ORGAN OF CORTT. 



123 



The tectorial membrane is the last special structure which remains to be 

 described in connection with the organ of Corti. It arises, as before stated, on 

 the limbus, not far from the line of origin of Reissner's membrane (fig. 131). 

 It overlies the projecting teeth at the edge of the limbus, and also the epithelium 

 between them : all this part of the membrane is thin and delicate, imperceptibly 



//. 



Fig. 140. VIEW OF A SMALL PART OF THE ORGAN 



OF CORTI OF THE HUMAN COCHLEA FROM ABOVE, 

 SHOWING THE LAMINA RETICULARIS. Much 



magnified. (Gf. Retzius. ) 



i.h, inner hair-cells, the hairlets being seen in 

 section ; i.r.h, heads of inner rods ; o.r.h, heads 

 of outer rods ; i.r.p, " olecranon " processes of inner 

 rods ; pk.pr, phalangeal processes of outer rods ; 

 ph l , pk 2 , ph z , first, second, and third series of 

 phalanges ; A 1 , A*, h 3 , A 4 , first, second, third, and 

 fourth series of outer hair-cells : //, cells of Hensen. 



Fig. 141. SURFACE VIEW OF A SMALL PIECE OF 



THE MEMBRANA TECTORIA OF THE HUMAN 



COCHLEA. (Gr. Retzius. ) 



7.~., limbus-zone of the membrane, showing 

 markings caused by the cells covering the limbus ; 

 0.2., 0.2., outer zone, showing a well-marked fibrous 

 structure ; II., stria described by Hensen in the 

 middle of the outer zone ; /. , reticular free border. 



shading off towards the inner edge of attachment. As the membrane projects over 

 the crest of the limbus, it swells out below into a pad-like projection (fig. 136) 

 which covers in and partly fills up the spiral groove, and rests below upon the rods 

 of Corti and contiguous structures. Towards its external edge the membrane again 

 thins out, and overlies the outer hair-cell region as a delicate film presenting a 

 somewhat reticular appearance, as if impressed by or moulded on the subjacent 

 structures. The thickened part of the membrane is distinctly fibrous in appearance 

 (the fibrillation extending obliquely from within out) (fig. 141), and after immersion 

 in weak solutions of omic acid, chromic acid, or bichromate of potash, it appears to 

 possess considerable toughness and elasticity. From its position the hairlets borne 

 by the hair-cells must necessarily be in contact with the under surface of this 



