anterior (P.A.) and posterior (P.V.) vertical semicircular canals; while the third, lying outside, and horizontally, is termed the external horizontal semicircular canal (H). One end of each of these canals is dilated into what is called an ampulla (A).

It is upon the walls of these ampullæ and those of the vestibular sac that the branches of the auditory nerve are distributed.

In each ampulla the nervous filaments may be traced to a transverse ridge caused by a thickening of the connective tissue which forms the walls of the canal (as well as of all other parts of the membranous labyrinth), and also by a thickening of the epithelium. Some of the epithelium cells are here prolonged into the fine hair-like processes described above. It is probable that these cells are specially connected with the terminations of the nerve filaments.

In the vestibule are similar but less marked ridges, or patches; here, however, the hair-like prolongations of the epithelium cells are absent or scanty, but, instead, otolithes are found in the fluid.

The fluid which fills the cavities of the semicircular canals and utriculus is termed endolymph. That which separates these delicate structures from the bony chambers in which they are contained is the perilymph. Each of these fluids is little more than water.

17. In the scala media1 of the cochlea the primitive bag is drawn out into a long tube, which is coiled two and a half times on itself into a conical spiral, and lies in a much wider chamber of corresponding form, excavated in the petrous bone in such a way as to leave a central column of bony matter called the modiolus. The scala media has a triangular transverse section (Fig. 66), being bounded above and below by the membranous walls which converge internally and diverge externally. At their convergence, the walls are fastened to the edge of a thin plate of bone, the lamina spiralis (L.S. Fig. 66), which winds round the modiolus. At their divergence they are

The true

I employ this term as the equivalent of canalis cochlearis. nature and connections of these parts have only recently been properly worked out, and the account now given will be found to be somewhat different from that in the first edition of this work. See particularly the explanation of Fig. 67.

fixed to the wall of the containing bony chamber, which thus becomes divided into two passages, communicating at the summit of the spire, but elsewhere separate. These two passages are called respectively the scala tympani and scala vestibuli, and are filled with perilymph.

The scala media, which thus lies between the other two scalæ, opens below, or at the broad end of the cochlea, by a narrow, duct into the sacculus hemisphericus, but at its opposite end terminates blindly. (Fig. 70.)

FIG. 66.-A SECTION THROUGH THE AXIS OF THE COCHLEA, MAGNIFIED THREE DIAMETERS.

Sc.M. scala media; Sc. V. scala vestibuli; Sc. T. scala tympani; L.S. lamina spiralis; Md. bony axis, or modiolus, round which the scale are wound; C.N. cochlear nerve.

That branch of the auditory nerve which goes to supply the cochlea, enters the broad base of the central column or modiolus, and there divides into branches, which, spreading out in a spiral fashion in channels excavated in the bony tissue, are distributed to the lamina spiralis throughout its whole length. They do not end here; but in any section of the lamina spiralis (Fig. 66, L.S.) they may be found running outwards from the central column across the lamina towards the angle of the scala media, in which indeed they become finally lost.

The upper wall of the scala media, that which separates it from the scala vestibuli, is called the membrane of Reissner. The opposite or lower wall, which separates it from the scala tympani, is the basilar membrane. The latter is very elastic, and on it rest the fibres of Corti (C C', Fig. 67), each of which is composed of two filaments

a, That end of the lamina spiralis which passes into the inner wall, pillar, or modiolus of the bony cochlea; c, the outer wall of the bony cochlea; Sca. T. the cavity of the scala tympani; Sca. M. the cavity of the scala media; d, the elastic basilar membrane which separates the scala media from the scala tympani; V. a vessel which lies in this, cut through; e, the so-called mem

joined at an angle. An immense number of these filaments are set side by side, with great regularity, throughout the whole length of the scala media, so that this organ presents almost the appearance of a key-board, if viewed from either the scala vestibuli or the scala tympani. These fibres of Ccrti lie among a number of epithelium cells forming the lining of the scala media at this part, and those cells which are close to the fibres of Corti have a peculiarly modified form. The ends of the nerves have not yet been distinctly traced, but they probably come into close relation either with these fibres or with the modified epithelium cells lying close to them, which are capable of being agitated by the slightest impulse.

18. These essential parts of the organ of hearing are, we have seen, lodged in chambers of the petrous part of the temporal bone. Thus the membranous labyrinth is contained in a bony labyrinth of corresponding form, of which that part which lodges the sac is termed the vestibule, and those portions which contain the semicircular canals, the bony semicircular canals. And the scala media is contained in a spirally-coiled chamber, the cochlea, which it divides into two passages. Of these, one, the scala vestibuli, is so called because at the broad end or base of the cochlea it opens directly by a wide aperture into the vestibule; by this opening the perilymph which fills the vestibule and bony semicircular canals and surrounds the membranous labyrinth, is put in free communication with the perilymph which fills the scala vestibuli of the cochlea, and, by means of the communication which exists between the two scale at the summit of the spire, with that of the scala tympani also.

In the fresh state, this collection of chambers in the petrous bone is perfectly closed; but in the dry skull there are two wide openings, termed fenestræ, or windows, on its outer wall; i.e. on the side nearest the outside of the skull. Of these fenestræ, one, termed ovalis (the

brane of Corti; C C', the fibres of Corti; VII. the filaments of the auditory nerve. It is doubtful whether the membrane of Corti really has the extent and connections given to it in this figure, which must not be taken for more than a general representation of the disposition of the parts. The membrane of Reissner, which separates the scala media from the scala vestibuli, is not represented.

oval window), is situated in the wall of the vestibular cavity; the other, rotunda (the round window), behind and below this, is the open end of the scala tympani at the base of the spire of the cochlea. In the fresh state, each of these windows or fenestræ is closed by a fibrous membrane, continuous with the periosteum of the bone.

FIG. 68.-TRANSVERSE SECTION THROUGH THE SIDE WALLS OF THE SKULL TO SHOW THE PARTS Of Ear.

Co. Concha or external ear; E.M. external auditory meatus; Ty.M. tympanic membrane; Inc.Mall. incus and malleus; A.S.C., P.S.C., E.S.C. anterior, posterior, and external semicircular canals; Coc. cochlea; Eu. Eustachian tube; I.M. internal auditory meatus, through which the auditory nerve passes to the organ of hearing,

The fenestra rotunda is closed only by membrane; but fastened to the centre of the membrane of the fenestra ovalis, so as to leave only a narrow margin, is an oval plate of bone, part of one of the little bones to be described shortly.

19. The outer wall of the internal ear is still far away from the exterior of the skull. Between it and the visible placed in a straight line,

opening of the ear, in fact, are