286 THE SPECIAL SENSES. 



determines the limitation of the localizing sense for different 

 regions of the skin. 



In the newer work of Head and Rivers, which has been referred to several 

 times, it will be recalled that they distinguish first of all between cutaneous 

 sensibility to pressure and a deep sense of pressure. When the cutaneous 

 fibers of a given area are all destroyed by degeneration, the area is still sen- 

 sitive to pressure applied so as to deform the skin inward. The spot so 

 stimulated can be localized accurately. This deep sense of pressure is mediated 

 by the deep nerve fibers which supply the muscles. According to these 

 authors the cutaneous pressure sensibility is mediated by two sets of fibers, 

 those which give us the power of tactile discrimination when the compass 

 points are applied simultaneously to the skin, and those which give us the 

 power of recognizing simply light pressures. In lesions of the spinal cord one of 

 these sensibilities may be lost and the other retained over a given skin area 

 (Head and Thompson, " Brain," 1906). In fact, the fibers of tactile discrimi- 

 nation are stated to pass up the cord (uncrossed) in the posterior funiculi, 

 while those of light pressures ascend in the lateral or anterolateral funiculi 

 and cross before reaching the medulla. 



The Pain Sense. Pain is probably the sense that is most widely 

 distributed in the body. It is present throughout the skin, and 

 under certain conditions may be aroused by stimulation of sensory 

 nerves in the various visceral organs, and indeed in all of the mem- 

 branes of the body. Our knowledge of the physiological properties 

 of the end-organs and nerves mediating this sense is chiefly limited 

 to the skin, and for cutaneous pain at least the evidence, as stated 

 above, is very strongly in favor of the view that there exists a special 

 set of fibers which have a specific energy for pain. All recent ob- 

 servers agree that the pain sense has a punctiform distribution in 

 the skin, the pain points being even more numerous than the pres- 

 sure points. The threshold stimulus of these points in various 

 regions may be determined by von Frey's stimulating hairs, and 

 experiments of this kind show, as we should expect, that it varies 

 greatly. The cornea, for instance, gives sensations of pain with 

 much weaker stimuli than in the case of the finger tips. In general, 

 however, the threshold stimulus is much higher for the pain than 

 for the pressure points. Histological examination of the pain points 

 indicates that there is no special end-organ, the stimulus taking 

 effect upon the free endings of the nerve fibers. Any of the usual 

 forms of artificial nerve stimuli may affect these endings if of suf- 

 ficient intensity, and, as is well known, stimuli applied to sensory 

 nerve trunks affect these fibers with especial ease. A temperature 

 of 50 to 70 C. applied to an afferent nerve will cause violent pain 

 sensations, but has no effect upon the motor nerve fibers in the same 

 trunk. Mechanical stimulation gives usually only pain sensations, 

 and the results of inflammatory changes, as in neuritis or neuralgia, 

 are equally marked. 



Localization or Projection of Pain Sensations. Under normal 

 conditions cutaneous pains are projected with accuracy to the point 

 stimulated, and it is possible that this result is due in part at least 

 to the training acquired in connection with concomitant (epicritic) 



