CHAPTER XVII 



THE SENSORY FUNCTIONS OF THE SKIN 



ASIDE from serving as the outer covering of the body and in addition to 

 what it does in the service of heat regulation, the skin has very important 

 sensory functions. Notwithstanding that much has been explained by work 

 done within the last decade, the intimate mechanism of these functions still 

 appears to be very enigmatical. We shall divide them into three different 

 groups, namely: (1) sensations of temperature, (2) sensations of pressure and 

 touch, (3) sensations of pain. 



1. SENSATIONS OF TEMPERATURE 



Temperature sensations are of two kinds, cold and heat sensations, and 

 both are probably related to the regulation of heat in our bodies. The nerves 

 which mediate these sensations produce reflex effects, which manifest them- 

 selves as variations in the intensity of combustion, in the distribution of 

 blood, and in the activity of the sweat glands. The conscious sensations of 

 temperature inform us how the thickness of our clothing and the temperature 

 of our rooms need to be changed one way or the other, although it must be 

 allowed that this information not infrequently leads us astray. 



Until a few years ago it was generally supposed that the diametrically 

 opposite sensations of heat and cold were mediated alike by all parts of the 

 skin and that only one kind of nerve fibers was concerned in both sensations. 

 Blix and Goldscheider independently of each other (1883, 1884) demon- 

 strated, however, that not all points on the skin are capable of arousing tem- 

 perature sensations of both kinds, but that the nerves which mediate sensations 

 of heat have their end organs at different points from those which mediate 

 sensations of cold. 



This proposition is proved by the following experiments. A metallic tube 

 drawn out to a fine point is filled with water of the desired temperature. When 

 cold water is used and the tip of the tube is applied to the skin, care being taken 

 not to exert pressure, one observes that the point can only be felt cold at certain 

 spots, while at others it produces no sensation of temperature at all. If the 

 experiment be repeated, using this time warm water instead of cold, we find that 

 sensations of heat can be received only from certain points. 



Marking off cold spots and heat spots on the skin with different colors, we 

 find that the two do not coincide, although it must be said that a perfectly exact 

 determination of their relative positions is very difficult or quite impossible, 

 owing to the conduction of heat by the skin (cf. Fig. 182). 

 458 



