THE ORGANS OF SPECIAL SENSE. 



mernbrana 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 homoge- 

 neous, 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 

 thickened and altered in character, forming what is called the ligamentum spirale. 

 It projects inward below as a triangular prominence, 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. The upper portion of 

 the ligamentum spirale contains numerous capillary loops and small blood-vessels, 

 arid forms what is termed the stria vascularis. 



The lamina spiralis ossea 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 

 lamince 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 

 ti/mpanicum, and is perforated by numerous foramina (foramina nervosd) for the 

 passage of the cochlear nerves. 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 auditory teeth. The basilar mem- 

 brane may be divided into two areas, inner and outer. The inner is thin, and is 

 named the zona arcuata : it supports the organ of Corti. The outer is thicker 

 and striated, and is termed the zona pectinata. The under surface of the mem- 

 brane 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 ; it lies below Corti's 

 tunnel. 



Organ of Corti. 1 This organ (Fig. 464) is situated upon the inner part of the 

 membrana basilaris, and appears at first sight as a papilla, winding spirally 



Membrana tectoria. 



Outer hair cells. 



Limbus. 



Nerve fibres. 



Cells of Deiters. 

 Outer rod. 

 Basilar membrane. 



FIG. 464. Section through the organ of Corti. Magnified. (G. Retzius.) 



throughout the whole length of the ductus cochlearis, from which circumstance it 

 has been designated the papilla spiralis. More accurately viewed, it is seen to 

 be composed of a remarkable arrangement of cells, which may be likened to the 

 keyboard of a pianoforte. Of these cells, the two central ones are rodlike bodies 

 1 Corti's original paper is in the Zeilschrift f. Wissen. Zool., iii., 109. 



