STEUCTUKE OF THE COCHLEA. 285 



lamina of bone (spiral lamina), partly by a flat membrane (basilar 

 membrane), into two parts or scalce ; the upper (supposing the cochlea 

 resting base downwards) being termed the scala vestibuli (fig. 319, s.v.), 

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

 by the membrane of the fenestra rotunda. The scalse 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 Reissner, 

 fig. 319, E), which springs from near the end of the spiral lamina, arid 

 passes upwards and outwards to the outer wall, thus separating a canal 

 (D C) triangular in section, which is lined by epithelium, 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. 319, /) ; (2) of the basilar membrane (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 are embedded in 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. 320). They rest by one ex- 

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

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

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

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

 feet may usually be seen the remains of the cells from which they have 

 been formed. The inner rods are narrower and rather more numerous 

 than the outer. Each outer rod has a process which extends outwards 

 and is known as the phalangeal process. This forms part of 



