Herrick, Size of Neme Fibers. 333 



a common mode. Morphologically, each system is defined by 

 the terminal relations of its fibers — by the organs to which 

 they are related peripherally and by the centers in which the 

 fibers arise or terminate." It so happens that throughout the 

 vertebrate series the peripheral fibers of each system have 

 certain tolerably uniform characteristics of caliber, medullation, 

 etc., by which they may be distinguished from those of other 

 systems. This fact lies at the basis of much of the recent 

 work on nerve components, in the course of which the several 

 systems of components have been followed through serial sec- 

 tions from their primary centers within the brain to their 

 peripheral termini. These fiber characteristics are, however, 

 by no means inflexibly fixed, but, as we have seen above, are 

 sometimes subject to wide and very confusing variations, partly 

 explicable as functional adaptations, partly as yet unexplained. 



These variations oppose very grave obstacles to the deter- 

 mination of the morphological rank of any organs on the basis 

 merely of the size of the nerve fibers innervating them. For 

 instance, the division of the body musculature into somatic and 

 visceral systems needs a much more secure foundation than that 

 afforded by studies on the caliber of the nerve fibers supplying 

 the several muscles such as have been made by Gaskell, Shore, 

 Edgeworth and others. This character is doubtless an impor- 

 tant aid, but it requires rigid embryological control. This is 

 clearly appreciated by some of these authors, who have followed 

 their measurements of nerve fibers by an embryological study 

 of the muscles innervated. 



By way of summary, then, we conclude that each func- 

 tional system of peripheral nerves has tolerably definite fiber 

 characteristics, the basis for which is as yet unknown ; that 

 these characteristics are by no means invariable, but that the 

 fibers of a given system may show considerable differences in 

 caliber and medullation in a single animal ; and that some of 

 these differences, at least, may be correlated with the degree of 

 functional development of the peripheral end-organ. In general, 

 highly developed muscle fibers, sense organs, etc. receive larger 



