CUTANEOUS AND INTERNAL SENSATIONS. 279 



anesthetic there is present a deep or subcutaneous sensibility to 

 pressure and movements, a sensibility 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 

 conveys sensations of pain and of extreme changes in tempera- 

 ture, but the sensations are imperfectly localized and the sensi- 

 bility 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, and it may be considered from the functional standpoint 

 as a defensive agency toward pathological changes in the tissues; 

 it is designated as protopathic sensibility. It is stated that the 

 glans penis possesses only this kind of sensibility. Protopathic 

 sensibility comprises three qualities of sensation and presum- 

 ably three sets of nerve-fibers, namely for pain, for heat (not 

 stimulated below 37 C.), and for cold (not stimulated above 

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

 by light pressures and small differences in temperature between 

 26 and 37 C., the range of temperature to which the tem- 

 perature nerves of the protopathic system are insensitive. These 

 fibers regenerate after lesions much more slowly than the pro- 

 topathic 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 corresponding 

 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. According to this classification we may 

 assume that the posterior roots of the spinal nerves carry into the 

 spinal cord the following varieties of afferent fibers : 



Heat (small differences). 



Epicritic 



Cutaneous sensory fibers <j 



Protopathic 



Cold (small differences). 

 Touch (light pressures). 

 Tactile discrimination. 

 Heat (extremes). 

 Cold (extremes). 



Pain. 



1 Pressure. 

 Pain. 

 Muscular (position). 



The paths taken by these fibers after entering the cord are de- 

 scribed on p. 174. 



