EXTERNAL EAR-TYMPANIC CAVITY. 405 hollows and elevations on the surface of the external ear of Man are not very apparent; but it is probable that they direct the sonorous undulations towards the entrance of the canal which leads to the middle ear. The form of the external ear in many Quadrupeds evidently adapts it to this purpose; and there are several which have the power of changing its direction by muscular action, in such a manner as to enable it to catch most advantageously the faintest sounds from any quarter. This is especially the case with animals that are naturally timorous, such as the Hare or the Deer; these have usually very large external ears. But it is among the Bat tribe whose residence in the dark recesses of caverns and excavations makes their eyes of comparatively little use to them, and causes them to depend greatly for guidance in their movements upon the sense of hearing-that we find the greatest development of the external ear (fig. 201). 516. The canal hollowed-out in the temporal bone (d, fig. 204) into which the external ear collects the sonorous vibrations, passes inwards until it is terminated by a membrane stretched across it, which is called the membrana tympani, or membrane of the drum of the ear (g). This forms the outside wall of a cavity (h) which constitutes the middle ear, and which is bounded on the inside by a bony wall that separates it from the internal ear. The cavity of the tym panum is not one of the essential parts of the organ; for nothing analogous to it exists either in Fishes or in the lower Reptiles. It contains air; and communicates with the back 406 TYMPANIC CAVITY AND CHAIN OF BONES. of the nasal cavity (n, fig. 200) by a canal termed the Eustachian tube (k, fig. 204). The partial or complete closure of this tube, occasioned either by swelling of its lining membrane or by the viscid secretion from it, produces the slight deafness common among those who are suffering from "colds." Within the cavity of the tympanum, there is a very curious apparatus of small bones and muscles, which serves to establish a connexion between the membrane of the drum and the small membrane covering the entrance to the internal ear. These bones are four in number; and are termed the malleus or a h 9 d Fig. 202.-BONES OF abc0 Fig. 203.-CAVITY OF THE TYMPANUM, WITH hammer (a, fig. 202); the incus, or anvil (b); the os orbiculare, a minute globular bone (c); and the stapes, or stirrupbone (d). These bones are connected together in the manner represented in fig. 203; where a a represents the wall of the tympanic cavity; b, the membrana tympani; c, one of the long processes of the malleus, which is attached to the membrane; d, the head of the malleus, which articulates with the incus; e, the other long process of the malleus, which is acted-on by the minute muscle f, that serves to tighten the tympanum; g, the incus, of which one leg is in contact with the wall of the cavity, whilst the other is connected with the orbicular bone h; i, the stapes, of which the upper end is connected with the orbicular bone, whilst the lower (which is of an oval form) is attached to the membrane that covers the entrance to the internal ear; and k is a small muscle which 'ACTION OF THE TYMPANIC APPARATUS. 407 acts upon this bone in such a manner as to relax the tympanum. 517. The use of this apparatus is evidently to receive the sonorous vibrations from the air, and to transmit them to the membrane forming the entrance to the internal or essential part of the organ of hearing; in such a manner, that the sonorous vibrations excited in the latter may be much more powerful than they would be if the air acted immediately upon it. The usual state of the membrane of the tympanum appears to be rather lax or slack; and when in this condition, Fig. 204.-VERTICAL SECTION OF THE ORGAN OF HEARING IN MAN. The internal portions are proportionately enlarged to make them more evident: a, b, c, the external ear; d, entrance to the auditory canal f; e, e, petrous portion of the temporal bone, in which the internal ear is excavated; g, membrane of the tympanum; h, cavity of the tympanum, the chain of bones being removed; i, openings from the cavity into the cells j excavated in the bone; on the side opposite the membrana tympani are seen the fenestra ovalis and rotunda; k, Eustachian tube; 7, vestibule; m, semicircular canals; n, cochlea; o, auditory nerve; P, canal by which the carotid artery enters the skull; q, part of the glenoid fossa which receives the head of the lower jaw; r, styloid process of the temporal bone. it vibrates in accordance with grave or deep tones. By the action of a small muscle lodged within the Eustachian tube, it 408 TYMPANIC APPARATUS :-INTERNAL EAR. may be tightened, so as to vibrate in accordance with sharper or higher tones; but