iSf^ ox THE SENSES. 



on t!ie other side of tlie face they approacli the other ear, the points will seem 

 anew to draw closer together, and the double impression will presently be again 

 lost in a single one. How are we to explain these singular effects, and especially 

 the remarkable illusion in the case last mentioned ? In the following manner : the 

 two points simultaneously applied always and necessarily produce only a single 

 sensation, whenever, by their pressure, they excite one and the same nerve-fibre; 

 when both, therefore, touch the skin within one of those circuits whose sensi- 

 bility is supplied by the same fibre. This pi'oposition is incontrovertible. The 

 skin is to be regarded as divided into a countless number of these spaces sup- 

 plied by a single nerve, which have received the name of circles of sensation, 

 (Empfindungskreise;) as a mosaic of such circles, whose size is regulated by the 

 variable affluence of different portions of the cuticle in sensitive fibres. ' Tho 

 further these are from one another, and the greater the division to which they 

 are destined for the supply of the sensibility of the part, the larger in that tract 

 of the skin are the circles of sensation. Within any one such circle all impres- 

 sions bear the same local token ; two, three, or more impressions, therefore, 

 which, within the limits of that circlf^ take efiTect simultaneously upon the skiu, 

 communicate but a single or blended sensation. 



It would seem now to be a necessary consequence of these facts, that in- 

 versely two simultaneous impressions should produce a double sensation and a 

 perception of local separation, when they take effect on two different circles of 

 sensation, and thus excite two different nerve-fibres ; that hence the two points 

 of the compasses should always be separately felt when each of them touches a 

 different circle. In this simple form, however, is the conclusion not tenable — 

 not consistent with the results of experience, as is shown by the following facts 

 and observations. If we make the experiment, for instance, on the skin of the 

 forearm, we find that the separation of the points of the compasses must amount 

 to at least 18 lines if we would produce a double sensation ; that a separation of 

 16 lines always produces a single one, let the points be placed where and in 

 whatever direction we will. Should we thence infer that the circles of sensa- 

 tion of this portion of the skin must have a diameter of 17 lines, we should en- 

 counter an inexplicable contradiction. If we imagine, for example, these divi- 

 sions to be squares of 17 lines lateral length, we know from the condition that 

 every point is sensitive, and that these squares border upon one another without 

 the least interval. If we conceive now two such coterminous squares, it is 

 readily seen that the points of the compasses need not be 18 lines apart to touch 

 at the same time two different squares, but that with a separation of i line we 

 can so place the points that one shall touch a certain square and the other a 

 second square close to the boundary at which these squares meet. There should 

 now arise a double sensation, since here the supposed condition of touching two 

 different circles of sensation is fulfilled, and we ought to be able to detect some 

 place in the forearm where, with a distance of ^ line, this would be the case. 

 But as this is not so, as a constant distance of IS lines in all directions is here 

 required for a double sensation, the supposed condition cannot be tenable, that 

 it sufiices, namely, to touch two contiguous circles in order to produce such a 

 sensation. To reconcile, therefore, the facts just cited with the theory of cir- 

 cles of sensation constituted by the spreading of single nerve-fibres, it has been 

 found necessary to resort to a further supposition, and to assume, as a condition 

 of the double sensation, that at least one or more circles which are not touched 

 must lie between those which are. This, indeed, is a proposition not directly 

 proved, and which, on close consideration, introduces some further difficulties 

 into the discussion, but it is the only one as yet by which the facts are recon- 

 ciled, all others which have been suggested being much less consistent with re- 

 cognized physiological facts and laws. Agreeably to this hypothesis, the ex- 

 planation for which we were seeking may bo concisely stated in the follov/ing 



