600 THE SENSES 



Palm of hand, . . 10 



Center of hard palate, . . 12 



Dorsal surface of first phalanges of fingers, . 14 



Back of hand, . .25 



Dorsum of foot near toes, . ..... 37 



Gluteal region, . -37 



Sacral region, . -37 



Upper and lower parts of forearm, . . ... 37 



Back of neck near occiput, . - .50 



Upper dorsal and mid-lumbar regions, . . . . . . .50 



Middle part of forearm, . .. . . . . . . 62 



Middle of thigh, . . . * . . ' 62 



Mid-cervical region, . . ...... 62 



Mid-dorsal region, . . . . . . . . .62 



In the case of the limbs, before the points are recognized as two, they 

 have to be further separated when the line joining them is in the long axis of 

 the limb than when in the transverse direction. 



According to Weber the mind estimates the distance between two points 

 by the number of unexcited nerve endings which intervene between the two 

 points touched. It would appear that a certain number of intervening un- 

 excited nerve endings is necessary before two points touched can be recognized 

 as separate, and the greater this number the more clearly are the points of 

 contact distinguished as separate. The delicacy of the sense of touch may 

 be very much increased by practice. A familiar illustration occurs in the 

 case of the blind, who, by constant practice, can acquire the power of reading 

 raised letters the forms of which are almost, if not quite, undistinguishable by 

 the sense of touch to an ordinary person. 



The different degrees of sensitiveness possessed by different parts may 

 give rise to errors of judgment in estimating the distance between two points 

 where the skin is touched. Thus, if blunted points of a pair of compasses 

 (maintained at a constant distance apart) be slowly drawn over the skin of 

 the cheek toward the lips, it is almost impossible to resist the conclusion that 

 the distance between the points is gradually increasing. When they reach 

 the lips they seem to be considerably farther apart than on the cheek. Thus, 

 too, our estimate of the size of a cavity in a tooth is usually exaggerated when 

 based upon sensations derived from the tongue alone. Another curious 

 illusion may here be mentioned. If we close the eyes, and place a small 

 marble or pea between the crossed fore and middle fingers, we seem to be 

 touching two marbles, figure 480. This illusion is due to an error of judg- 

 ment. The marble is touched by two surfaces which, under ordinary cir- 

 cumstances, could be touched only by two separate marbles, hence the mind, 

 taking no cognizance of the fact that the fingers are crossed, forms the con- 

 clusion that two sensations are due to two marbles. 



Sense of Temperature. The whole surface of the body is more or 

 less sensitive to differences of temperature. The sensation of heat is distinct 



