282 THE SPECIAL SENSES. 



signs of two adjacent fibers may- not be distinct enough for us to 

 recognize them as separate and that practically there must be a 

 number of intervening unstimulated areas, the number varying 

 according to the sensitiveness of the area. Von Frey has, however, 

 given a new method of testing the localizing sense of the skin, the 

 results of which seem to accord with this anatomical explanation. 

 If instead of applying the two points simultaneously they are 

 applied in succession, at an interval of one second, the individual can 

 distinguish the difference when two neighboring pressure points are 

 stimulated. Each pressure point in the skin, therefore, has a local 

 sign, which enables us to distinguish it from all others, and by this 

 method the ultimate sensory circles on the skin become much 

 smaller than when measured by the usual method of Weber. The 

 center of each is a pressure point and the area is determined by the 

 distance from this center at which an isolated stimulation of this 

 point can be obtained. It seems probable, moreover, that each of 

 these pressure points is connected to the brain by a separate nerve 

 path, possibly a single fiber, and that this anatomical arrangement 

 determines the limitation of the localizing sense for dnferent 

 regions of the skin. 



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 deterixdned 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 appUed to sensor}^ 

 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, 



