CUTANEOUS AND INTERNAL SENSATIONS. 275 



anesthetic there is present a deep or subcutaneous sensibihty to 

 pressure and movements, a sensibihty which must be mediated 

 through sensory fibers contained in the nerves to the muscles. In 

 the skin itself there are present two systems of sensory fibers which 

 regenerate at different times in a nerve that has been severed, 

 and may be studied separately by this means. One system con- 

 veys sensations of pain and of extreme changes in temperature, 

 but the sensations are imperfectly locahzed and the sensibihty is 

 low, or, to express the same idea in another way, the threshold 

 is high. This kind of sensation is found in the viscera also; it is 

 designated as protopathic sensibility. It is stated that the glans 

 pen-s possesses only this kind of sensibility. Protopathic sen- 

 sibility comprises three qualities of sensation, namely, pain, 

 heat (not stimulated below 37° C), and cold (not stimulated above 

 26° C). The second system of fibers responds to stimulations by 

 hght pressures and small differences in temperature between 26° 

 and 37° C. These fibers regenerate after lesions much more slowly 

 than the protopathic variety, and since the sensations mediated 

 by them are localized very exactly, they furnish us the means for 

 making fine discriminations of touch and temperature. For this 

 reason they are described as an epicritic system, and the corre- 

 sponding sensations are designated as epicritic sensibility. This 

 system of fibers is not found in the other organs, and it constitutes, 

 therefore, the special characteristic of the skin area. In this 

 system there are included separate fibers for heat, for cold, for 

 light pressures, and for tactile discrimination. It is through the 

 sensations mediated by these fibers that we recognize the shape 

 and size of objects. The paths taken by these fibei-s after enter- 

 ing the cord are described on p. 173, According to this description 

 it would seem that in the central nervous system the protopathic 

 and epicritic paths are not kept separate, but are integrated in 

 some way, so that at the central end, in the brain, the distinction 

 between them is not recognized in consciousness under normal 

 conditions. 



The distinction made by these authors between the cutaneous 

 and deep sensations, especially between the cutaneous and deep 

 sensations of pressure, is very important and must be borne in 

 mind in all studies of supposed anesthetic areas of skin. Whether 

 the distinction made by them between protopathic and epicritic 

 sensibility will be estabhshed by further observations is at present 

 an open question. Two other sets of observers* have used their 

 method of study, namely, the section and regeneration of a cu- 

 taneous nerve in man; but, unfortunately, they do not corroborate 

 the findings of Head and Rivers in regard to the different qualities 



* Boring, "Quarterly Journal of Exp. Physiology," 1916, 10, 1 ; and Trotter 

 and Davies, "Journal of Physiology," 1909, 38, 109. 



