FUNCTIONS OF THE BRAIN 909 



changed. Rotation of the head to the right may be supposed to 

 cause the endolymph in the right external canal, in virtue of its 

 inertia, to lag behind the movement, and to press upon the anterior 

 surface of the ampulla. The disorders of movement after lesions of 

 the canals may be explained as the result of the withdrawal of 

 certain of these afferent impulses, and the consequent overthrow of 

 that equipoise of excitation necessary for the maintenance of equi- 

 librium. An experiment of Kreidl on a crustacean (palaemon) has 

 made it probable that the otoliths by their weight may mechani- 

 cally affect the hair-cells, and so increase their sensitiveness to 

 changes of position. This animal has the peculiarity that in moult- 

 ing the inner lining of the otocysts, in which the otoliths lie and 

 which open to the exterior, are shed along with the otoliths. When 

 moulting is over, the animal by means of its claws conveys fine 

 sand grains into the otocysts, where they function as otoliths. 

 Kreidl placed the animal after moulting upon finely powdered iron, 

 some of which was conveyed into the otocyst instead of sand. It 

 was now found possible to obtain definite reactions from the animal 

 in the presence of a magnet, which, of course, tended to attract the 

 ferruginous otoliths, and so to alter their position with reference 

 to the hairs. The way in which the animal changed its position in 

 response to the magnet could be satisfactorily accounted for on the 

 hypothesis that normally the contact of the otoliths with the hairs 

 is altered under the influence of gravity when such changes of posi- 

 tion occur. Even in man there is evidence of the existence of some 

 mechanism not depending on the muscular sense or on impressions 

 passing up the channels of ordinary or special sensation, by which 

 orientation (the determination of the position of the body in space) 

 is rendered possible. For a man lying perfectly still, with eyes shut, 

 on a horizontal table which is made to rotate uniformly, can not 

 only judge whether, but also in what direction, and approximately 

 through what angle, he is moved. The phenomena of pathology 

 afford weighty additional testimony in favour of the equilibratory 

 function of the semicircular canals. For many cases of vertigo are 

 associated with changes in the internal ear (Meniere's disease). 

 And while nearly every normal individual becomes dizzy when 

 rapidly rotated, 35 per cent, of deaf-mutes are entirely unaffected 

 (James), and the proportion seems to be much higher among con- 

 genital deaf-mutes. Kreidl and Bruck, too, have found that ab- 

 normalities of locomotion and equilibration are much more common 

 in deaf-and-dumb children than in others. Now, in these cases 

 the defect is usually in the internal ear. We must conclude, then, 

 that the co-ordination of muscular movements necessary for equili- 

 brium is achieved in some centre, to which afferent impulses pass 

 from the internal ear by the vestibular branch of the auditory nerve, 

 and from which efferent impulses pass out to the muscles. If, as 



