CHAPTER XV. 

 CUTANEOUS AND INTERNAL SENSATIONS. 



General Classification. According to the older views, the 

 sensory nerves of the skin give sensations of touch. Modern 

 physiology has shown, however, that these nerves mediate at 

 least four different qualities of sensation namely, pressure, 

 warmth, cold, and pain. Our so-called touch sensations are 

 usually compound, consisting of a pressure and a temperature 

 component and also very frequently an element of muscle sense 

 when muscular efforts are involved, as, for instance, in measuring 

 weights or resistances. The four sensory qualities enumerated 

 constitute the cutaneous senses, and they are present, or, to 

 speak more accurately, the nerves through which these senses 

 are mediated are present not only over the general cutaneous 

 surface but also in those membranes such as the mucous 

 membrane of the mouth and the rectum (stomodeum and 

 proctodeum) which embryologically are formed from the 

 epiblast. The surfaces in the interior of the body, on the 

 contrary such as the membranes of the alimentary canal, 

 muscles, fasciae, etc. have only nerves of 'pain, but no sense 

 of touch or temperature. Of these cutaneous senses, three 

 pressure, warmth, and cold may be grouped with the exterior 

 senses, the sensations being projected to the exterior of the 

 body, into the substance causing the stimulation; although, as 

 was mentioned above, the temperature sensations under con- 

 ditions fever, vascular dilatation, etc. may be projected to 

 parts of the skin itself and be felt as changes in ourselves. The 

 temperature sensations are, in fact, projected to the exterior- 

 whenever they are combined with pressure sensations, the latter 

 serving, as it were, as the dominant sense. The pain sense, on 

 the other hand, belongs to the group of interior senses, the 

 sensations being always projected into our own body and being 

 felt as changes in ourselves. 



Protopathic, Epicritic, and Deep Sensibility. In the matter of 

 the classification of the cutaneous senses and, indeed, the body senses 

 in general, a new point of view has been suggested by Head and 

 Rivers.* These authors made a careful study of the loss of sensa- 

 tions after division of the cutaneous nerves, and of the subsequent 

 gradual and separate return of these sensations following upon suture 

 of the divided ends. They find that in skin areas made completely 

 * Head and Rivers, " Brain," 1905, 99, and 1908, 323. 

 278 



