THE SENSE OF LOCALITY. 



831 



rliere the cutaneous 



arranged in a linear chain, which usually radiates from the hair -follicles. The 

 " tickling-spots " coincide with the pressure and pain-spots. The feeling of tickling 

 corresponds to the feeblest stimulation of a nerve-fibre, and pain to the strongest. 

 The pain-spots can be isolated by means of a needle, or electrically, especially in 

 the cutaneous furrows, in which the pressure-sense is absent. 



Goldscheider removed from his own body small pieces of skin, in which he had previously 

 ascertained the presence of these " spots," and then investigated the excised skin microscopic- 

 ally. At each such spot he found a rich supply of nerves ; at the pressure-spots, there were 

 no touch-corpuscles. 



[By means of the skin, impressions are supplied also to the brain, whereby we become con- 

 scious of the amount and direction of a body moved in contact with the skin. Indeed, the 

 discriminative sensibility is more acute for motion than for touch ; but the liability to error in 

 judging of the distance and direction is great (Hall).] 



[Very complex sensations are obtained by means of the combined action of the skin and 

 muscles, e.g. , those known as "feelings of double contact." These sensations are of the 

 greatest advantage in acquiring the use of instruments and tools. If we touch an object with 

 a rod, we seem to feel the object at the point of the rod, and not in the hand 

 nerves are actually stimulated. With a walking stick, we feel the ground 

 at the end of the stick. Touch the tips of the hair, or a tooth, and the 

 sensation is referred to the tips of the hair in the one case, and the crown 

 of the tooth in the other (Ladd).] 



426. SENSE OF LOCALITY. We are not only able to 

 distinguish differences of pressure or temperature by our sensory 

 nerves, but we are able to distinguish the part which has been 

 touched. This capacity is spoken of as the sense of space or 

 locality. 



Methods of Testing. 1. Place the two blunted points of a pair of com- 

 passes (fig. 615) upon the part of the skin to be investigated, and determine 

 the smallest distance at which the two points are felt only as one impres- 

 sion. Sieveking's aesthesiometer ma) T be used instead (fig. 616) ; one of the 

 points is movable along a graduated rod, while the other is fixed. 2. 

 The distance between the points of the instrument being kept the same, 

 touch several parts of the skin, and ask if the person feels the impression 

 of the points coming nearing to or going wider apart. 3. Touch a part of 

 the skin with a blunt instrument, and observe if the spot touched is cor- 

 rectly indicated by the patient. 4. Separate the points of two pairs of 

 compasses unequally, and place their points upon different parts of the skin, and ask the person 

 to state when the points of both appear to be equally far apart. A distance of 4 lines on the 



Fig. 615. 

 iEsthesiometer. 



Fig t>16. 

 .32sthesiometer of Sieveking 



forehead appears to be equal to a distance of 2*4 lines on the uppei lip. This is Fechner's 

 ' methods of equivalents." 



The following results have been obtained. The sense of locality of a part of the 

 skin is more acute under the following conditions : 



1. The greater the number of tactile nerves in the corresponding part of the skin. 



2. The greater the mobility of the part, so that it increases in the extremities 

 towards the fingers and toes. The sense of locality is always very acute in parts of 

 the body that are very rapidly moved ( Vierordt). 



