THE INTERNAL EAR. S011ial is conducted to the labyrinth either by the external and middle ear, as described above (*air conduction') ; or through the bones of the head ('bone conduc tion') ; normally air conduction is better than bone eonduction. In diseases of the middle ear or in obstructions of the external auditory meatus, air conduction is not so good as hone conduction. .\dvantage is very frequently taken of this fact in the differential diagnosis between diseases of the internal ear and those of the rid dle or of the external ear. To test the air con duction a vibrating tuning-fork is held close to the ear: to test the hone conduction the handle of the tuning-fork is brought in contact with the teeth, forehead, mastoid process, etc., in turn. As stated. it is the base of the stapes that vont munieates the vibrations to the perilymph; this would he impossible but for the existence of the fenestra rotund«, which nets as a vent for the vibrations of the perilymph and endolymph; every time the membrane of the fencstra (walk bulges in. that of the fenrstra rotunda bulges out. and vice versa. The interim] ear also performs the most essential function connected with hear ing: that of perception of auditory impressions by means of the eoehlear and vestibular te•mina tions of the auditory nerve. It remains for us now to consider the function of the setnieircular canals. It is now a generally recognized fact that they are not connected with the sense of hearing. They have to do with the maintenance of the equilibrium of the body. This fact was established by E‘sald in To maintain equilibrium in the various positions of rest and especially in locomotion, the accurate co6rdina don of a great ninny muscles is necessary. l'he
cerebellum is the organ which sends nut the nerve-impulses to these muscles; this action is an unconscious one, being purely reflex in char acter. The afferent impulses which bring about these reflexes arc, besides others of secondary importance, the ones conveyed to the cerebelluto front the semicircular canals. When these canals are removed in animals, or when they are dis eased in man, as in Aleniere's disease. there are invariably profound disturbances of equilibrium, the subject being unable to stand or even to sit, without swaying or falling, to say nothing of such complicated acts as walking. jumping, etc.
The range of hearing, like that of vision. varies remarkably in different persons. Some persons are insensible to sounds which others can read ily hear. The ordinary range of human hearing includes, according to Wollaston, more than nine octaves. Technically stated, the normal ear per ceives vibrations as musical notes from 16 vibra tions per second to about 32,500. The sensation of sound, like that of light. frequently lasts longer than the exciting cause. We have fa miliar proof of this fact in the noise which re mains in the cars after a long journey in a coach or railway ear; and this fact was (dearly demon strated by So cart, who found, in his experiments on toothed wheels, that the removal of one tooth (lid not produce any interruption of the sound.