78 STRUCTURE OF THE HUMAN EAR. fied by being conveyed from the tympanic membrane to one which is twenty times smaller. Behind the B D B E Fig. 46,-Diagram of human ear (after Bernstein). D, Auditory canal; E, mouth of Eustachian tube; cc, tympanic membrane; B, tympanic cavity; o, fenestra ovalis ; ", fenestra rotunda; s, semicircular canals; A, cochlea. k Am. Am. k Am. I fenestra ovalis is the labyrinth, which is filled with fluid, and on which the final filaments of the auditory nerve are distributed. This fluid is thrown into vibrations by those of the stirrup, but as it is enclosed in a bony case, the vibrations would be greatly curtailed if it were not for the second membrane, or fenestra rotunda. This round membrane, therefore, acts as a counter opening, for if the fluid is St S Fig. 47.-Ossicles of the ear. H, Hammer; Am, anvil; Am. k, shorter process of the anvil; Am. 1, longer process of the anvil; S, stirrup; St, long process of the hammer. compressed in one place, it must claim more room in another. The labyrinth consists mainly of two parts, STRUCTURE OF THE HUMAN EAR. 79 the cochlea and the semicircular canals. The semicircular canals are three in number, and stand at right angles to one another. No satisfactory explanation of their function has yet been given; but there is some evidence that, in addition to, or apart from, hearing, they are affected by the position of the head, and thus serve as organs for maintaining the equilibrium of the body. Each of the canals commences with an oval dilatation, or ampulla. In the ampulla is a projecting ridge, on which are long, stiff, delicate, hair-like processes, the vibrations of which probably give certain sound-sensa tions. In the canals certain parts bear shorter hairs, over which are minute earstones, or otolithes, consisting of carbonate of lime, embedded in a gelatinous substance. The cochlea contains, moreover, a compli Fig. 48.-Section through the ampulla (after Bernstein). N, Nerve; z, terminal cells; h, auditory hairs. cated and wonderful organ, discovered by Count Corti. This appears to be, in fact, a microscopic musical instrument, composed of some four thousand complex arches, increasing regularly in length and diminishing in height from the base to the summit of the cochlea. The waves of sound have been supposed to play on this organ, almost like the fingers of a performer on the keys of a musical instrument. Fig. 49.-Tympanal wall of the ductus cochlearis, from the dog. Surface view from the side of the scala vestibuli, after the removal of Reissner's membrane, 399. I. Zona denticulata Corti. II. Zona pectinata Todd-Bowman: 1, Habenula sulcata Corti; 2, Habenula denticulata Corti; 3, Habenula perforata Kölliker. III. Organ of Corti: a, portion of the lamina spiralis ossea (the epithelium is wanting); b and c, periosteal blood-vessels; d, line of attachment of Reissner's membrane; e and e,, epithelium of the crista spiralis; f, auditory teeth, with the interdental furrows; 9, 9, large-celled (swollen) epithelium of the sulcus spiralis internus, over a certain extent shining through the auditory teeth; from the left side of the preparation they have been removed; h, smaller epithelial cells near the inner slope of the organ of Corti; k, openings through which the nerves pass; i, inner hair cells; 1, inner pillars; m, their heads; o, outer pillars; n, their heads; p, lamina reticularis; q, a few mutilated outer hair cells; r, outer epithelium of the ductus cochlearis (Claudius's cells of the author's); removed at s in order to show the points of attachment of the outer hair cells. After Waldeyer, in Stricker's Manual of Histology." MODE OF ACTION OF AUDITORY ORGANS. 81 The fibres of Corti, according to Helmholtz, may be distributed among the seven octaves which are in general use, so that there will be 333 fibres to every semitone, and 400 to each octave. Weber has estimated that a skilful ear can perceive a difference even of the of a tone, or nearly four thousand sounds, and 64 this would agree fairly well with the number of fibres. But why, it may be asked, should a given musical sound act more on one of these "keys" than another ? If several tuning-forks which sound different notes are placed on a table, and another in vibration be brought near them, the one sounding the same note is thrown into vibration, while the others are unaffected. A second tuning-fork would affect its own fellow, but no other, and so on.* A very slight change in the tuning-fork, such, for instance, as would be made by fastening a piece of wax to one of the prongs, is sufficient to destroy the sympathetic vibrations. The sound of the human voice has been known to break a bell-shaped glass by the agitation thus caused. The difficulty is to hit the pitch with sufficient precision, and retain the tone long enough. It is probable, therefore, that each of Corti's arches is set for a particular sound, and sensitive to it alone. This suggestion derives additional probability from the observations of Hensen (see p. 93) on the auditory hairs of Crustacea. We thus obtain a glimpse, though but a glimpse, of the manner in which the arches of Corti may possibly act. There are many problems still to be solved, but it is at least easy to see that so complex an organ may be capable of conveying very complex sensations. *Helmholtz, "Sensations of Tone." G 82 ORGANS OF HEARING IN THE LOWER ANIMALS. ON THE ORGANS OF HEARING IN THE LOWER ANIMALS. The semicircular canals in the human ear (see p. 79) have been supposed by some, in addition to, or apart from, their functions as organs of hearing, to assist in maintaining the equilibrium of the body; at all events, when they are injured, the movements frequently become disorderly, and the otolithic organs of the lower animals appear, at any rate in certain cases, to perform a similar function.* Otolithes, as we have seen, are present in our own ears, but they play a much more important part in those of the lower animals. In the lowest, the soundwaves may be considered to produce a certain effect upon the general tissues. The soft parts of the body are, however, not well calculated to receive such impressions. Their effect would be heightened by the presence of any solid structures, whether spicules, as in sponges, etc., or solid hairs projecting from the general surface, as in a great many of the lower animals. The Medusa (jelly-fishes, Fig. 50) present round the edge of the umbrella certain "marginal bodies," with reference to which there have been great differences of opinion. O. F. Müller, by whom they were discovered, regarded them as orifices for the exclusion of digested food, Rosenthal and Escholtz considered them to be glands, Milne Edwards as ovaries; but it seems now clearly established that some are organs of hearing, * Delage, "Sur une fonction nouvelle des Otocystes," Arch. d. Zool. Exp., 1887. Engelmann, "Ueber d. Function der Otolithen,” Zool. Anz., 1887. |