THE LABYRINTHINE SENSATIONS 607 



normal position against the weight of the bullet. No such phenomena are 

 presented by a normal bird. 



The same absence of tone is seen in mammals. A dog with both laby- 

 rinths destroyed may jump down from a. ta.hlp. nnr,^ hi]f f w j]l not, rp.pp.a,t the 

 experiment, since the nmsrles of the fore limbs arp fnn toneless to support 

 the head against the shock of the jump, and he knocks his head against the 

 ground as his legs collapsejunder him. If only one canal be put out of action, 

 as, for instance, by stopping it with dentist's amalgam, the head is thrown 

 into oscillations in a corresponding plane, or perhaps rather we should say 

 that when the head oscillates in this plane there are no correspondingjjensa- 

 tions set up whuJLtpmrl to inhibit th? mnYpjnntg The same effect may be 

 produced temporarily by painting any one of the canals with cocaine so as 

 to paralyse its nerve- endings. The converse experiment of isolated stimula- 

 tion of one canal has also been effected 

 by Ewald. For this purpose Ewald, 

 by means of a dentist's burr, opened 

 one bony canal at two spots. By the 

 hole furthest away from the ampulla 

 he introduced an amalgam stopping, 

 so as to prevent any current of fluid 

 backwards through the canal. Over 

 the second hole he fixed, by means of 

 plaster of Paris, a tube which was 

 connected by a flexible rubber tube 

 with a rubber ball. By this means, 

 while the bird was sitting quietly on 

 its perch, he could suddenly blow upon 

 the exposed membranous canal with- FIG. 315. 



out disturbing the bird in any way. 



By the air pressure thus produced on the canal a stream of endolymph was 

 caused in the direction of the ampulla. Every time this was done he found 

 that the animal moved its head and eyes in the direction of the current and 

 always exactly in the plane of the canal which was being stimulated. By 

 this means proof was brought of the correctness of the theory put forward 

 by Breuer and Mach, viz. that the specific stimulus of the nerve-endings in 

 the ampulla is afforded by the current of the endolymph in the semicircular 

 canals. 



Since the endolymph is a fluid with inertia it will not immediately follow 

 a rotational movement of the bony walls of the semicircular canals. Thus a 

 sudden turning of the head from left to right will cause movement of endo- 

 lymph towards, and therefore increased pressure on, the ampullary nerve- 

 endings of the right horizontal canal, and movement of endolymph away 

 from, and therefore diminished pressure on, the corresponding ampulla of 

 the left side. In this way, for movement in any given plane, the two 

 corresponding semicircular canals of the two sides are synergic, and unite in 

 sending impulses which guide the equilibrating centres, and inform us of 



