THE PROPERTIES OF EACH PART OF THE REFLEX ARC 795 



at the tip of the tongue and the tip of the fingers; it is least on the 

 relatively immobile skin of the back. These distances are much less when 

 the points rest on two touch corpuscles. Under these conditions, for in- 

 stance, the distance for the volar side of the finger tip or even for the 

 palm of the hand may be only one-tenth of a millimeter; and for the 

 arm and back it may become reduced to half a millimeter. 



Localization of touch is a very accurate process, at least in the most 

 sensitive parts of the skin, but nevertheless it is very probably a mat- 

 ter of education. An evidence of this is the fact that in the much more 

 highly specialized retina the power of localization of objects in the visual 

 field is a process of education and experience. For this reason a person 

 from whom a congenital cataract has been removed, can not locate the 

 objects which he sees until after he has learned by his experience of touch, 

 taste, etc., to associate the portion of the retina stimulated with a certain 

 part of the visual field. If this is true for the retina, it is also probably 

 true for touch. The famous experiment of Aristotle is explicable on the 

 same basis. If the fingers are crossed and a marble placed between the 

 crossed fingers, it will be felt as double, since now it touches two skin 

 surfaces which have not been accustomed to touch the same object, but 

 educated to feel different objects. Experience associates those two skin 

 areas with different objects, not with the same object. 



The Pain Sense. It was at one time thought that the sensation of pain 

 was due to overstinralation of any kind of receptor, but it is now known that 

 for this, as for^ other skin sensations, there exist special receptors. Thus, 

 it is found that in certain parts of the body, such as the cornea, and to a 

 certain extent in the glans penis, pain receptors alone are present, and 

 in disease the sense of pain may be entirely abolished, whereas that of 

 touch remains, this condition being called analgesia. Overstimulation of 

 a touch spot does not, as we have seen, cause pain but only a sense of 

 pressure. Although pain is appreciated by special receptors, the charac- 

 ter of the pain is dependent on the other sense receptors simultaneously 

 excited; for example, a throbbing pain is due to the simultaneous pres- 

 sure produced by dilated blood vessels, etc. A sensation of pain accom- 

 panies certain reflexes of a protective nature (nociceptive reflexes, page 

 825) , and when the reflex is absent the part is likedy to suffer damage. On 

 this account the pain nerves may be regarded as trophic nerves. The 

 sense of pain may also occur in structures which are devoid of ordinary 

 sensibility, such as the intestine and the ureter. 



