634 THE SENSES 



Tip of nose 6 mm. 



Middle of dorsum of tongue 8 mm. 



Palm of hand 10 mm. 



Center of hard palate 12 mm. 



Dorsal surface of first phalanges of fingers 14 mm. 



Back of hand 25 mm. 



Dorsum of foot near toes 37 mm. 



Gluteal region 37 mm. 



Sacral region 37 mm. 



Upper and lower parts of forearm 37 mm. 



Back of neck near occiput 50 mm. 



Upper dorsal and mid-lumbar regions 50 mm. 



Middle part of forearm 62 mm. 



Middle of thigh 62 mm. 



Mid-cervical region 62 mm. 



Mid-dorsal region 62 mm. 



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 recog- 

 nized 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. 



