262 THE SPECIAL SENSES. 



some recognizable difference in consciousness. This difference is 

 spoken of as the local sign. We may believe that every sensitive 

 point upon the skin has its own distinctive local sign or quality, and 

 that by experience we have learned to project each local sign more 

 or less accurately to its proper place on the skin surface. Two points 

 on this surface that are a great distance apart are easily recognized 

 as different ; but as we bring the points closer together the difference 

 becomes less marked and finally disappears when the distance 

 corresponds to the area of the sensory circle for the part of the skin 

 investigated, for instance, 1 mm. for the tongue, 22 mms. for the 

 forehead, etc. The ultimate limit of the power of discrimination 

 was assumed by Weber to depend upon the area of distribution of 

 a single nerve fiber. Assuming that each nerve fiber at its termi- 

 nation spreads over a certain skin area, it was suggested that the 

 size of this area forms a limit to the power of discrimination, 

 since two stimuli within it would affect a single fiber and therefore 

 would give a single sensation. 



This view, however, has not been supposed to accord with the 

 facts even when the additional supposition was made that the local 

 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 different 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 



