54 PHYSIOLOGY CHAP. 



EDITOKIAL NOTE 



This chapter would not be complete for English readers at least without a 

 reference to the investigations of Head and Kivers on the mechanism of 

 peripheral sensibility. Their conclusions were drawn mainly from a study of 

 the sensory changes produced by section of a small cutaneous nerve in Head's 

 arm and the observation of the sensory phenomena that occurred during its 

 regeneration, but they were supported by numerous clinical examinations 

 made in conjunction with Sherren. 



In addition to those fibres, concerned with cutaneous sensibility, they 

 described a system that subserves deep sensibility (see Chapter II.) the end- 

 organs of which respond to pressure either by sensations of contact or of pain 

 if the pressure is excessive, and to the movements of joints, tendons, and 

 muscles. The sensations of pressure evoked can be accurately localised, and 

 the direction of the movement appreciated correctly, even though the over- 

 lying skin is totally insensitive, but two compass points applied simultaneously 

 over the part cannot be discriminated by this system alone. The nerves of 

 this deep sensibility run mainly with the muscular nerves and are not 

 destroyed if all the cutaneous fibres are cut. Head and Thompson, however, 

 suggest that fibres belonging to the deep system also reach the skin. 



Cutaneous sensibility was divided into two separate systems ; the one 

 called " protopathic " is capable of reacting to all painful cutaneous stimuli of 

 every nature, and to the more extreme degrees of heat and cold, that is, to 

 thermal stimuli above 40 C. and below 24 C., but the sensations produced are 

 diffuse, unnaturally intense, and unaccompanied by a definite recognition of 

 the locality of the spot stimulated. 



Through the second system, styled " epicritic" light contact and the inter- 

 mediate degrees of heat and cold are appreciated, and on it, in addition, 

 depends cutaneous localisation, the discrimination of the compass points, and 

 the appreciation of size. 



The protopathic system is essentially one of punctate sensibility, as 

 sensations of pain, heat, and cold can be excited only from the corresponding 

 spots ; the sensation of warmth depends probably on nerve-endings that lie 

 between the sparsely scattered heat spots, and the appreciation of coolness, as 

 contrasted with cold, may be due to end-organs other than those of the cold 

 spots. 



It is suggested that these three peripheral systems were developed at 

 different phylogenetic periods. The two cutaneous systems, the so-called 

 " epicritic " and the " protopathic," can be, according to Head, studied separ- 

 ately on an area of skin after section of a sensory nerve, as the loss of the 

 protopathic elements is usually less extensive than the epicritic, and during 

 regeneration and recovery of function, since the " protopathic " sensibilities 

 reappear first. On the glans penis, too, there is only "protopathic" 

 sensation. 



These ingenious and elaborate observations have not been, however, 

 verified or supported by authoritative independent investigations, and the 

 researches of Trotter and Davies, especially, have thrown much doubt on them 

 and on the accuracy of the conclusions drawn from them. These workers 

 investigated the effects of the section of seven cutaneous nerves in themselves ; 

 they found that this operation produces a central area of profound sensory 

 loss, an intermediate zone of moderate extent surrounding this of partial 

 loss, and a larger zone in which qualitative changes only can be detected. 

 When regeneration sets in, the return of all sensory functions begins about 

 the same time, but is irregular. They failed to discover that identity 

 in the states of sensation in the intermediate zone of partial loss and 

 in the central area during the progress of recovery, which is an essential 



