OTOLITH SACS OF THE INNER EAR 479 



C. ARTIFICIAL STIMULATION OF THE* SEMICIRCULAR CANALS 



Breuer (1874) made the first experiments of this kind and they were 

 extended later by Ewald. In the following discussion we shall follow in the 

 main Ewald's results. 



The anatomical structure of the semicircular canals, as Breuer and Mach 

 have pointed out, make it highly probable that the specific stimulus for the 

 nervous end organs in the ampulla? consists of currents in the endolymph. 



When a ring-shaped tube containing a fluid is rotated in the plane of its 

 curvature the fluid remains for a time at rest on account of its inertia i. e., a 

 current is set up in the opposite direction relative to the walls of the tube, until 

 the fluid has had time to acquire the speed of the tube. Such a current must 

 result as often as a change in the speed or direction of rotation takes place. 

 The same phenomena evidently must occur in the semicircular canals with every 

 rotation of the head. But the effect in the different canals will depend upon 

 their position with reference to the axis of rotation. Rotation about the vertical 

 axis acts almost exclusively on the two external canals. If the head is turned 

 to the right there arises in the external canal of the right side a current directed 

 toward the end of the canal containing the ampulla, in that of the left side a 

 current away from the ampulla. And thus there is in the different pairs of semi- 

 circular canals a current of a certain strength in a certain direction correspond- 

 ing to every turn of the head. The sensory hairs of the maculae are moved by 

 these currents and in this way the corresponding nerve endings are excited. 



These conclusions are capable of experimental proof by producing move- 

 ments of the endolymph in a given direction. For this purpose Ewald opened 

 a bony semicircular canal at two points. Into the opening farther from the 

 ampulla he introduced a plug so that the movement of fluid in that direction 

 was prevented. He adjusted to the other opening a small apparatus by means 

 of which he could exert pressure on the naked membranous canal. Since the 

 fluid could not move away from the ampulla, when pressure was applied a 

 current of endolymph was naturally produced toward the ampulla. With 

 every stimulus of pressure the animal (pigeon) invariably moved its head 

 and eyes in the direction of the current and exactly in the plane of the canal 

 stimulated. When the pressure was not released the animal brought its head 

 back after a time to the starting point. If now the pressure was released 

 and thus a current in the opposite direction was produced, the head and eyes 

 were again turned, but this time in the opposite direction i. e., always in 

 the direction of the current of endolymph and in the plane of the canal 

 stimulated. 



Proof that the currents of endolymph give the normal stimulus to the 

 semicircular canals is furnished also by rotation experiments. To prevent 

 complications with the sense of sight the animal must be blindfolded. If a 

 pigeon be placed in a rotation apparatus in such a way that it is rotated to the 

 right about a vertical axis, it turns its head in the horizontal direction to 

 the left, that is, in the same direction as the current produced by the inertia 

 of the endolymph. When the head has been turned a certain distance to the 

 left, it moves a certain distance to the right toward the median position, then is 

 again rotated to the left, and so on. In this way the head swings incessantly to 



