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AUDITORY NERVE. 



take place, and then in both ears even when only one side is affected. The reac- 

 tion-formula resulting under such circumstances conforms entirely with Pniiger's 

 law, namely kathodal closure causes ringing in the ears and anodal opening deeper 

 roaring. While the current is closed, a permanent reaction exists even when the 

 strength of the current is slight. Even in the presence of complete deafness this 

 typical reaction may persist. 



Pathological. Increased irritability of the auditory nerve at any point in its 

 course, its centers, or its terminal distribution causes nervous acuity of hearing 

 (hyperacusis) , which is generally a symptom of widespread increase in nervous 

 irritability, for example in hysterical persons. If present in particularly marked 

 degree it may give rise to a distinctly painful sensitiveness, which may be designated 

 acoustic hyperalgia. Irritation of the area named causes auditory perceptions, 

 among which nervous roaring or ringing in the ears (tinnitus} is due to the fact 

 that either the vascular noises in the ear are abnormally loud or the auditory 

 nerve is hyperesthetic. In this way is explained the tinnitus following large doses 

 of quinin or salicylates in consequence of vasomotor influences upon the laby- 

 rinthine vessels, which may increase to the degree of causing rupture of a vessel. 

 Frequently, in the presence of roaring in the ears, the reaction is increased upon 

 applying the galvanic current. Less commonly there is a so-called paradoxical 

 reaction, that is, upon applying the galvanic current to one ear there appears, 

 in addition to the reaction in this ear, the opposite in the ear through which the 

 current is not passed. This phenomenon can be explained in the sense of trans- 

 ference. In other cases of lesions of the auditory nerve noises rather than musical 

 notes can be excited by the current. In addition, various deviations from the 

 formula of Brenner have been observed, and even complete reversal of this formula. 

 Excitation, particularly of the cortical center of the auditory nerve, especially in 

 the insane, may cause auditory hallucinations. If the irritability of the auditory 

 nerve is diminished or even destroyed, nervous impairment of hearing (hypac^^sis) 

 and nervous deafness (anacusis) develop. Often disease of one ear is attended 

 by a compensatory relation to the other. 



The Semicircular Canals of the Labyrinth. After division of the canals, espe- 

 cially if bilateral, marked disorders of equilibrium appear. The oscillating 

 movement of the head in the direction of the plane of the injured canal is charac- 

 teristic. If the horizontal canal is divided the head (of the pigeon) is rotated 

 alternately to the right and the left. The rotation appears chiefly when the 

 animal attempts to make movements; during rest, the movements cease. The 

 phenomenon may persist for months. Injury to the posterior vertical canal 

 causes marked upward and downward nodding movements, in connection with 

 which the animal occasionally falls forward or backward. Injury, finally, of the 

 upper vertical canal causes likewise oscillatory vertical movements of the head, 

 frequently with falling forward. Destruction of all of the canals is frequently 

 followed by various oscillatory movements of the head that often render standing 

 impossible. Breuer observed on mechanical, thermic and electrical irritation of 

 the canals analogous rotation of the head. On applying salt-solution with a 

 brush to the exposed canals Landois likewise observed the oscillatory movements 

 described, which occasionally disappeared after having persisted for some time. 

 The instillation of a 25 per cent, solution of chloral into a rabbit's ear will in 

 the course of fifteen minutes have an effect similar to that due to destruction of the 

 canals. Division of the auditory nerves in the skull has the same effect. 



Goltz considers the canals as the sensory mechanism for maintaining the 

 equilibrium of the head. Mach considered them as a mechanism for appreciating 

 the movements of the head. According to Goltz, the endolymph, with every 

 position of the head, exercises a maximum degree of pressure upon a given portion 

 of the semicircular canals, and in this way stimulates the nerve-terminations in the 

 ampullae in varying degree. According to Breuer there ocdur in the semicircular 

 canals on rotation of the head currents in the endolymph that stand in fixed rela- 

 tions to the direction and extent of the movement of the head, and which, there- 

 fore, if perceived constitute a delicate means for estimating the movement of the 

 head. The nervous end-organs of the ampullae are adapted to execute this per- 

 ception. If, therefore, the semicircular canals act as a mechanism, in a measure 

 as a static sense-organ, for the sense of equilibrium, the appreciation of the 

 position or of the movements of the head, their destruction or irritation will 

 modify these perceptions and thus give rise to abnormal oscillations of the head. 

 Breuer, as a result of his experiments, reaches the conclusion that the labyrinth 

 is intended as a means of orientation in space, and, particularly, that the semi- 



