6i6 'Journal of Comparative Neurology and Psychology. 



is tru3, it can scarcely be said that the caliber of the fibers depends 

 on their functional importance to the organism. The greater 

 diameter of the gustatory fibers as compared with the general 

 visceral may fall under Herrick's principle of "a correlation 

 between the diameter of the fiber and the functional importance 

 of the fiber, or the physiological importance of its terminal organ, 

 as compared with other organs of the same system" (Herrick 

 1902). The case of the velar nerve, however, seems directly 

 opposed to this principle. Of the functional importance of the 

 velum as an organ and of its sensory nerve supply, which is very 

 rich, there can be no doubt. Yet the fibers and their ganglion cells 

 are much smaller than the average of the general cutaneous system, 

 to which, so far as we know, the fibers of the velar nerve belong. 

 Apparently the fibers of the cutaneous system which are charac- 

 teristically coarse have been reduced in caliber in response to the 

 same influences which have determined the caliber of the visceral 

 sensory fibers. Fibers supplying the internal surfaces of the body 

 are as a rule finer than those supplying the external surface. It 

 seems probable that this is due to some differences in the con- 

 ditions of stimulation, and this is in agreement with the principle 

 expressed above that the caliber of dendrites is determined by the 

 receptive functions of the neurone. Until we have further experi- 

 mental data upon the conditions and modes of stimulation of 

 sensory fibers, it would be useless to speculate further concerning 

 the controlling factors here. 



The method of specialization. 



In the course of the evolution of vertebrates there has been a 

 change from segmental masses of muscle (myotomes) to special 

 muscles. On the functional side the cause of this change is 

 found in the increased number, variety and combinations of mus- 

 cular movements required of the higher animals by their conditions 

 of life and their habits. Whereas the movements of the adult 

 lamprey consist chiefly in wriggling, sucking, rasping and breath- 

 ing, we must perform a thousand sorts of movements, massive 

 and minute, complex and clever, in order to earn our daily bread. 

 For all the movements of the lamprey a few large masses of muscle 

 arranged in antagonistic pairs or groups and alternating in their 

 contraction suffice. For us, a great number of special muscles 



