Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/616

594 594 EAR in. the longitudinal direction, but different segments may be stretched radially with different degrees of tension so as to resemble a series of tense strings of gradually increasing length. Each string would then respond to a vibration of a particular pitch communicated to it by the hair-cells. The exact mechanism of the hair-cells and of the membrana reticularis, which looks like a damping appa ratus, is unknown. II. Physiological Characters of Auditory Sensation. 1. Under ordinary circumstances auditory sensations are referred to the outer world. When we hear a sound, we associate it with some external cause, and it appears to originate in a particular place, or to come in a particular direction. This feeling of exteriority of sound seems to require transmission through, the membrana tympani. Sounds which are sent through the walls of the cranium, as when the head is immersed in, and the external auditory canals are filled with, water, appear to originate in the body itself. It is probable, however, that the external character of ordinary auditory sensations may be more the result of habit than due to any anatomical peculiarity of the ear itself. 2. An auditory sensation lasts a short time after the cessation of the exciting cause, so that a number of separate vibrations, each capable of exciting a distinct sensation if heard alone, may succeed each other so rapidly that they are fused into a single sensation. If we listen to the puffs of a syren, or to vibrating tongues of low pitch, the single sensation is usually produced by about 30 or 35 vibrations per second ; but there can be no doubt, as was first pointed out by Helmholtz, that when we listen to beats of consider able intensity, produced by two adjacent tones of sufficiently high pitch, the ear may follow as many as 132 intermis sions per second. 3. The sensibility of the ear for sounds of different pitch is not the same. It is more sensitive for acute than for grave sounds, and it is probable that the maximum degree of acuteness is for sounds produced by about 3000 vibra tions per second, that is near/a 5 |. Sensibility as to pitch varies much with the individual and with the training to which he has subjected himself. Thus some musicians may detect a difference of I0 ] 0o th of the total number of vibra tions, while other persons may have difficulty in appreciat ing a semitone. This power of appreciating differences of pitch is termed a correct or just ear, and there can be no doubt of its improvement by cultivation. 4. Hearing with two ears does not appear materially to influence auditive sensation, but probably the two organs are enabled, not only to correct each other s errors, but also to aid us in determining the locality from whence a sound originates. It is asserted by Fechner that one ear may perceive the same tone at a slightly higher pitch than the other, but this may probably be due to some slight patho logical condition in one ear. If two tones, produced by two tuning forks of equal pitch, are produced one near each ear, there is a uniform single sensation ; if one of the tuning forks be made to revolve round its axis in such a way that its tone increases and diminishes in intensity, neither fork is heard continuously, but both sound alternately, the fixed one being only audible when the re volving one is not. It is difficult to decide whether excita tions of corresponding elements in the two ears can be dis tinguished from each other. It is probable that the resulting sensations may be distinguished, provided one of the generating tones differs from the other in intensity or quality, although it may be the same in pitch. 5. Hitherto we have considered only the audition of a single sound, but it is possible also to have simultaneous auditive sensations, as in musical harmony. It is difficult to ascertain what is the limit beyond which distinct auditory sensations may be perceived. We have in listening to an orchestra a multiplicity of sensations which produces a total effect, while, at the same time, we can with ease single out and notice attentively the tones of one or two special instruments. Thus the pleasure of music may arise partly from listening to simultaneous, and partly from the effect of contrast or suggestion in passing through suc cessive, auditory sensations. The principles of harmony belong to the subject of music, but it is necessary here briefly to refer to these from the physiological point of view. If two musical sounds reach the ear at the same moment, an agreeable or disagreeable sensation is experienced, which may be termed a concord or a discord, and it can be shown by experiment with the syren (see ACOUSTICS) that this depends upon the vibra- tional numbers of the two tones. The octave (1:2), the twelfth (1:3), and double octave (1:4), are absolutely consonant sounds; the fifth (2:3) is said to be perfectly consonant ; then follow, in the direction of dissonance, the fourth (3: 4), major sixth (3: 5), major third (4:5), minor sixth (5:8), and the minor third (5:6). Helm holtz has attempted to account for this by the application of his theory of beats. Beats are observed when two sounds of nearly the same pitch are produced together, and the number of beats per second is equal to the difference of the number of vibrations of the two sounds. Beats give rise to a peculiarly disagreeable intermittent sensation, com parable to what is experienced on watching a flickering light, and the painful sensation may arise from intermittent irritation of the auditory nerve filaments. The maximum roughness of beats, according to Helmholtz, is attained by 33 per second; beyond 132 per second, the individual impulses are blended into one uniform auditory sensation. When two notes are sounded, say on a piano, not only may the first, fundamental, or prime tones beat, but partial tones of each of the primaries may beat also, and as the difference of pitch of two simultaneous sounds augments, the number of beats, both of prime tones and of harmonics, augments also. The physio logical effect of beats, though these may not be individually distin guishable, is to give roughness to the ear. If harmonics or partial tones of prime tones coincide, there are no beats ; if they do not coincide, the beats produced will give a character of roughness to the interval. Thus in the octave and twelfth, all the partial tones of the acute sound coincide with the partial tones of the grave sound ; in the fourth, major sixth, and major third, only two pairs of the partial tones coincide, while in the minor sixth, minor third, and minor seventh, only one pair of the harmonics coincide. For details, see Helmholtz, On Sensations of Tone as a Physiological Basis for the Theory of Music, translated by Alexander J. Ellis, London, 1875. DISEASES OP THE EAE. Deafness may arise irorn obstruc tion of the external ear occasioned by disease of various kinds ; from ulceration, thickening, or perforation of the membrana tympani ; from inflammatory affections, both acute and chronic, of the middle and internal ear ; from obstruction of the Eustachian tube caused by inflammation of its lining membrane, leading to thickening and accumu lation of mucus or pus ; from diseases of the throat blocking up the end of the Eustachian tube; and, lastly, from disease of the auditory nerve or of the terminal apparatus con nected with it in the membranous labyrinth. Otitis, or ear-ache, is an inflammation, usually of a rheumatic nature, of some portion of the external auditory canal. Most fre quently occurring in weakly individuals, it causes intense pain, which shoots over the head on the affected side. It may lead to the formation of a small abscess in one of the wax glands found in the passage. Hot applications by fomentations or warm poultices give relief, and if an abcess forms, it ought to be carefully lanced. Otorrhcca is a muco-purulent discharge, often of a foetid odour, from the ears of scrofulous children. It frequently occurs during teething, and it may be one of the sequelae of scarlet fever, or measles, or small-pox. When pus flows from the ear, it may come from the membrane lining the deeper portion of the external meatus, or from the middle ear by a hole in the membrana tympani, or from diseased portions of bone near the middle, or internal ear. The treatment, of course, varies according to the cause, but generally the discharge may be lessened in quantity, and at all events rendered less offensive, by the use of weak in-