INTERNAL EAR, DISORDERS OF.— The percipient apparatus of the ear is relatively rarely affected and furnishes but 2 to 10 per cent. of the cases in the statistical tables,—the larger figure em bracing apparently every case which gives evidence of nerve-involvement, however secondary in fact and impor tance to tympanic trouble. It comprises the congenital defects as well as the cen tral lesions, such as nerve-atrophy in tabes, word-deafness from cortical lesion, and many other rare cerebral affections; but the group which most concerns us in this practical review is made up largely of lesions of the labyrinth due to the specific affections, including syphilis.
Tuning-fork Tests.—The diagnosis of these affections is largely from negative evidence, much of it furnished by the tuning-fork tests of the function: and these had better be here considered.
Tuning-forks can be conveniently used, giving tones due to vibrations of from 50 to 2000 per second, and much can be learned by use of A = 213 v. s. or C = 520 v. s. alone; but it is not best tc trust to any one tone. The lower forks must usually have clamps to dampen the overtones (such can be im provised by slipping bits of rubber-tub ing over the ends), and in the absence of such will often give the notes one or two octaves higher coincidentally with the fundamental. For this reason and for its convenient duration of vibration I prefer the A = 213 v. s., of medium size, more often found in the shops. Such a fork, struck upon some rather soft sur face by falling its own length, should generally be heard some 90 seconds through the air when held before the ear; while with its handle resting upon the mastoid or other portion of the skull or face it should be audible slightly less than half as long. It should be heard equally in each ear from points in the middle line of the head; and the sound waves should escape from each canal, as can be heard through the auscultation tube. Stopping the canal with the finger should increase the sound in the closed ear to a degree that extinguishes its per ception in the other and makes the sound again audible by bone-conduction after it has been lost normally. Low tones are
heard better relatively by bone; high tones by air; so high-pitched forks should have long handles if their use on the mastoid is to be free from possible fal lacy. Low-toned forks should be lightly struck to test bone-conduction, lest their vibration on the head should be oppres sively loud.
If we place the vibrating A-fork on one mastoid it should be heard for some 40 seconds, as stated, and for some 50 mere when transferred to the front of the canal; and each other fork has its fairly-definite proportion for a normal ear, equal on the two sides. But in deaf ears the finding will be different and dis crepant perhaps on the two sides. Lesion of the conducting-apparatus will impede alike the entrance of sound-waves by air and their escape from the tympanum when awakened there through bone-con duction. IIearing by air-conduction will be subnormal, by bone-conduction it will be exaggerated; the proportion changing from 90:40 to perhaps 30:50, bone-con duction preponderating. This is Rinne's or Schwabach's test,—modified by Roosa very practically by merely noting whether it is "louder front? or back?" as almost any patient can rightly decide.
If the deafness be due to the percipient apparatus, the normal preponderance of air-conduction will continue, bone-con duction being relatively worse, or, per haps, totally lost. The proportion may now be A. C. 40: B. C. 10. So, too, from the middle line of the head the hearing will be worse in the worse internal ear, whereas if the trouble be in the conduct ing apparatus the more obstructed ear will be the one hearing louder by bone conduction. This is Weber's test.
Gardiner Brown modified Weber's test by resting the tuning-fork on the bridge of the nose and having the patient raise his finger just when he ceased to hear its vibration. As this should be exactly when the vibrations ceased to be felt by the fingers of the examiner, a. rough, but practical, measure is gained (for each ear if unequal) of the increase or de crease of the bone-conduction, and the result is conveniently stated as + 3 sec onds, — 4", etc.