seem to predispose to some extent to this so-called boilermakers' disease of the labyrinth. But, on the other hand, it has been noticed that whenever more advanced, one-sided middle-ear lesions interfere with sound-conduction, the ear of that side does not suffer so much in the end as its mate. The labyrinth is shown to be the seat of boilermakers' deafness by the loss along the upper end of the auditory scale and the impaired bone-conduction. Tinnitus and vertigo are not usually complained of. It is not quite settled whether the use of firm cotton plugs in the ear affords protection against the influence of such loud dins. 406. Drug action has in rare instances injured the labyrinth. Large doses of quinin produce a characteristic roaring, which ordinarily ceases within less than twentyfour hours. There are on record, however, a number of instances of more or less complete permanent deafness, always bilateral, traceable to excessive quantities of quinin. Experiments on animals have shown that poisonous doses may cause effusion of blood into the labyrinth. The same has been found true of salicylate of sodium. Clinically, however, the latter has very rarely done any permanent damage to the hearing. Yet in rare instances patients claim that a moderate tinnitus, due to middle-ear disease, has been permanently intensified by large doses of salicylate of sodium. 407. Labyrinthine involvement, mainly confined to the auditory nerve-ends (partial deafness and tinnitus), is observed in rare instances in the course of various infectious diseases, such as typhoid, typhus, pernicious anemia, etc. There are, however, some forms of systemic disease in which the labyrinth suffers with greater frequency. In leukocythemia complete or incomplete attacks of Ménière's disease have been observed repeatedly. Mumps is known to localize itself at times in the internal ear, resulting in complete deafness, from which very few recoveries have been recorded. This metastasis, one-sided oftener than bilateral, occurs rarely at the beginning, oftener during the course or after the termination of mumps. There may be with it some pain and considerable tinnitus. 408. Syphilis is accused often as a cause of labyrinthine disease, but according to personal experience, as well as published reports in literature, the localization in the labyrinth is really very rare in the acquired form of the disease. It is apparently more frequent in congenital syphilis, occurring in females more than in males, and usually between the ages of eight to twenty years. Congenital syphilis localizing itself in the labyrinth coincides often with or follows syphilitic keratitis, and the subjects generally show the malformation of the upper incisor teeth known as Hutchinson's teeth. In the acquired form a few casual observations have shown the labyrinth filled with an inflammatory non-purulent exudate, presumably preceded by syphilitic disease of the blood-vessels. Autopsies at a later stage have shown osseous transformation of the labyrinth with disappearance of the nerve-fibers. The labyrinthine affection in both the acquired and the congenital forms may assume the apoplectic type, or may in an acute manner destroy rapidly the hearing without vertigo. In other instances the affection has been of a slower character and sometimes did not proceed to complete deafness. As a rule, it is double-sided. Specific treatment has had no positive effect in most instances. In the slower forms it is not even possible to arrest the disease with certainty. Sometimes, however, moderate recovery has been observed, which by some surgeons has been attributed to the use of pilocarpin in addition to specific treatment. 409. A typical labyrinthine affection is a frequent unfortunate outcome of cerebrospinal meningitis. percentage varies somewhat with the type of the disease. After severe epidemics of cerebrospinal meningitis its victims constitute a large part of the deaf-mutes of the locality. The disease, as well as its labyrinthine complication, occurs principally in children. Its beginning Its is usually overlooked on account of the severity of the primary disease and the somnolence or coma so frequently present. As the patient recovers, he finds himself completely deaf and generally distressingly dizzy. The vertigo improves, the staggering gait becomes steadier, after a while incoordination occurs only under trying circumstances, but the deafness remains. Postmortems have shown the lesion to be a suppurative inflammation extending along the auditory nerve into the labyrinth and destroying both the trunk of the nerve and its ends. A primary inflammation of the labyrinth with fever, headache, dizziness, and deafness has been described by Voltolini in children. This affection, however, is now generally believed to be an abortive form of cerebrospinal meningitis. CHAPTER XLVII. DISEASES OF THE AUDITORY NERVE.—DEAF MUTISM. 410. Anatomy of the Auditory Nerve.-The eighth cranial nerve pursues a short course from its emergence at the side of the medulla oblongata underneath the pons to the internal meatus in the middle of the posterior surface of the petrous pyramid. It is joined by the seventh or facial nerve in its course, which leaves it in the depth of the internal meatus to pass over the vestibule into the Fallopian canal. The auditory nerve consists of two branches really representing separate nerves, the cochlear branch or posterior lateral root, and the vestibular nerve, or anterior median root. The independence of these two branches is shown both by their separate peripheral distribution and their separate central course. Besides, the vestibular nerve receives its medullary investment earlier during embryonic life than the cochlear nerve. The cochlear nerve-fibers pass through the spiral ganglion in the interior of the modiolus. The first neuron of this nerve consists of the fibers in the lamina spiralis, peripheral to the spiral ganglion. The second neuron begins in the latter and terminates in the anterior or ventral and posterior or dorsal auditory nuclei in the medulla. Thence the further path is by means of fibers in the trapezoid body and striæ acusticæ, which, decussating, pass forward in the lateral inferior fillet (lemniscus) to the posterior corpora quadrigemina. Through the subthalamic region and posterior part of the internal capsule they finally reach the cortex of the temporal lobe, in which they terminate. The vestibular nerve is interrupted by a ganglion in the internal meatus, whence its fibers enter the vestibule and ampullæ. The nerve ends in a dorsal nucleus in the floor of the fourth ventricle. Its subsequent connections have been traced into the cerebellum, but are not yet completely known. 411. Affections of the auditory nerve central to the labyrinth are quite rare and are oftener seen by neurologists than by otologists. As in labyrinthine disease, the deafness is characterized by impairment of bone-conduction as well as air-conduction, but, unlike the latter, it is stated that the perception of the highest notes does not suffer first. Tinnitus is, as a rule, not so pronounced as in labyrinthine disease or is even absent. The deafness, too, is not necessarily-indeed, rarely-associated with vertigo. The diagnosis, however, must be based largely on other concomitant nervous symptoms. The auditory nerve trunk suffers from degeneration in a small proportion of patients with tabes. The atrophy is of the same character as the lesion of the optic nerve, which latter, however, is much more common. It is not certain whether there ever occurs a true neuritis of the auditory nerve, except as a complication of adjoining inflammatory processes. Deafness, sometimes one-sided, does occur when basilar disease, localized meningitis, or especially syphilitic disease involves the auditory nerve, but all this is very rare. The intracerebral nerve path may suffer from hemorrhagic effusions, tumors, and abscesses. On the basis of the involvement of the auditory nerve, a localizing diagnosis cannot be made, since its symptoms have been observed in consequence of disease of the cerebellum, as well as of the corpora quadrigemina, subthalamic region, internal capsule, and temporal lobe. When the process is distinctly one-sided, it is the opposite ear which becomes deaf. This has a special bearing in the case of abscesses of otitic origin situated in the temporal lobe. Besides actual deafness disease of the temporal lobe may result in what has been termed word-deafness, or amnesic aphasia. The patient hears, but does not understand the significance of the words. He can repeat the |