THE SENSE OF HEARING. 



379 



coclilea. It is known as the membrane of Reissner. The periosteal con- 

 nective tissne of the bony wall of the cochlea is generally well developed 

 within the area enclosed between the membrane of Reissner and the membrana 

 basilaris; it is particularly thick at the line of division between the scala media 

 and the scala tympani, where it forms a projecting ridge at the outer attach- 

 ment of the basilar membrane. This ridge is the spiral ligament; an exten- 



..o.r.h 



Fig. 195.— Diagram of the constituents of the organ of Corti (from Foster, after Retzius) : a, inner hair 

 cell; a', the head, seen from above; B, inner, b', outer, rod of Corti; pA, in each, is the phalangar pro- 

 cess; c, the twin outer hair-cell ; Cc, the cell of Corti; h, its auditory hairs; n, its nucleus; x, Ilensen's 

 body; D.c, cell of Deiters ; n' , its nucleus; ph.p, its phalangar process;/?/, the cuticular filament; m.b, 

 basilar membrane; m.r, reticulate membrane; c', the head of a cell of Corti, seen from above; d, the 

 organ of Corti, seen from above ; i.hc, the heads of the inner hair-cells ; i.r.h, the head and phalangar pro- 

 ecu- <>f the inner rod ; o.r.li, the head of the outer rod, with ph.p, its phalangar process, covered to t! 

 hand by the inner rods, but uncovered to the right; o.h.c, the heads of the cells of Corti, supported by 

 the rings of the reticulate membrane; pli, one of the phalangse of tin' reticulate membrane. 



sion from it, gradually decreasing in thickness, reaches into both the vestibular 

 and the tympanic scala. 



A thick layer of both columnar and cuboidaJ epithelium lines the con- 

 nective tissue forming the outer wall of the canal is cochlearis. This epithe- 

 lium is peculiar in that the blood-vessels of the underlying connective tissue 

 penetrate between the epithelial cells themselves. The tectorial membrane 

 (membrana tectoria) is a sheet of radially-fibrillated tissue, thin at its poinl of 

 attachment to the vestibular lip of the limbus, and becoming thicker and then 

 thinner again as it stretches out over the organ of Corti, reaching as tin- as the 

 most external row of hair-cells. It is said to lie in actual contact with the 

 rods of Corti and the free ends of the hair-cells, and it has been presumed to 

 serve as a damper for the vibrations imparted to the organ of ( 'orti. 



