INNERVATION OF THE HUMAN SKIN 225 



Hensen, and expresses himself as follows : " It is suggested that the 

 development of the actual nerve fibril is simply the coming into view 

 of a pathway produced by the repeated passage of nerve impulses 

 over a given route." 1 (Italics not in original.) 



A passage from Professor McDougall's Physiological Psychology 

 may also be referred to at more length than it was in Chapter III., 

 page 25. Speaking of the automatization of voluntarily acquired 

 actions which have been explained by the view that purely reflex 

 actions carried out by mechanisms of the spinal level were also 

 originally acquired by our original ancestors as voluntary actions, 

 he says, " This view is usually associated with the name of Wundt, 

 who has forcibly advocated it. It implies, of course, the assumption 

 that acquired characters are in some degree transmitted from one 

 generation to another, a proposition which most biologists at 

 the present time are inclined to deny because they cannot conceive 

 how such transmissions can be effected. Nevertheless, the rejection 

 of this view leaves us with insuperable difficulties when we attempt 

 to account for the evolution of the nervous system, and there are no 

 established facts with which it is incompatible. If, therefore, we 

 accept this view we shall regard the congenital neural dispositions, 

 both those that determine pure reflexes and those that determine 

 instinctive actions, as having been acquired and consolidated under 

 the guidance of individual experience, with the co-operation, to 

 a degree which we cannot determine, of natural selection." 2 



These three statements from a physiologist, a zoologist, and 

 a psychologist, all of great eminence, though they differ in particular 

 problems studied, tell very strongly in favour of the position here 

 put forward as to initiative in the production of specialised 

 innervation of the skin. 



Origin of Cold, Warm, Pain and Touch Spots. 



The hair-clad skin of primitive man provided ample raw 

 material for the eventual differentiation of both end-organs and 

 sensorial areas which is found to-day. Not only did he possess 

 what is called Common Sensation in his skin but in the indi ridual 

 hairs lay a delicate tactile structure, which, though probably 

 inferior in delicacy, serves a similar purpose to that of the vibrissse 

 on the muzzle of Felidse. Each hair, being deeply inserted into 

 the skin and supplied with fine nerve fibrils, when it is bent, acts 

 as a lever communicating an impulse to an afferent nerve trunk 



1 Text Book of Embryology. Vertebrata with the exception of Mammalia. 

 Vol. II., 1919, p. 106. 



2 Physiological Psychology. W. McDougall, p. 156 (1911). 



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