THE ORGAN OF HEARING. 39! 



likewise the undoubted epithelial character of the innermost cells 

 next the endolymph, but uncertainty exists concerning the true 

 nature whether epithelium or endothelium of the cells filling the 

 intercapillary spaces and lying between the epithelial layer and the 

 adjacent connective tissue. 



The tympanic wall of the cochlear duct consists of two por- 

 tions, the limbus, which includes the wall from the attachment 

 of Reissner's membrane to the end of the lamina spiralis, and the 

 basilar membrane, which extends from the end of the bony 

 spiral lamina to the basilar crest on the outer wall. 



The limbus corresponds to a conspicuous local increase in the 

 periosteum and the fibrous coat at the point where the apical border 

 of the cochlear duct is attached to the bony spiral lamina. The 

 greatest thickening of the periosteal tissue occurs within the half 

 of the limbus next the membrane of Reissner, the half adjoining 

 the basilar membrane exhibiting an abrupt decrease in the layer, 

 marked by a sharp edge overhanging the sulcus spiralis, the con- 

 cavity formed by the receding border of the suddenly-diminished 

 stratum ; the upper and lower edges of the recess constitute the 

 superior and inferior labia. 



The thicker portion of the limbus between the membrane of 

 Reissner and the superior labium is remarkably modelled, since 

 its surface is broken by clefts and furrows, which become deeper 

 as well as larger towards the margin of the superior labium ; this 

 peculiar arrangement culminates in the deeply-cleft edge of the 

 superior labium, where irregular tongue-like processes separated by 

 lateral clefts form the so-called auditory teeth, the entire number 

 of which in the human cochlea has been estimated at about 2500. 



The epithelium covering the limbus differs in various parts ; 

 flat polyhedral cells cover the elevated portions, including the 

 auditory teeth, the intervening furrows and clefts being clothed by 

 columnar elements. The epithelium lining the sulcus spiralis 

 consists of a single layer of low cuboidal or flattened polyhedral 

 cells continuous with the investment of the auditory teeth on the 

 one hand and with the highly-specialized elements of Corti's organ 

 on the other. 



The basilar membrane, the outer zone of the tympanic wall, 

 stretches from the end of the osseous spiral lamina to the basilar 

 crest of the spiral ligament of the outer wall. The membrane bears 

 upon part of its surface directed towards the cochlear duct the re- 

 markably modified neuro-epithelium constituting the organ of 

 Corti, and is consequently divided into the inner zona tecta, over 

 which this end-organ lies, and the outer zona pectinata, covered 

 with the more usual epithelium. 



