THE LABYKINTHINE SENSATIONS 679 



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

 labyrinths destroyed may jump down from a table once, but will 

 not repeat the experiment, since the muscles of the fore limbs are 

 too toneless to support the head against the shock of the jump, and 

 he knocks his head against the ground as his legs collapse under 

 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 oscilla- 

 tions in a corresponding plane, or perhaps rather we should say that 

 when the head oscillates in this plane there are no corresponding 

 sensations set up which tend to inhibit the movements. The same 

 effect may be produced 

 temporarily by painting any 

 one of the canals with 

 cocaine so as to paralyse 

 its nerve-endings. The con- 

 verse experiment of isolated 

 stimulation 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 without dis- 

 turbing 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 endo- 

 lymph 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 endolymph towards, and therefore increased 

 pressure on, the ampullary nerve-endings of the left horizontal canal, 



. 313. 



