pressure is diminished by swallowing with the mouth and nose held shut (Toynbee's experiment).

The rhythmic movements transmitted to the foot-plate of the stapes by the chain of ossicles are continued onward to the membranous labyrinth. The stapes (see Fig. 14, Plate 9) as it is forced inward displaces the incompressible perilymph and sets up a current that passes from the vestibule up the scala vestibuli and down over the apex of the cochlea, through the helicotrema, flowing down the scala tympani and causing the membrane over the fenestra cochleæ to bulge in the same degree as the stapes is forced inward. At the same time that the current flows through the scala vestibuli the yielding basilar membrane and cochlear duct that it supports are forced out against the scala tympani and rub against the auditory hairs of the hair-cells in Corti's membrane. The stimulus thus conveyed to the fibers of the auditory nerve is conducted for the most part to the temporal lobe of the opposite side, with the cooperation of association tracts in other portions of the cortex, as, for example, the visual sphere in the occipital lobe, when the stimulus is interpreted as a sound perception (Gad).

The fibers of the basilar membrane, according to Helmholtz, represent a series of strings of different pitch, and every nerve-fiber that terminates on the basilar membrane produces a special note. Thus when the note C is produced, only that portion of the basilar membrane intended for the production of the tone C bulges and is thrown into vibration. Corresponding to the varying lengths of the basilar fibers the highest tones are produced by vibrations in the first turn; the deepest, in the apex of the cochlea. When a complex tone is produced, as many different portions of the basilar membrane as there are components in the sound are thrown into vibration; hence a practised ear is able to analyze a sound. If, for instance, the vowel e is sounded, the fibers b3, fi (b), which are tuned to produce its component parts, are chiefly thrown into vibration. Sounds

of from 16,000 to 50,000 vibrations fall within the limit of hearing (C, to e). From D, to h' constitute the musical range. Human speech lies between C, (sound of R) and c" (sound of S). The ability to hear a tone depends on two factors: vibration of the corresponding fiber of the basilar membrane and irritability of the corresponding nerve-fiber. If, for instance, the fibers b3, f', b, have been destroyed by some morbid process, the vowel sound

FIG. 18.-Normal auditory image in the basilar membrane (Helmholtz's hypothesis): Scale of musical tones from D' to h'. Scale necessary for speech perception, bl to g2.*

e cannot be heard. Speech perception is possible even after extensive destruction of the basilar membrane, providing the range from b' to g2 is capable of function (Bezold).

The irritability of the auditory nerve is subject to periodic variations, and when listening intently we note

*h in the German scale is equivalent to b English; b in the German scale is equivalent to b-flat English.

an alternate increase and decrease in the sounds. Once the auditory nerve has been excited, perception is at first intensified, but soon becomes fatigued if the irritation is continued for any length of time. Visual impressions (seeing colors), twitching of the ears, vertigo, and starting of the whole body may be produced reflexly through the auditory nerve. The subjective auditory sphere in binaural hearing lies in the head (Urbantschitsch). Irritation of one ear increases the perceptive power of the other. The maculæ and crista acustice are not affected by sound, but are by movements of the body, especially of the head, the movements being transmitted to the endolymph, which displaces either the otolith membrane on the hair-cells of the maculæ acustica, or the cupola of the crista acusticæ. The stimulus is then conducted through the vestibular nerve to the cerebellum, where, with the coöperation of the visual and tactile senses, the equilibrium is regulated by reflex alternations in the innervation of the muscles, and the position of the head is ascertained. Rectilinear movements are regulated by the maculæ acustica; rotary movements, by the crista acustica (Mach, Breuer).

Ordinarily the maintenance of equilibrium is an unconscious act and we become aware of it only when, owing to some disturbance, the crista and macula acusticæ are irritated. Thus, lesion of the external semicircular canal is followed by pendulum movements of the head from side to side; in lesion of the superior canal the head moves backward and forward and tends to fall forward. In lesion of the posterior canal there are nodding movements and the head tends to fall backward (Flourens). If a stimulus ordinarily produced only by movement of the head excites the apparatus of equilibration without the head having moved, the false interpretation of the position of the head leads to vertigo and staggering gait. When a stimulus continues to act for a long time, as in protracted rotary movements, vertigo likewise results.

The vestibular apparatus is also connected by reflex paths with the visual centers, so that irritation of the

apparatus-as, for instance, in long-continued rotary movements-is followed by movements of the eyes. Irritation of the external semicircular canal is followed by horizontal nystagmus; of the posterior canal, by vertigo; of the superior canal, by diverging nystagmus (Cyon). If the vestibular apparatus is completely destroyed, the possibility of such stimulation is lost. In deaf-mutes, in whom the semicircular canals are often absent, rotary movements about the axis of the body not infrequently produce neither vertigo nor nystagmus. The vestibular apparatus of the two ears are entirely equivalent, so that one can replace the other. If both are completely destroyed, the equilibrium is regulated by means of the visual and tactile senses (Gad).

III. EXAMINATION.

THE object of the aural surgeon is the prevention, cure, or amelioration of diseases of the organ of hearing. Hence proper treatment can be instituted only after making the diagnosis. The diagnosis is based on the patient's statement (anamnesis) and the data obtained by examination.

Ordinary data, such as name, age, and social condition, may be obtained during the examination. Childhood and work in the open-air predispose to inflammatory disease of the internal ear; old age and occupation amid noisy surroundings (boiler-makers) predispose to diseases of the auditory nerve.

The duration and course of the aural trouble are next inquired into. The prognosis is more favorable if the aural disease is recent and the deafness sudden than when the onset has been insidious. In chronic conditions incorrect statements are often obtained, because the disease began during childhood, or because it did not at first give rise to any subjective disturbances. Much may be learned by inquiring what the patient's power of hearing was at school, or when he was in the army, etc.

The causes of the disease are next inquired into. The prognosis is more favorable in simple otitis media—such as follows a cold, for instance—than after general diseases, such as scarlet fever or tuberculosis; it is better in deafness following the use of drugs, such as salicylic acid and quinin, and intoxications, as by phosphorus, than in deafness due to injury or exposure to an intense sound impression, such as an explosion. Again, the prognosis is better in deafness due to nasal obstruction (mouth-breathing, dryness of the mouth on awakening,