i2oo THE EAR. 



by Mach, 1 Crum Brown, 2 and Breuer, 3 and they have been led to emit 

 theories similar to each other, and all based on the notion of the canals 

 being organs for sensations of acceleration of movement or for the sense 

 of rotation. Mach was the first to point out that the phenomena of 

 Purkinje, already described, were in all probability related to the semi- 

 circular canals. He showed that when the body is moved in space, in a 

 straight line, we are not conscious of the velocity of motion, but of 

 variations in this velocity. Similarly, if the body is rotated round a 

 vertical axis, we perceive only angular acceleration, and not angular 

 velocity. The sensations produced by angular acceleration last longer 

 than the acceleration itself, and the position of the head during the 

 movements enables us to determine direction. By a special apparatus, 

 consisting of a chamber rotating round a vertical axis and surrounded 

 by a scale, Mach investigated the nature of vertigo. He examined 

 critically all the organs that might be supposed to be the seat of the 

 sensations of vertigo, or rather its cause, such as the nerves, skin, con- 

 nective tissues, bones and muscles, and the displacement of the mass of 

 blood caused by rotation, and found it impossible thus to account for 

 the phenomena. It will be remembered that Purkinje attached great 

 importance to the movements of the eyeballs, and he supposed that 

 vertigo consists in a displacement of the visual field, produced by the 

 movements of the eyeballs, and which we refer to external objects that 

 appear to be moving, just as we judge of the changes in position of 

 external objects by the sensations excited by contractions of the ocular 

 muscles, as we follow objects with the eye. This explanation did not 

 satisfy Mach, and he inclined to the explanation offered by Breuer, 

 namely, that the movements of the eyeballs are of a reflex character, 

 and that the reflexes are excited by irritation of the nerves distributed 

 to the ampullae. The movements are, in a sense, compensatory, and 

 they are in the opposite direction to the rotation of the head. When 

 the movement of the head stops, then the eyeballs return to their 

 normal position ; but, during this movement backwards, external objects 

 will appear to move in the visual field in a direction opposite to the 

 direction of the rotation of the eye. A sensation is then excited of 

 objects moving in a direction opposite to that of the body, and vertigo 

 is the result. Mach found, however, that this did not fully explain 

 the phenomena, as in certain experiments there was, after rotation, 

 an apparent movement of external objects, even when the retina 

 was stationary, and he finally put forward the following theory: — 

 Suppose in a body, B, there is a cavity on the walls of which there are 

 nerve-endings, and that this cavity contains a solid or liquid, A. By its 

 weight, A will exercise a greater pressure on one part of the walls of the 

 cavity than on the others, and it will thus determine the position of the 

 body, B, relative to the vertical. With each acceleration of B, A will 

 press in the opposite direction, and this contrapressure will be added 

 to the acceleration due to weight, so that the direction of the pressure 

 as well as its intensity will change in the cavity. In like manner, with 

 each angular acceleration communicated to B, A will oppose a rotation 

 in the opposite direction. Thus, B will obtain knowledge both of its 



1 Sitzungsb. d. k. Akad. d. Wissensch., Wien, 1874 ; also " Grundlinien der Lehre von 

 den Bewegungsemptindungen," Leipzig, 1875. 



2 Journ. Physiol., Cambridge and London, vol. viii. p. 327. 



3 Med. Johrb., Wien, 1874, 1875, S. 72, 87. 



