1170 



TOUCH. 



selected, whose relative position is subject to 

 variation (such as the two eyelids, the two 

 lips, &c.), and the two points of the pair of 

 compasses be applied respectively to these two 

 surfaces, the distance which separates them 

 will seem to be much greater than if the two 

 points rest at the same time on one or the 

 other surface. 6. The same holds good, ac- 

 cording to Weber, when the two points are 

 applied to parts of the surface, which, though 

 in continuity with each other, differ remark- 

 ably, either in structure, in function, or in the 

 use habitually made of them ; thus, the points 

 will be more clearly distinguishable, and will 

 therefore seem to be more distant from each 

 other, when one is applied to the inner surface 

 and the other to the red outer surface of the 

 lips, than when they are both applied to the 

 latter, although its discriminating power is 

 much greater than that of the former ; and the 

 same holds good of the margin and dorsum of 

 the tongue, the palmar and dorsal surfaces of 

 the last phalanges of the fingers, &c. 7. The 

 discriminating sense is more acute in the in- 

 teguments of the head, than in those of the 

 trunk ; and on the face, its acuteness di- 

 minishes as the distance from the mouth in- 

 creases. 8. The tactile discrimination of the 

 integuments of the limbs augments with the 

 distance of the part from the axis of the 

 body j it is less in the integuments of the 

 trunk, than in those of the members. 



The power of tactile discrimination may be 

 conceived to depend in part upon the mode 

 in which the ultimate nerve-fibres are dis- 

 tributed in the skin, being greater in propor- 

 tion as contiguous parts are supplied from 

 distinct central sources, and less when the 

 central terminations of their nerve-tubes are 

 the same. Thus, if two impressions be made 

 along the course of the same nerve-tube, they 

 will not be felt as two, but as one , and this 

 probably holds good of the parts of the in- 

 tegument supplied with branches from the 

 central axis of any one tube. On the other 

 hand, whenever two impressions be made 

 upon two distinct nerve-tubes, or on the 

 branches proceeding from them, they will 

 probably be felt to be double ; and the dis- 

 tance at which these impressions seem to the 

 tactile sense to be, appears to have a relation 

 to the distinctness of the central connections 

 of these nerve-tubes, as appears from the 

 fact that the " limit of confusion " is less 

 across the median line than on either side of 

 it ; that it is less between two parts (such as 

 the lips and eyelids) whose nervous supply is 

 known to be distinct, than on either part 

 separately ; and that it is less between two 

 parts whose nervous supply may be presumed, 

 from their difference of function, to be dis- 

 tinct (as the inner and outer surfaces of the 

 lips), than on either part separately. More- 

 ever, it would not seem to be improbable that 

 one use of the plexuses from which the limbs 

 are supplied, is to produce such an inter- 

 mingling of the fibres from different gan- 

 glionic centres, that contiguous portions of 

 the integument shall be connected with cen- 



tres very remote from each other, and their 

 discriminating power thus augmented. No 

 such intermingling takes place in the nerves 

 which supply the trunk, and the tactile dis- 

 crimination of its integument is (as we have 

 seen) vastly inferior to that of the extremities. 

 Thus it may happen that the common sensi- 

 bility of two parts may be the same, whilst 

 their power of tactile discrimination may differ 

 considerably; and we may even have the 

 common sensibility greatest where the tactile 

 discrimination is least, as we experience, for 

 example, on the integument of the face, which 

 is far more sensitive to a blow, and especially 

 to a " fillip," than is the integument of the 

 palmar surface of the fingers, although greatly 

 inferior to them in discriminating power. 

 The actual nervous supply, and the conse 

 quent sensibility, of a part, may be greater in 

 such cases ; but the unity, or close approxi- 

 mation, of the source from which this pro- 

 ceeds, may prevent its discriminating power 

 from augmenting in the same proportion. 



In like manner, we find that the tactile dis- 

 crimination of different parts bears no relation 

 whatever to that peculiar modification of 

 common sensibility (which yet appears the 

 exclusive attribute of the external integuments) 

 through which the feeling of " tickling " is 

 excited. For the parts which are most sus- 

 ceptible to this feeling, such as the axillae and 

 the soles of the feet, are possessed of a very 

 low degree of discriminating power, and those 

 which possess this power in the highest 

 degree (such as the tips of the fingers) are 

 the least " ticklish." Further, it is worthy of 

 notice that the parts through which that 

 peculiar sensation, which we have termed the 

 genital sense, is specially excited, namely, the 

 penis and the mammary areola, are remark- 

 able rather for the obtuseness than for the 

 acuteness of their power of tactile discrimina- 

 tion. 



That it is only through the skin and those 

 parts of its internal reflexions which are in 

 closest proximity with it (especially the lining 

 of the mouth and nostrils), that we can dis- 

 criminate tactile impressions, appears from 

 this ; that although the internal mucous and 

 serous surfaces, the fibrous membranes, and 

 the parenchyma of many organs, &c. &c., 

 are all capable of becoming acutely sensible 

 to pain when irritated or inflamed, yet no 

 foreign substance is ever distinctly felt by the 

 touch through these parts. Thus, although a 

 sensation of a pleasing or a painful nature is 

 excited by certain substances immediately 

 upon being swallowed, all consciousness of 

 their presence (so far as it is dependent upon 

 the sense of touch) soon ceases, and cannot 

 be again recalled by the utmost exertion of the 

 will. Further, a foreign substance, lodged in 

 the alimentary canal, or in the trachea, may 

 give rise to the greatest possible distress, 

 through the irritation it produces ; but though 

 it thus acts upon the nerves of the parts im- 

 mediatelj 7 in contact with it, these nerves 

 convey no idea to the sufferer of the shape 

 or size of the body, or of any other of its 



