1146 



THE ORGANS OF SPECIAL SENSE 



GANGLION SPIRAL BETWE EN TH E TWO LA Y E HS OF 

 SPIRALE FIBRES LAM I N A S PI R ALIS OSSEA 



to the outer wall of the cochlea, and completes the roof of the scala tympani. 

 A second and more delicate membrane, the membrane of Reissner (membrana 

 vestibularis [Reissner i]) (Fig. 852), extends from the thickened periosteum covering 

 the lamina spiralis ossea, at an angle of about 45 degrees, to the outer wall of 

 the cochlea, to which it is attached at some little distance above the membrana 

 basilaris. A canal is thus shut off between the scala tympani below and the 

 scala vestibuli above ;'^his is the membranous canal of the cochlea (ductus cochlearis 

 or scala media} (Fig. 853). It is triangular on transverse section, its roof being 

 formed by the membrane of Reissner, its outer wall by the periosteum which lines 

 the bony canal, and its floor by the membrana basilaris, and the outer part of 

 the lamina spiralis ossea, on the former of which is placed the organ of Corti. 

 Reissner's membrane is thin and homogeneous, and is covered on its upper and 

 under surfaces by a layer of epithelium. The periosteum, which forms the outer 



wall of the ductus cochlearis, is greatly 



NCHVC-FIBRES PASSING OUT thickened &nd altered in character; 



forming what is called the spiral liga- 

 ment of the cochlea (ligamejitum spirale 

 cochleae] (Fig. 852). It projects in- 

 ward below as a triangular promi- 

 nence, the crista basilaris, which gives 

 attachment to the outer edge of the 

 membrana basilaris, and immediately 

 above which is a concavity, the sulcus 

 spiralis externus (Fig. 852). The 

 upper portion of the ligamentum 

 spirale contains numerous capillary 

 loops and small bloodvessels within 

 the epithelium, and forms what is termed the stria vascularis. The stria is limited 

 below by a prominence (prominentia spiralis}, in which a bloodvessel (vas promi- 

 nens) is distinctly visible. 



The lamina spiralis ossea (Fig. 853) consists of two plates of bone extending 

 outward; between these are the canals for the transmission of the filaments of the 

 auditory nerve. On the upper plate of that part of the osseous spiral lamina 

 which is outside Reissner's membrane the periosteum is thickened to form the 

 limbus laminae spiralis, and this terminates externally in a concavity, the sulcus 

 spiralis internus, which presents, on section, the form of the letter C; the upper 

 part of the letter, formed by the overhanging extremity of the limbus, is named 

 the labium vestibulare ; the lower part, prolonged and tapering, is called the labium 

 tympanicum, and is perforated by numerous foramina (foramina nervosa] for the 

 passage of the component parts of the cochlear nerve. Externally, the labium 

 tympanicum is continuous with the membrana basilaris. The upper surface 

 of the labium vestibulare is intersected at right angles by a number of furrows, 

 between which are numerous elevations; these present the appearance of teeth 

 along the free margin of the labium, and have been named by Huschke the audi- 

 tory teeth. The basilar membrane may be divided into two areas, inner and 

 outer. The inner is thin, and is named the zona arcuata or zona tecta (Fig. 852); 

 it supports the organ of Corti. The outer is thicker and striated, and is termed 

 the zona pectinata. The under surface of the membrane is covered by a layer of 

 vascular connective tissue. One of these vessels is somewhat larger than the rest, 

 and is named the vas spirale (Fig. 856); it lies below Corti's tunnel. 



The organ of Corti 1 (organon spirale [Cortii]) (Figs. 856 and 857) is situated 

 upon the inner part of the membrana basilaris that is, the part directed toward 



FIG. 855. Part of the cochlear nerve, highly magnified. 

 (Henle.) 



1 Corti's original paper is in the Zeitschrift /. Wissen. Zool., iii., 109. 



