Page:Mind (Old Series) Volume 12.djvu/212

THE PERCEPTION OF SPACE. (ll.) 199 Should we be tempted to object to this supposition of the advocate of perception by muscular feelings, that we have learned the spatial significance of these feelings by reiterated experiences of seeing what figure is drawn when each special muscular grouping is felt, so that in the last resort the muscular space-feelings would be de- rived from retinal-surface feelings ; our opponent might imme- diately hush us by pointing to the fact that in persons born blind the phenomenon in question is even more perfect than in ourselves. If we suggest that the blind may have originally traced the figures on the cutaneous surface of cheek, thigh or palm, and may now remember the specific figure which each present movement formerly caused the skin-surface to perceive, he may reply that the delicacy of the motor per- ception far exceeds that of most of the cutaneous surfaces that in fact we can feel a figure traced only in its differentials, so to speak, a figure which we merely start to trace by our finger-tip, a figure which traced in the same way on our finger-tip by the hand of another is almost if not wholly un- recognisable. The champion of the muscular sense seems likely to be triumphant until we invoke the articular cartilages, as internal surfaces whose sensibility is called in play by every movement we make, however delicate the latter may be. To establish the part they play in our geometrising, it is necessary to review a few facts. It has long been known by medical practitioners that, in patients with cutaneous anaes- thesia of a limb, whose muscles also are insensible to the thrill of the faradic current, a very accurate sense of the position into which the limb may be flexed or extended by the hand of another may be preserved. 1 On the other hand, we may have the sense of attitude impaired when the tactile sensibility is intact. That the pretended feeling of outgoing innervation can play in these cases no part, is obvious from the fact that the movements by which the limb changes its position are passive ones, imprinted on it by the experiment- ing physician. The writers who have sought a rationale of the matter have been driven by way of exclusion to assume the articular surfaces to be the seat of the perception in ques- tion. 2 That the joint-surfaces are sensitive appears evident from 1 See for example Duchenne, Electrisation localisee, pp. 727, 770, Ley den ; Vir chow's Archiv, Bd. xlvii. (1869). 2 E.g., Enlenburg, Lehrb. d. NervenJcrankheiten, Berlin, 1878, i. 3.