STRUCTURE OF THE COCHLEA 237 



the lower the scala tympani (s.t.} ; the latter is closed at its larger 

 end by the membrane of the fenestra rotunda. The scalae are lined by 

 endosteum, and are filled with perilymph, continuous with that of the 

 rest of the osseous labyrinth at the commencement of the scala vestibuli ; 

 they communicate at the apex by a small opening, the helicotrema. 



The scala vestibuli does not occupy the whole of that part of the 

 bony tube of the cochlea which is above the partition. Its outer third 

 is cut off by a delicate connective-tissue membrane (membrane of 

 Eeissner, fig. 276, B), which springs from near the end of the 

 spiral lamina, and passes upwards and outwards to the outer wall, thus 

 separating a canal (D C) triangular in section, which is lined by epithe- 

 lium, and represents the membranous labyrinth of the cochlea (canal 

 of the cochlea}. 



Canal of the cochlea. The floor of the canal of the cochlea is 

 formed (1) of the extremity of the spiral lamina, which is thickened 

 above by a peculiar kind of connective tissue, forming an overhanging 

 projection known as the limbus (fig. 276, I) ; (2) of the basilar mem- 

 brane (b.m.), which stretches across from the end of the bony lamina 

 to the outer wall, and is attached to this by a projection of reticular 

 connective tissue termed the spiral ligament (l.sp.) 



The basilar membrane is composed of stiff, straight fibres, which 

 extend from within out, and themselves rest on a homogeneous stratum. 

 It is covered below by a layer of connective tissue continuous with the 

 endosteum of the scala tympani ; above the modified epithelium which 

 forms the organ of Corti rests upon it. It becomes gradually broader 

 in the upper turns of the cochlea (rather more than twice as broad in 

 the uppermost as in the lowermost turn), and its constituent fibres 

 become therefore gradually longer. 



The organ of Corti consists of the following structures : 



1. The rods of Corti, two series (inner and outer) of stiff, striated 

 fibres of a peculiar shape, the inner rods somewhat like a human ulna, 

 the outer like a swan's head and neck (fig. 277). They rest by one ex- 

 tremity (the foot) on the basilar membrane a short distance apart, and 



FIG. 277. A PAIR OF RODS OF CORTI, FROM THE RABBIT'S COCHLEA, IN SIDE 

 VIEW. (Highly magnified.) 



&, b, basilar membrane ; i.r. inner rod ; e.r. outer rod. The nucleated protoplasmic masses at 

 the feet are also shown. 



are inclined towards one another, their larger ends (heads) being jointed 

 together ; the series of rods thus enclose a sort of tunnel, the floor of 

 which is formed by a part of the basilar membrane. Close to their 



