398 ORGANS OF THE SENSES. 



tent of the surface of the skin in which the impression made 

 by the two points of the dividers (aesthesiometer) forms only 

 one, we find that the extent of these circles differs greatly 

 according to the parts of the body under consideration; the 

 limit is very small at the tip of the tongue, but increases 

 greatly in the dorsal regions of the trunk; anatomical data 

 also show that this extent is in inverse ratio to the quantity 

 of tactile corpuscles contained in the skin. We must not, 

 however, decide absolutely from this that a circle of sensa- 

 tion has an anatomical breadth or size, such, for instance, as 

 the space enclosed by the ramifications of a nerve fibre : in 

 order to prove the contrary, it is sufficient to remember that 

 the extent of a circle of sensation varies according to atten- 

 tion, exercise, habit, and other influences. Since the space 

 which is between the points of the dividers, in certain parts, 

 contains more than twelve of Krause's corpuscles, while in these 

 parts two circles of sensations meet, or even partly overlap 

 each other, so that they cannot be separated by perception ; 

 we must admit that these are phenomena of radiation, that 

 is to say, that the excitation of sensory nerve fibre is trans- 

 mitted toother adjacent fibres; and since attention, habit, 

 and exercise can diminish this radiation, we must conclude 

 that it is not an instance of peripheral impression, but of 

 central perception. 



In regard to the skin of the various segments of the limbs, 

 especially of the arm, Vierordt has arrived, by means of 

 numerous and careful experiments, at the conclusion that the 

 sensibility (sense of touch or sense of localization) varies in 

 proportion to the distance between the point considered and 

 the articulation immediately above it, going back as far as to 

 the root of the limb. The comparative values of the delicacy 

 of the sense of localization also form the sum of two breadths : 

 the one, which is constant, being the sensibility of the skin 

 in the axis of the articulation ; the other, which is variable, 

 being in proportion to the distance between the point under 

 consideration and the articulation situated below it, and, 

 therefore, in proportion to the extent of the movements of 

 localization around the articulation. 



One remarkable circumstance, which may be easily ex- 

 plained by referring to our study of the nervous system is, 

 that prolonged sensations of pressure last for a considerable 

 time after the causes producing them have ceased to act : 

 persons who wear spectacles feel them still after taking them 

 off; and after holding an object in the hand, we sometimes 



