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chain of ossicles in man, does actually vibrate as a whole, and at the same rate as the membrane of the drum, when aërial vibrations strike upon the latter.
26. Thus, there is reason to believe that when the tympanic membrane is set vibrating, it causes the process of the malleus, which is fixed to it, to swing at the same rate ; the head of the malleus consequently turns through a small arc on its pivot, the slender process. But the turning of the head of the malleus involves that of the head of the incus upon its pivot, the short process. In consequence the long process of the incus swings through an arc which has been estimated as being equal to about two-thirds of that described by the handle of the malleus. The extent of the push is thereby somewhat diminished, but the force of the push is proportionately increased ; in so confined a space this change is advantageous. The long process, however, is so fixed to the stapes that it cannot vibrate without, to a corresponding extent and at the same rate, pulling this out of, and pushing it into, the fenestra ova.is. But every pull and push imparts a corresponding set of shakes to the perilymph, which fills the bony labyrinth and cochlea, external to the membranous labyrinth and scala media. These shakes are communicated to the endolymph and fluid of the scala media, and, by the help of the otolithes and the fibres of Corti, are finally converted into impulses, which act as irritants of the ends of the vestibular and cochlear divisions of the auditory nerve.
27. The difference between the functions of the membranous labyrinth (to which the vestibular nerve is distributed) and those of the cochlea are not quite certainly made out, but the following view's have been suggested :-
The membranous labyrinth may be regarded as an apparatus whereby sounds are appreciated and distinguished according to their intensity or quantity ; but which does not afford any means of discriminating their qualities. The vestibular nerve tells us that sounds are weak or loud, but gives us no impression of tone, or melody, or harmony.
The cochlea, on the other hand, it is supposed, enables the mind to discriminate the quality rather than the quantity or intensity of sound. It is suggested that the excitement of any single filament of the cochlear nerve gives rise, in the mind, to a distinct musical impression ; and that every fraction of a tone which a well-trained ear is capable of distinguishing is represented by its separate nerve-fibre. Under this view the scala media resembles a key-board, in function, as well as in appearance, the fibres of Corti being the keys, and the ends of the nerves representing the strings which the keys strike.
If it were possible to irritate each of these nerve-fibres experimentally, we should be able to produce any musical tone, at will, in the sensorium of the person experimented upon, just as any note on a piano is produced by striking the appropriate key.
28. A tuning-fork may be set vibrating, if its own particular note, or one harmonic with it, be sounded in its neighbourhood. In other words, it will vibrate under the influence of a particular set of vibrations, and no others. If the vibrating ends of the tuning-fork were so arranged as to impinge upon a nerve, their repeated minute blows would at once excite this nerve.
Suppose that of a set of tuning-forks, tuned to every note and distinguishing fractions of a note in the scale, one were thus connected with the end of every fibre of the cochlear nerve; then any vibration communicated to the perilymph would affect the tuning-fork which could vibrate with it, while the rest would be absolutely, or relatively, indifferent to that vibration. In other words, the vibration would give rise to the sensation of one particular tone, and no other, and every musical interval would be represented by a distinct impression on the sensorium.
29. It is suggested that the fibres of Corti are competent to perform the function of such tuning-forks ; that each of them is set vibrating to its full strength by a particular kind of wave sent through the perilymph, and by no other ; and that each affects a particular fibre of the cochlear nerve only. But it must be remembered that the view here given is a suggestion only which, however probable, has not yet been proved. Indeed recent inquiries have rather diminished than increased its probability.
The fibres of the cochlear nerve may be excited by internal causes, such as the varying pressure of the blood and the like : and in some persons such internal influences do give rise to veritable musical spectra, sometimes of a very intense character. But, for the appreciation of music produced external to us, we depend upon the intermediation of the scala media and its Cortian fibres.
30. It has already been explained that the stapedius and tensor tympani muscles are competent to tighten the membrane of the fenestra ovalis and that of the tympanum, and it is probable that they come into action when the sonorous impulses are too violent, and would produce too extensive vibrations of these membranes. They therefore tend to moderate the effect of intense sound, in much the same way that, as we shall find, the contraction of the circular fibres of the iris tends to moderate the effect of intense light in the eye.
The function of the Eustachian tube is, probably, to keep the air in the tympanum, or on the inner side of the tympanic membrane, of about the same tension as that on the outer side, which could not always be the case if the tympanum were a closed cavity.
THE ORGAN OF SIGHT.
1. IN studying the organ of the sense of sight, the eye, it is needful to become acquainted, firstly, with the structure and properties of the sensory expansion in which the optic nerve, or nerve of sight, terminates ; secondly, with the physical agent of the sensation; thirdly, with the intermediate apparatus by which the physical agent is assisted in acting upon the nervous expansion.
The ball, or globe, of the eye is a globular body, moving freely in a chamber, the orbit, which is furnished to it by the skull. The optic nerve, the root of which is in the brain, leaves the skull by a hole at the back of the orbit, and enters the back of the globe of the eye, not in the iniddle, but on the inner, or nasal, side of the centre. Having pierced the wall of the globe, it spreads out into a very delicate membrane, varying in thickness from etth of an inch to less than half that amount, which lines the hinder two-thirds of the globe, and is termed the retina. This retina is the only organ connected with sensory nervous fibres which can be affected, by any agent, in such a manner as to give rise to the sensation of light.
2. If the globe of the eye be cut in two, transversely, so as to divide it into a i anterior and a posterior half, the retina will be seen lining the whole of the concave wall of the posterior half as a membrane of great delicacy, and, for the most part, of even texture and smooth surface. But, exactly opposite the middle of the posterior wall, it presents a slight circular depression of a yellowish hue,
Diagrammatic views of the nervous (A) and the connective (B) elements of the retina, supposed to be separated from one another. A, the nervous structures-6, the rods; C, the cones; b c', the granules of the outer layer, with which these are connected; dd, interwoven very delicate nervous fibres, from which fine nervous filaments, bearing the inner granules, ff', proceed towards the front surface ; 88, the continuation of these fine nerves, which become convoluted and interwoven with the processes of the ganglionic corpuscles, h h'; ii, the expansion of the fibres of the optic nerve. Ď, the connective tissue-aa, external or posterior limiting membrane ; e e, radial fibres passing to the internal or anterior limiting membrane ; d'é, nuclei; dd, the intergranular layer ; 88, the molecular layer ; 1, the anterior limiting membrane.
(Magnified about 250 diameters.)