THE SENSES 



579 



the essential organs of hearing, or Organ of Corti, is formed by 

 the membrana basilaris, which connects the outer wall of the 

 cochlea to the lamina spiralis ; a cover to the organ of Corti is 

 formed by the membrana tectoria. The cochlear canal is the con- 

 tinuation of the membranous labyrinth. The upper passage of 

 the cochlea — i.e., the scala vestibuli- — is continuous with the 

 lymphatic peri-lymph space of the vestibule, whilst the scala 

 tympani, or lower passage, ends at the base of the cochlea in a 

 blind extremity in which is a membranous window, the fenestra 

 rotunda, which separates the scala tympani from the cavity of the 

 tympanum. The cochlear canal terminates suddenly at the summit 

 of the cochlea, and at this point the two scalap, which in their 

 windings have been decreasing in size from base to apex, meet 

 and communicate by a small opening, the helicotrema, and the fluid 

 of the one is thus in connection with that of the other. 



Organ of Corti. — This consists of a triangular-shaped tunnel 

 (Fig. 183), the base of which rests on the basilar membrane ; the 

 tunnel is composed of certain rods arranged side by side, inclined 



Fig. 183. — Diagrammatic Transverse Section of a Turn of the Cochlea. 



from both sides towards each other, and meeting superiorly like an 

 inverted V. At this point the rods, known as the rods of Corti, 

 fit into each other in a peculiar manner. Flanking either side of 

 the tunnel are certain cells of two distinct kinds ; those nearest to 

 the tunnel are somewhat flask-shaped, and having hairs growing 

 from their summit, are spoken of as the inner and outer hair cells ; 

 external to the outer hair cells are some tall conical cells known as 

 Hensen's cells. The auditory nerve ascends the axis of the cochlea, 

 giving off branches which in their passage ramify over the lamina 

 spiralis, at the outer edge of which the above-described organ of 

 Corti exists ; having reached this the fibres lose their medulla, and 

 the naked axis cylinders pass into the cells flanking the triangular 

 tunnel, some fibres crossing the tunnel to reach the cells on the 

 opposite side. How the nerve terminates in the hair cells — for it 

 is to these that it is distributed — is unknown, but that the hair 

 cells are the organs of hearing is undoubted ; Hensen's cells are. 

 probably only of a nutritive nature and unconnected with auditory 

 impulses. 



