THE PERCEPTION OF SPACE. 191 



tion far exceeds that of most of the cutaneous surfaces ; 

 that, in fact, we can feel a figure traced only in its differen- 

 tials, so to speak, — a figure which we merely stmi 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 in- 

 ternal surfaces whose sensibility is called in j)lay 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 

 anaesthesia 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.* On the other hand, 

 we may have this sense of movement impaired when the tac- 

 tile 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, f 



That the joint-surf aces are sensitive appears evident from 

 the fact that in inflammation they become the seat of excru- 

 ciating pains, and from the perception by everyone who 

 lifts weights or presses against resistance, that every in- 

 crease of the force opposing him betrays itself to his con- 

 sciousness principally by the starting-out of new feelings 

 or the increase of old ones, in or about the joints. If the 

 structure and mode of mutual application of two articular 

 surfaces be taken into account, it will appear that, granting 

 the surfaces to he sensitive, no more favorable mechanical 



* See for example Duchenne, Electrisation localisee, pp. 727, 770, Ley- 

 den; Vircbow's Archiv, Bd. xlvii. (1869). 



f E.g., Eulenburg, Lehrb. d. Nervenkrankheiten (Berlin), 1878, i. 3. 



