144 



ESSENTIALS OF PHYSIOLOGY. 



with the saccule of the posterior part of the membranous labyrinth by 

 a fine tube, the canalis reuniens. 



The Organ of Corti. The end-organ for hearing lies in the canal 

 of the cochlea and is called the organ of Corti. It consists of a 

 specialised epithelium resting on the basilar membrane (fig. 46). On 

 section the epithelium is seen to be arranged in relation with two rows 

 of rod-like cells, the rods of Corti, which are inclined towards each other 

 in such a way as to form a tunnel. The basal ends of the rods are 

 expanded, and in the angle which each forms with the basilar membrane 

 is a small nucleated mass of protoplasm. The free end of each outer 

 rod is shaped like the head of a swan, the back of it fitting into the 

 end of the inner rod, which resembles the proximal end of the ulna. 



FIG. 46. Structure of organ of Corti (diagrammatic). 



a, membrana tectoria ; b, hair cell ; c, basilar membrane ; d, rods of 

 Corti ; e, cochlear nerve fibres and (on the left) spiral ganglion. 



It is estimated that there are about 6000 inner, and about 4000 outer, 

 rods. The heads of the outer rods are continued outwards as 

 phalangeal processes which unite with corresponding processes on the 

 outer supporting cells to form the reticular membrane. 



On either side of the rods of Corti are hair cells, a single row on 

 the inner side, four or five rows on the outer side ; the free ends of 

 the outer cells occupy the apertures in the reticular membrane. The 

 hair cells are supported by the cells of Deiters. Beyond the hair cells 

 the supporting cells become shorter, and the epithelium is continuous 

 with the flattened cells lining the canal of the cochlea. 



A thick membrane, the membrana tectoria, extends from the limbus 

 laminae spiralis so as to rest upon the organ of Corti. 



The fibres of the cochlear nerve occupy the centre of the modiolus 

 and are distributed along the spiral lamina. They are the central 

 processes of the bipolar cells of the spiral ganglion, which lies in the 



