364 SPECIAL PHYSIOLOGY. 



the points are applied in the longitudinal direction. The two points 

 can, moreover, be distinguished at much shorter distances when they 

 are applied consecutively, than when they are made to touch simul- 

 taneously. (Czermak.) 



Accompanying the double sensation of the two points, is a distinct 

 feeling of interspace between them ; and if the points of the compasses 

 be drawn, with a certain rapidity, over a tract of skin, they always 

 appear to the mind of the person experimented upon, to be further 

 apart, as they pass over regions possessing a relatively greater sensi- 

 bility or sense of space. When the compasses are drawn in the oppo- 

 site direction, the points seem to approach each other. Not only, 

 therefore, are the points felt distinct at shorter distances in the more 

 sensitive parts, but they seem to be more distant ; this distance also 

 seems greater when they are applied consecutively and not simul- 

 taneously. At a certain short distance the feeling of interspace dis- 

 appears, but gives way to the sensation of an elongated body, and 

 finally to that of a single point. If one point be cold and the other 

 warm, they are felt, even at short distances, as double, although their 

 relative position cannot be recognized. 



The thinness of the epidermis, under certain circumstances, favors 

 the acuteness of the tactile sense, as is shown in the comparison be- 

 tween the outer and inner aspects of a limb; but the delicacy of touch 

 in the fingers is proverbial, although their epidermis is very thick. 

 Touching or irritating the naked cutis causes pain, not a tactile sen- 

 sation. 



It is most probable that the delicacy of touch, and the power of 

 discriminating distance, are proportional to the number of nerve-fibres 

 supplying the skin, and indirectly, therefore, to the number of papillae. 

 If two impressions be made on a part sparingly supplied with separate 

 nerve-fibres, such impressions may travel to the sensorium, only along 

 one fibre, in which case, only a single impression will be perceived. 

 It has been assumed, that each cutaneous nerve-fibre ends in a pencil 

 of delicate filaments, for the supply of a definite circular or oval area 

 of the skin, the diameter of which has been estimated at probably about 

 4fl(jth of an inch; but the filaments of adjacent nerve-fibres are sup- 

 posed to pass into contiguous areas, so that the exact spot of the body, 

 which is the seat of a single impression, is recognized by the aid of 

 compound impressions. On this view, a theory is offered as to the 

 sense of locality possessed by the skin, viz., that the smaller these 

 areas i. e.^ the more numerous and closer the nerve-fibres the greater 

 the acuteness of this sense of space. (Fick.) 



The curious observation has been made, that a part endowed with a 

 finer sense of space, feels a part less endowed in that respect, and not 

 the latter, the former; when, e. g., the finger touches the forehead, it 

 is the finger which feels the forehead, and not the forehead the finger. 

 For this experiment, the two parts of the skin must be of the same 

 temperature; for when two regions of the skin of different tempera- 

 tures, are brought in contact, a double sensation is produced. 



A useful instrument named an cesthesiometer, consisting of a. gradu- 

 ated bar, having a fixed and movable point attached to it, has been 



