CHAPTER XXIII. 



INNERVATION OF THE HUMAN SKIN. 



For at least seventy years the surface of the human skin has been 

 the subject of so much physiological observation and experiment 

 that Professor Sherrington considers the literature connected with 

 it to be probably greater than in any other branch of physiology. 

 Most of this study centres round the skin as a receptive field and 

 problems of the nervous system. It is easy to see why this should 

 be in the case of an organ so great as the skin, covering all the other 

 structures and organs and exposed through ages of evolution to the 

 vicissitudes of an inconceivable number of stimuli. And one 

 outcome of this study is to show that, metaphorically speaking, 

 the skin is a mosaic, and not the confused and blurred production 

 of a child of four years old who has been given a sheet of paper and 

 a paint-box. There is order in this field, and even without calling 

 in final causes, plan and purpose. Beside the protective function 

 exercised by the skin it plays a large part, through its nervous 

 endowment, in the processes by which the brain is made aware of 

 the surrounding phenomena, thus conveying intelligence to the 

 centre of life only less important than that of the special senses. 

 It is maintained here that the residt of the various physical stimuli, 

 of which pain, cold, warmth and touch are the chief, is that certain 

 functions and structures of the skin have arisen in response to them. 

 This is, no doubt, to beg the question of origin, and if the 

 balance of evidence be seen to be against this view the order of 

 events would need to be stated differently. But the position is 

 clear, whether correct or not, and if it be shown to be erroneous 

 it will at least have good "lighthouse value." 



Observed Facts. 



Briefly stated the facts of the innervation of the skin are of 

 two orders, anatomical and physiological ; the former examined 

 by the aid of the microscope, the latter by physiological experiments 

 of a varied kind. The chief aspect in wilich these are viewed here 

 is the mode of distribution of these two groups of fact, and it is 

 held that this strongly suggests without proving it, the alleged 

 mode of origin of both. 



