604 THE SEMICIRCULAR CANALS. 



Pathological. Increased sensibility of the auditory nerve in any part of its course, its centre, 

 or iwripheral expansions causes the condition known as hyperakusis, which usually is a sign of 



Sreatly increased nervous excitability, as in hysteria. When excessive, it may give rise to 

 istinctly painful impressions, which condition is known as acoustic hyperalgia (Eulenburg). 

 Stimulation of the parts above-named causes sensations of sound, the most common being the 

 sensation of singing in the ears, or tinnitus. This condition is often due to changes in the 

 amount of blood in the blood-vessels of the ear either amemic or hyperaemic stimulation. 

 There is well-marked tinnitus after large doses of quinine or salicin, due to the vaso-motor 

 effect of these drugs upon the vessels of the labyrinth {Kirchner). Not unfrequently, in cases 

 of tinnitus, the reaction due to the galvanic current is often increased. More rarely there is 

 the so-called "paradoxical reaction" i.e., on applying the galvanic current to one ear, in 

 addition to the reaction in this ear, there is the opposite result in the non-stimulated ear. In 

 other cases of disease of the auditory nerve, noises rather than musical notes are produced by 

 the current ; stimulation, especially of the cortical centre of the auditory nerve, chiefly in 

 lunatics, may cause auditory delusions ( 378, IV.). According as the excitability of the 

 auditory nerve is diminished or abolished, there is the condition of nervous hardness of hearing 

 hypakusis), or nervous deafness (anakusiB). 



The Semicircular Canals of the Labyrinth. Section or injury to these canals 

 does not interfere with hearing, but other important symptoms follow their injury, 

 such as disturbances of equilibrium due to a feeling of giddiness, especially when 

 the injury is bilateral (Flourens). This does not occur in fishes (Kieselbach). The 

 pendulum-like movement of the head, in the direction of the plane of the injured 

 canal, is very characteristic. If the horizontal canal be divided, the head (of the 

 pigeon) is turned alternately to the right and left. The rotation takes place, especially 

 when the animal is about to execute a movement : when it is at rest, the movement 

 is less pronounced. The phenomenon may last for months, and injury to the 

 posterior vertical canals causes a well-marked up and down movement or nodding of 

 the head, the animal itself not unfrequently falling forwards or backwards. Injury 

 to the superior vertical canals also causes pendulum-like vertical movements of the 

 head, while the animal often falls forwards. When all the canals are destroyed, 

 various pendulum-like movements are performed, while standing is often impossible. 

 Breuer found that electrical stimulation of the canals caused rotation of the head, 

 while Landois, on applying a solution of salt to the canals, observed pendulum- 

 like movements, which, however, disappeared after a time. A 25 per cent, solu- 

 tion of chloral dropped into the ear of a rabbit causes, after fifteen minutes, a 

 similar destruction of the canals ( Vulpian). Section of the acoustic nerves within 

 the cranium has the same result (Bechterew). 



Explanation. Goltz regards the canals as organs of sense for ascertaining the equilibrium or 

 position of the head in space ; Mach, as an organ for ascertaining the movements of the head. 

 According to Goltz's statical theory, every position of the head causes the endolymph to exert 

 the greatest pressure upon a certain part of the canals, and thus excites in a varying degree the 

 nerve-terminations in the ampulla?. According to Breuer, when the head is rotated, currents 

 are produced in the endolymph of the canals, which must have a fixed relation to the direction 

 and extent of the movements of the head, and these currents, therefore, when they are perceived, 

 afford a means of determining the movement of the head. The nervous end-organs of the 

 ampulla* are arranged for ascertaining this perception. If the semicircular canals are an appa- 

 ratus in fact, " sense-organs " for the sensation of the equilibrium, and if their function is 

 to determine the position or movements of the head, necessarily their destruction or stimulation 

 must alter these perceptions, and so give rise to abnormal movements of the head. Vulpian 

 regards the rotation of the head as due to strong auditory perceptions (?) in consequence of 

 affections of the canals. Bottcher, Tomaszewicz, and Baginslcy regard the injury to the cere- 

 bellum as the cause of the phenomena. The pendulum-like movements, however, are so 

 characteristic that they cannot be confounded with disturbances of the equilibrium which result 

 from injury to the cerebellum. 



[Kinetic Theory. In 1875 Crum Brown pointed out that, if a person be rotated passively, 

 his eyes being bandaged, he can, up to a certain point, indicate pretty accurately the amount 

 of movement, but after a time, this cannot be done, and if the rotation, as on a potter's wheel, 

 be stopped, the sense of rotation continues. Crum Brown suggested that currents were produced 

 1 J the endolymph, while the terminal hair-cells lagged behind, and were, in fact, dragged through 

 t v! e it He .P mted out that tne ri ght posterior canal is in line with the left superior, and 

 the left posterior with the right superior, a fact which is readily observed by looking from 



