924 



SENSE OF SPACE. 



irritants disturb normal tactile sensations by over-stimulation, and, 

 therefore, excite only pain. 



v. Vintschgau discovered that if two electrical tactile stimuli are applied to 

 the middle of the forehead in succession, a shorter interval of time is usually 

 required to perceive them as separate (0.022 sec.) than if they are applied to 

 the dorsal surface of the lower arm (0.033 sec.). 



If rapidly interrupted electrical or mechanical stimuli which can still 

 be perceived as separate irritations, are permitted to act on the skin, 

 and if the stimuli are then suddenly withdrawn, a new sensation arises 

 after a short interval of rest. This secondary sensation appears as a short 

 stinging sensation. It is supposed to result as a summation within 

 the cells of the sensory paths in the spinal cord, and to be identical 

 with the phenomenon of delayed sensation of pain. 



The law of specific energies presupposes the existence among the 

 cutaneous nerves of different fibers with different end-organs, which con- 

 duct the various forms of sensation (pressure, temperature, pain). In 

 fact Blix and Goldscheider have found such fibers. Electrical stimula- 

 tion causes different sensations in different minute punctate areas of the 

 skin: in one place pain alone is perceived, in another cold, in a third 

 heat, and in a fourth the sensation of pressure. At each temperature- 

 point there is insensitiveness to pain or pressure. The pressure-points 

 are much closer together and usually more numerous than the tempera- 

 ture-points. There are also special pain-points and ticklish points. 

 These sensory points are arranged in linear chains, which usually radiate 

 from the hair-papillae. Ticklish points coincide with the pressure-points 

 and pain-points. The sensations of tickling and itching correspond to 

 the feeblest irritation of the nerve -fiber, that of pain to the strongest 

 irritation. The pain -points may be shown by the needle and by 

 electricity, especially in the wrinkles of the skin, in which the pressure- 

 sense is absent. 



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

 determined the various points, and examined the tissues microscopically. At 

 every sensory point be found an extraordinary number of nerves; at the pressure- 

 points there were no tactile corpuscles. 



The best way of testing the tactile sense in general, according to E. Hering, is 

 by means of numerous rods, wrapped with wire of different size. The coarse wire is, 

 naturally, the easiest to distinguish on account of its unevenness, while fine wire 

 appears, on the contrary, almost smooth when the tactile sense is not acute. 

 :erent portions of skin exhibit varying degrees of tactile delicacy, and they 

 my be arranged in the following order, from the most delicate to the least delicate : 

 nger-tips, palm of the hand, inferior surface of the toes, back of the hand, flexor 

 irlace ol the forearm, buttocks, extensor surface of the forearm, leg, upper arm, 

 thigh, scapular region. 



SENSE OF SPACE. 



Man is able not only to distinguish differences in pressure or in 

 ;emperature and also pain as such, by means of his nerves, but also 

 the s C atial ^en mt ^^ ^ impression is made '> this faculty is designated 



dwJSS ?! Testing.(i) Two blunt compass-points are placed at different 



is d^erminS? nt ^ P r !l 0n skl " tO be examin ed, and the greatest distance 



minea at which the two points are still perceived as one. Instead of the 



^fixed ^rltl^ ^ esthesl meter may be used. This consists of two points, 



ed, and the other movable on a scale like a cobbler's measure. (2) With 



