THE SENSES. 165 



of intervening unexcited nerve-endings are 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. By 

 practice the delicacy of a sense of touch may be very much increased. 

 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 power of correctly localizing sensations of touch is gradually 

 derived from experience. Thus infants when in pain simply cry, but 

 make no effort to remove the cause of irritation, as an older child or adult 

 would, doubtless on account of their imperfect knowledge of its exact 

 situation. By long experience this power of localization becomes perfected, 

 till at length the brain possesses a complete "picture" as it were of the 

 surface of the body, and is able with marvellous exactness to localize each 

 sensation of touch. 



Illusions of Touch. The different degrees of sensitiveness pos- 

 sessed 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 further apart than on the cheek. Thus, too, our estimate 

 of the size of a cavity in a tooth is usually exaggerated when based upon 

 sensation 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. This illusion is due to an error of judgment. The marble is 

 touched by two surfaces which, under ordinary circumstances, could only 

 be touched by two separate marbles, hence the mind, taking no cogni- 

 zance of the fact that the fingers are crossed, forms the conclusion that 

 two sensations are due to two marbles. 



(b) Pressure. It is extremely difficult to separate touch proper from 

 sensations of pressure, and, indeed, the former may be said to depend upon 

 the latter. If the hand be rested on the table and a very light body such 

 as a small card placed on it, the Only sensation produced is one of contact; 

 if, however, an ounce weight be laid on the card an additional sensation 

 (that of pressure) is experienced, and this becomes more intense as the 

 weight is increased. If now the weight be raised by the hand, we are 

 conscious of overcoming a certain resistance; this consciousness is due 

 to what is termed the "muscular sense" (p. 119, Vol. II.). The estimate 

 of a weight is, therefore, usually based oh two sensations, (1) of pressure on 

 the skin, and (2) the muscular sense. 



