied by one motor fiber than in muscles concerned 

 in massive movements. 



The question of specialization in other organs is of course sub- 

 ject to study along similar lines. 



