HEARING. 795 



to the external wall of the canal. It consists of two layers; the superior is 

 the membrane of Corti, or membrana tectoria ; the inferior the membrana 

 basilaris, which is attached externally to the planum semilunare. These 

 membranes are placed parallel with each other and contain between them 

 the organ of Corti, which rests on the basilary membrane. (Fig. 190.) 



701. The scala vestibuli 1 communicates below with the vestibule by the 

 aperturce scalw vestibuli cochleae ; the lower passage, or scala tympani, com- 

 municates with the tympanum by the fenestra rotunda. These scalse com- 

 municate at the apex of the cochlea by an opening termed the hiatus or 

 helicotrema, which exists in consequence of a deficiency of the last half turn 

 of the lamina spiralis. 



The osseous portion of the lamina spiralis has on its superior external 

 portion a denticulated cartilaginous substance called the lamina denticulata. 

 From the superior surface of the spiral lamina, and internal to the lamina 

 denticulata, is a delicate membrane extending upward and outward at an 

 angle of about 45 degrees to the external wall of the scala. This is called 

 the membrane of Reissner. It divides the scala into two passages, the lower 

 of which is the ductus cochlearis. This duct ends in the apex of the cochlea 

 in a coeca, and communicates at the base with the saccule by the ductus 

 reuniens ; it contains the essential portion of the auditory apparatus of the 

 cochlea, and is a part of the membranous labyrinth. 



FIG. 190. 



A Diagram of a Section of the Tube of the Cochlea, enlarged. (Modified from Henle.): SV, 

 scala vestibuli ; ST, scala tympani ; CC, canal of the cochlea ; 1, membrane of Reissner; 2, coch- 

 lear branch of the auditory nerve; 3, lamina spiralis ossea; 4, planum semilunare; a, lamina 

 denticulata ; &, sulcus spiralis ; c, tympanic lip of the sulcus spiralis ; d, inner rods of Corti ; e, outer 

 rods of Corti ; /, lamina reticularis ; i, inner hair-cells ; mb, membrana basilaris ; me, membrane of 

 Corti ; p, outer hair-cells ; sm, central space between the rods. 



702. The organ of Corti rests upon the basilary membrane. It consists 

 of the inner and outer hair-cells, and two rows of elongated cells, placed 

 parallel with each other, having an inclining position so that their free ex- 

 tremities rest against each other and thus form the arch of Corti, which covers 

 the central space. (Fig. 190.) These rows are called' the inner and outer 

 rods or pillars of Corti. From the superior extremity of both the inner and 



1 The upper scala is divided into two parts by a membranous partition, the upper of 

 which is called the scala vestibuli ; the other, ductus cochlearis (Fig. 190). 





