40 PHYSIOLOGY CHAP. 



an increase, in the second a decrease, of the action set up by the 

 mechanical stimulus within the skin. 



VIII. When any part of the skin is excited by a small surface 

 or a blunt point, we are able, even with the eyes shut, to indicate 

 more or less exactly the place of excitation. Weber termed this 

 capacity of localising cutaneous excitation the " sense of locality " ; 

 others have termed it the "spatial sense." These terms are ill 

 chosen, as they suggest that there is a specific localising sense in 

 the skin, other than those of contact or pressure, and of heat and 

 cold, to which our sensations of locality or space must be referred. 

 In reality, these sensations are only the perceptual signs of the 

 cutaneous sensations which have already been discussed ; in other 

 words, we perceive not only contacts, and positive or negative 

 changes in temperature, but also their seat, that is the area or 

 surface of the skin that is altered by tactile and thermal stimuli. 



There are two methods of determining cutaneous localisation, 

 both of which were employed by Weber : 



(a) The skin of a blindfolded subject is touched with a blunt 

 point, and the subject must at once indicate the spot touched. 

 The degree of error is measured in millimetres and indicates the 

 degree in which the region is sensitive to localisation. 



(&) The blunt ends of a compass or other instrument with a 

 scale (Fig. 22 aesthesiometer of Weber, v. Frey, Giesbach, Binet, 

 Ponzo, and others) are applied lightly and simultaneously to the 

 skin. The blindfolded subject has to say if two separate contacts 

 are perceived, or only one. The power of localisation in the region 

 under examination is measured by the minimal distance at which 

 the two points of the compass are perceived separately, and not as 

 a single contact. 



According to Vierordt the delicacy of tactile spatial perception 

 seems at many points of the body-surface to be in a certain 

 relation with their mobility, in so far as it corresponds to the 

 variety and rapidity, direction and range of the movement. 

 Spatial discrimination is maximal at the tip of the tongue, which 

 is able to move rapidly in all directions. In the skin of the limbs 

 it increases from the proximal towards the distal regions, and is 

 greatest at the finger-tips, the most distal segments, where the 

 range of movements of the limb is maximal ; these are also the 

 parts usually employed as tactile organs. 



Both in the skin of the limbs and that of the trunk tactile 

 discrimination is more developed in the transverse than in the 

 longitudinal axis, and on the flexor surfaces than on the extensor 

 surfaces (Weber) ; in the intercostal spaces the errors are mainly 

 in the direction of these spaces, from which it appears that the 

 direction of the nerves has some influence upon the direction of 

 errors in localisation (Ponzo). 



In young people tactile discrimination is better developed 



