790 THE CENTRAL NERVOUS SYSTEM 



nerve is stimulated (chemically, mechanically or electrically) during its 

 passage across the tympanum, the sensation evoked is that of taste. 

 And so with the receptor; whatever the means by which it is excited, 

 whether by the particular kind of stimulus for which it is adapted or by 

 excessive intensities of other stimuli, excitation always evokes the same 

 sensation. If the optic nerve or retina is mechanically stimulated, as 

 by pressure against the outer canthus of the eye or by an electric cur- 

 rent, the sensation is that of light. Applying these facts to less well- 

 known receptors, such as those of heat and cold, it is interesting to note 

 that stimulation of a "cold spot" by extreme heat or by mechanical 

 or electrical stimuli brings out the sensation of cold. 



Properties of Epicritic and Protopathic Receptors 



A valuable grouping of receptors of the skin has been demonstrated by 

 Head and his pupils by experiments on himself. Head found after sec- 

 tion of the skin nerves of the radial nerve, for example that deep 

 pressure and pain were still present in the area supplied by the nerve, 

 indicating that these deep sensations are carried by the sensory fibers 

 present in the muscular nerves. In such a paralyzed sensory region the 

 power of general localization is fairly good, although light, touch, tem- 

 perature and superficial pain are entirely absent in the overlying skin. 



In the case of the fingers the nerves of deep sensibility run in the ten- 

 dons of the finger muscles, so that after severance of the cutaneous 

 nerves and tendons of the hand, all sensibility is gone. 



During the regeneration of the cut nerve the cutaneous sensations re- 

 appear at two periods: one group, called the protopathic, begins to ap- 

 pear in from seven to twenty-six weeks, whereas the other, called epicritic, 

 does not fully appear for one or two years. 2 The protophatic sensations 

 are of a distinctly lower order than the epicritic. When they alone are 

 present, there is the sensation of pain, "but not that of fine touch; tem- 

 perature sensations are felt when extreme degrees of heat or cold above 

 38 C. or below 20 C. are applied to the skin, but not for slight de- 

 grees; the power of discriminating between two points is almost entirely 

 absent ; and the sense of localization is very imperfect. For example, the 

 person will often refer the point that has actually been stimulated to a 

 neighboring normal portion of skin. Protopathic sensibility is more or 

 less distributed in spots, and it is strongly "affective" in character, caus- 

 ing an intense subjective sensation. A stimulus that causes only moderate 

 pain under normal conditions produces in a "protopathic area" a pain 

 that may be intense. 



The epicritic sensation, as will be inferred from the foregoing, responds 



