CHAP, ii.] THE BRAIN. 1011 



In mammals (rabbits) section of the canals also produces a 

 certain amount of loss of coordination, but much less than that 

 witnessed in birds ; and the movements of the head are not so 

 marked, peculiar oscillating movements of the eyeballs, differing in 

 direction and character according to the canal or canals operated 

 upon, becoming however prominent. In the frog no deviations of 

 the head are seen, but there is some loss of coordination in the 

 movements of the body. In fishes no effect at all is produced. 



Injury to the bony canals alone is insufficient to produce the 

 symptoms; the membranous canals themselves must be divided 

 or injured. The characteristic movements of the head may 

 however be brought about in a bird without opening the bony 

 canal, by suddenly heating or cooling a canal, especially its 

 ampullar terminations, or by the making or breaking of a con- 

 stant current directed through the canal. 



There can be no doubt that these characteristic movements 

 of the head are the result of afferent impulses started in the 

 nervous endings of the auditory nerve over the ampulla of the 

 canal, and conveyed to the brain along that nerve. And that 

 injury to or other stimulation of each of the three canals should 

 produce in each case a different movement of the head, the 

 direction of the movement being different according to the plane 

 in which the canal lies, shews that these impulses are of a peculiar 

 nature. This is further illustrated by the following experiment. 

 If the horizontal canal be carefully laid bare, and the membranous 

 canal opened so as to expose the endolymph, blowing gently over 

 the opened canal with a fine glass cannula will produce a definite 

 movement of the head, which is turned to the one side or to the 

 other, according as the current of air drives the endolymph 

 towards or away from the ampulla. From this it is inferred 

 that a movement of the endolymph over, or an increased pressure 

 of the endolymph on, the nervous endings in the ampulla gives 

 rise to afferent impulses which in some way determine the issue 

 of efferent impulses leading to the movement of the head. It is 

 further suggested that since the planes of the three canals lie in 

 the three axes of space, any change in the position of the head 

 must lead to changes in the pressure of the endolymph on the 

 walls of the ampullae or to movements of endolymph over those 

 walls, and so must give rise to impulses passing up the auditory 

 nerve ; and that since every change of position will affect the three 

 canals differently (whereas the changes of pressure of the endo- 

 lymph involved in a " wave of sound" will affect all three ampullae 

 equally) those impulses will differ according to the direction of 

 the change. A still further extension of this view supposes that 

 since in any one position of the head the pressure of the endo- 

 lymph will differ in the three ampullae, mere position of the head, 

 as distinguished from change of position, is adequate to generate 

 afferent impulses differing in the different positions. 



