688 THE SENSES 



So, with regard to the ideas derived from sensations of touch combined 

 with movements, it is doubtful how far the consciousness of the extent of 

 muscular movement is obtained from sensations in the muscles themselves. 

 The sensation of movement attending the motions of the hand is very slight; 

 and persons who do not know that the action of particular muscles is neces- 

 sary for the production of given movements, do not suspect that the move- 

 ment of the fingers, for example, depends on an action in the forearm. 

 The mind has, nevertheless, a very definite knowledge of the changes 

 of position produced by movements; and it is on this that the ideas which 

 it conceives of the extension and form of a body are in great measure 

 founded. 



There is no marked development of common sensibility to be made out 

 in muscles: they may be cut without the production of pain. On the 

 other hand, there is no doubt that afferent impulses must pass upward 

 from muscles and tendons to the brain on the basis of which we become 

 conscious of their condition. This, then, must be a special sense. It has 

 been suggested that the minute end-bulbs of Golgi found in tendons, and 

 that the Pacinian corpuscles in the neighborhood of joints, are the terminal 

 organs of this special sense. 



Cutaneous Sensibility and Differential Innervation of the Skin. 

 Studies on the regeneration of cutaneous or sensory nerve trunks after 

 section or after degeneration from disease have revealed the interesting 

 fact that the skin possesses two types of sensory nerves. In the recovery 

 of sensibility to touch and temperature and to pain Head and Rivers have 

 discovered that after a relatively short time, from seven to twenty-six 

 weeks, a certain degree of sensitiveness appears. However, the sensations 

 are different from ordinary sensibility less distinct, more diffuse, different 

 in quality and cannot be localized with the usual definiteness. The stimu- 

 lus to evoke them must be coarser and more general. Only relatively wide 

 extremes of heat are perceived, heat above 38 C. or below 24 C. Pain 

 sensations are more disturbing, of a peculiar type. This type of sensibility 

 has been designated as protopathic to distinguish it from the usual type or 

 epicritic sensibility. In regeneration epicritic sensibility returns much 

 later, in two years more or less, after nerve section or degeneration. With 

 the return of the epicritic sensory function the usual accurate discrimina- 

 tion of temperature variations within the narrower limits between 24 

 and 38C., and of delicate touch sensibility, accurate spatial localization, 

 and discrimination replace the protopathic type. 



These experiments lend strength to the view that cutaneous sensibility 

 depends on a double or at least a differential innervation. However, 

 Head's later experiments seem to show that the two classes of nerves 

 probably run in common tracts in the cord and brain stem. In cord lesions 

 that affect the epicritic sensibility the protopathic is also lost. 