it will then be less able to receive the impressions of deeper sounds. This state we may artificially produce either by holding the breath and forcing air into the Eustachian tube, so as to make the membrane bulge-out by pressure from within; or by exhausting the cavity by an effort at inspiration with the mouth and nostrils closed, which will cause the membrane to be pressed inwards by the external air. In either case the hearing is immediately found to be imperfect; but it will be observed that while the experimenter thus renders himself deaf to grave sounds, acute sounds are heard even more distinctly than before. There is a different limit to the acuteness of the sounds of which the ear can naturally take cognizance, in different persons. If the sound be so acute (or high in pitch) that the membrana tympani will not vibrate in unison with it, the individual will not hear it, although it may be loud; and it has been noticed that some persons cannot hear the very shrill tones produced by particular Insects, or even by Birds, which are distinctly audible to others. There is good reason to think that the two muscles which have been mentioned (§ 516) as tightening and relaxing the tympanum, exert a regulative influence upon its tension analogous to that which the contractile fibres of the iris possess in regard to the diameter of the pupil (§ 534); preparing it to be acted-on by faint sonorous undulations when we are listening, and moderating the concussion of very loud sounds which are anticipated. 518. The internal ear is composed of various cavities that communicate with each other; of these the vestibule (l, fig. 204) may be regarded as the centre, whilst from it there pass-off on one side the three semicircular canals, m, and on the other the cochlea, n. The vestibule is the part which corresponds with the simple cavity that constitutes the whole organ of hearing in the lower animals (§ 512), and the others may be regarded as extensions of it for particular purposes. It communicates with the cavity of the tympanum by a small orifice in the bony wall that separates them, termed the fenestra ovalis (oval window); but this orifice is closed by a membrane, to which the lower end of the stapes is attached. The three semicircular canals are passages excavated in the solid bone, and lined by a continuation of the same membrane as that STRUCTURE OF THE INTERNAL EAR. 409 which lines the vestibule; each passes-off from the vestibule and returns to it again. The cochlea, n, also is a cavity excavated in the hard bone, and lined by a continuation of the same membrane; its form is almost precisely that of the interior of a snail-shell (whence its name), being a spiral canal which makes about two turns and a half round a central pillar. This canal is divided into two, however, by a partition that runs along its whole length; which partition is partly formed by a very thin lamina of bone, and partly (in the living state) by a delicate membrane. The two passages do not communicate with each other except at the top or centre; at their lower end (corresponding to the mouth of the snail-shell) they terminate differently; for whilst one freely opens into the vestibule, the other communicates with the cavity of the tympanum, by an aperture termed the fenestra rotunda (round window), which is closed by a membrane.1 Thus the internal ear communicates with the cavity of the tympanum by two minute orifices only, -the fenestra ovalis and the fenestra rotunda, both of them closed by membranes, against the former of which the stapes abuts, whilst the latter is free. 519. The whole internal ear is lined by a delicate membrane, on which the auditory nerve (o, fig. 204) is very minutely distributed, especially on the membranous portion of the partition between the two passages of the cochlea. The cavities are completely filled with fluid, which is set in vibration by the movements of the stapes, communicated through the membrane of the fenestra ovalis; and these vibrations are probably rendered more free by the existence of the second aperture-the fenestra rotunda. It is by the influence. of these undulations upon the expanded fibrils of the auditory nerve, that the sensation of sound is produced; but in what way the different parts of the labyrinth (as this complex series of cavities is not unaptly called) contribute to the performance of this function, is not yet known. In all Fishes but the lowest, the three semicircular canals exist; they have, however, no vestige of a cochlea. In the true Reptiles, a rudiment of the cochlea may be generally discovered. In Birds, this cavity is more completely formed, though the passage is not spiral, but is nearly straight; of its real 1 There is a double spiral staircase constructed exactly on this plan in Tamworth church. |