Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/954

Rh 894 ANATOMY [ORGANS OF SENSE plate of bone, the osseous spiral lamina, which, springing from the modiolus, winds spirally around it, and projects into the tube. When the membranous cochlea is in its place the division is completed by a membrane, the mem branous spiral lamina, or basilar membrane, which bridges across the interval between the free edge of the osseous spiral lamina and the outer wall of the tube, to which it is attached by the spiral cochlear ligament. These passages are called scala tympani and scala vestibidi. But another membrane, the membrane of Eeissner, also arises from a denticulated spiral crest, limbus or crista spiralis, attached to the vestibular border of the free edge of the osseous spiral lamina, and extends to the spiral liga ment at the outer wall of the tube, on the vestibular aspect of the basilar membrane, so as to enclose a passage between it and the basilar membrane, called scala intermedia or ductus cochlearis. The membrane of Reissner is formed of delicate vascular connective tissue, with au endothelial Via. 83. Transverse section through the tube of the cocWeo. m, modiolus; 0, outer wall of cochlea; SV, scala vestibuli; ST, Bcala tympani; DC, ductus cochlearis; mR, membrane of Reissner; bm, basilar membrane; c*, crista epiralis; /, spiral ligament ; tg, spiral ganglion of auditory nerve ; oc, organ of of Corti. layer on each of its two surfaces. The scala tympani or lower passage, widest at the base of the cochlea, begins at the inner wall of the tympanum, into which it would have opened through the fenestra rotunda, had not the fenestra been closed up by a membrane. The ecala vestibuli or upper passage, also widest at the base, communicates with the cavity of the osseous vestibule. At the apex of the cochlea these two scalas communicate with each other through a small hole, the helicotrema. As the scala vestibuli opens into the osseous vestibule, the perilymphis continued into it, and through the helicotrema into the scala tympani. The ductus cochlearis is the membranous cochlea, and its walls are formed of the basilar membrane next the scala tympani, of the membrane of Eeissner next the scala vestibuli, and of the spiral ligament next the wall of the cochlea, which connects the two membranes together. It follows the spiral windings of the cochlea, terminates at the apex of the spiral in a closed end, whilst at the base it communicates with the sacculus of the membranous vesti bule by a slender tube, the canalis reuniens ; hence the membranous cochlea contains endolymph. The termina tion of the cochlear branches of the auditory nerve and the arrangement of the peripheral end-orgaus in relation to them are to be looked for in the basilar membrane. These parts have been repeatedly investigated and described in elaborate monographs, the titles of which are given as an appendix to Waldeyers article on the cochlea in Strieker s Handbuch der Lehre von den Geweben, Leipsic, 1872. The general results only of these investigations will be given here, and the original memoirs may be referred to for further details. On the surface of the basilar membrane directed to the Orga ductus cochlearis a remarkable arrangement of cells exists, which presents an appearance that has been compared with the key-board of a pianoforte, and has been named the organ of Corti ; it consists of the following parts: Some of these cells, distinguished by their elongated curved form, are arranged in two groups, an inner and an outer. The cells of the inner group rest by a broad foot on the inner part of the basilar membrane, close to its attachment to the spiral lamina, project obliquely forwards and outwards, and expand into a dilated head : the cells of the outer group also rest by a broad foot on the same membrane, incline forwards and inwards, and fit into a depression in the head of the cells of the inner group : these two groups of cells form the rods or pillars of Corti, and by their juxtaposition arch over an excessively minute canal enclosed between them and the basilar membrane, which may be named the canal of Corti. The inner rods are, however, more numerous than the outer, and Pritchard has shown that the rods increase in length from the base to the apex of the cochlea. Immediately internal and almost parallel to the inner group of these rods, and adjacent therefore to the crista spiralis, is a row of compressed conical cells, which possess at their anterior ends short stiff hair-like processes; they are the inner hair cells of Deiters. Immediately external and almost parallel to the outer group of rods are four or five rows of hair cells, the outer hair cells, which are attached by their bases to the basilar membrane, whilst from the opposite extremity a brush of hairs projects through the reticular membrane. The outer hair cells are, according to Waldeyer, relatively of large size in man. The reticular membrane of Kolliker is a delicate framework perforated Fio. 84. Vertical transverse section through the baellar membrane and organ of Corti. bm, ct, il, &amp;lt;fcc., as in fip. 83; t, inner hair cell; t r, inner, and or, outer rod of Corti ; o, outer hair cells; ip, supporting colls; en, cochlear nerve; ec, canal ot Corti; rm, reticular membrane; mt, membrana tectoria. (Adapted from Waldeyer.) by rounded holes. It extends parallel to the basilar mem brane from the inner rods of Corti to the external row of outer hair cells, and through the holes in it the hairs of the latter project. It obviously acts as a support for the anterior ends of these cells, and binds together these important elements of the organ of Corti. The interval between the outer hair cells and the spiral ligament is occupied by cells of a more or less columnar form, the supporting cells of Hensen. Covering over the organ of