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 derived from retinal-surface feelings, our opponent might immediately 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 perception 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 unrecognizable. * * * * * 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 geometrizing, it is necessary to review a few facts. It has long been known by medical practitioners that, in patients with cutaneous anæsthesia of a limb, whose muscles also are insensible to the thrill of the faradic current, a very accurate sense of the way in which the limb may be flexed or extended by the hand of another may be preserved.[190] On the other hand, we may have this sense of movement impaired when the tactile sensibility is well preserved. 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 experimenting physician. The writers who have sought a _rationale_ of the matter have consequently been driven by way of exclusion to assume the articular surfaces to be the seat of the perception in question.[191]