940 THE CENTRAL NERVOUS SYSTEM 



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. 



Summary. We must conclude, then, that the co-ordination of 

 muscular movements necessary for equilibrium is achieved in some 

 centre or centres 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 this centre or one of these centres 

 is situated in the cerebellum, the efferent path is, as already suggested, 

 partly an indirect one (perhaps by commissural fibres to the Rolandic 

 area, and then out along the pyramidal tract), or more probably to 

 lower centres, which control such massive co-ordinated movements as 

 those concerned in walking and the maintenance of the normal attitude, 

 and thence out along certain tracts that connect the thalamus to the 

 spinal cord (p. 886). 



The influence of the labyrinth on the eye movements through its 

 connection with the oculomotor nuclei in the mid-brain has been 

 previously mentioned as a factor in the phenomena which follow 

 its destruction or stimulation. It would, however, be erroneous to 

 assume that either the cerebellum or the labyrinth is essential to the 

 crude maintenance of that postural muscular tonus which is the reflex 

 foundation of the act of standing. For in the dog and cat the posture 

 persists after removal of the fore-brain, the mid-brain back to the 



