THE LABYRINTHINE SENSATIONS 



655 



FIG. 326. After complete destruction of 

 the two labyrinths the neck muscles become 

 so weak that they are unable to overcome the 

 weight of a bullet attached to the beak. 



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 amal- 

 gam, the head is thrown into oscil- 

 lations in a corresponding plane, 

 or perhaps rather we should say 

 that when the head oscillates in 

 this plane there are no correspond- 

 ing 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 converse 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 qf 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 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 



