THE SENSES. 553: 



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 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 towards 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 con- 

 siderably 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 

 cognizance 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 de- 

 pend 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." The esti- 

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

 pressure on the skin, and (2) the muscular sense. 



The estimate of weight derived from a combination of these two sen- 

 sations (as in lifting a weight) is more accurate than that derived from 

 the former alone (as when a weight is laid on the hand); thus Weber 

 found that by the former method he could generally distinguish 19 oz. 

 from 20 oz., but not 19f oz. from 20, while by the latter he could at 

 most only distinguish 14^ oz. from 15 oz. 



It is not the absolute, but the relative, amount of the difference of 

 weight which we have thus the fa.culty of perceiving. 



