256 Mr. R. Moon on the Functions of the Membranous 



most part, similar in kind to that of cutting the horizontal canals, 

 though more violent in degree ; and the explanation in all the 

 different cases is in its main features the same. 



When the ears are perfect, the effect of throwing the head 

 back, upon the principles already explained, would be to force 

 the extremity nearest to the vestibule of each posterior vertical 

 ampulla into the corresponding portion of the surrounding os- 

 seous ampulla, and thus would be given a signal upon which, 

 subject to the disposition of the controlling will, the muscles by 

 which the head is moved forwards would be called into play. 



On the other hand, when either of the last-named membra- 

 nous canals is cut, on the principles above laid down, the effect 

 would be to throw the end furthest from the vestibule of its 

 ampulla into the corresponding portion of the osseous envelope; 

 so that in the latter case a signal would be given of a character 

 opposite to that given in the former ; i. e. the tendency would be 

 to make the animal fall backwards. 



The mode in which, when both the posterior vertical canals 

 were cut, the animal showed a disposition to turn its head upside 

 down so as to rest the top of it on the ground, may be accounted 

 for by supposing that in this way it placed in equilibrium the 

 membranous ampullse of the canals in question. 



The mode of explanation in the case of the anterior vertical 

 canals is obviously the same, mutatis mutandis, as in the case 

 last considered. 



If the views above set forth be correct, it is evident that we 

 may look upon the membranous labyrinth in the light of a seventh 

 sense*, by which intelligence of the amount and direction of 

 the motions of the head are communicated to the mind or sub- 

 ordinate governing principle. That there is a necessity for some 

 such special provision as regards the motion of the head is suf- 

 ficiently obvious when we reflect that we cannot judge of the 

 position of the head with regard to surrounding objects in the 

 way in which we can judge of that of the hand, the foot, or 

 any part of our frame which can be contemplated by the eye. 

 It might have been anticipated that the muscular effort neces- 

 sary to move the head in a given direction would have afforded 

 a sufficiently accurate measure of the effect produced by such 

 effort. I think that M. Flourens's experiments prove conclu- 

 sively that this is not so, and that a much more refined mode of 

 measurement and adjustment has been provided for us — one, in 

 fact, which takes cognizance of the effects actually produced, and 

 not merely of the efforts made to produce them. 



It will probably have been observed, however, that the mode 

 of adjustment, so far as hitherto explained, is in one respect de- 

 * The sense of muscular exertion being counted as one. 



