756 THE EAR. 



ment. From this point the canal makes two turns and a half round the 

 central pillar (from left to right in the right ear, and in the opposite 

 direction in the left ear), and ends by an arched and closed extremity called 

 the cupola, which forms the apex of the cochlea. The first coil, being much 

 the widest in its curve and composed of the largest portion of the tube, 

 nearly hides the second turn from view ; and bulging somewhat into the 

 tympanum, forms the round elevation on the inner wall of that cavity called 

 the promontory. 



Fig. 511. 



Fig. 511. DIAGRAMMATIC VIEW OF THE CANAL OF THE COCHLEA LAID OPEN, f 



1, modiolus or central pillar; 2, placed on three turns of the lamina spiralis ; 3, scala 

 tympani ; 4, scala vestibuli. 



Fig. 512. VIEW OF THE OSSEOUS COCHLEA DIVIDED THROUGH THE MIDDLE 

 (from Arnold), f 



. 1, central canal of the modiolus ; 2, lamina spiralis ossea ; 3, scala tympani ; 4, scala 

 vestibuli ; 5, porous substance of the modiolus near one of the sections of the canalis 

 spiralis modioli. 



The modiolus (columella cochleae) forms the central pillar or axis round 

 which turn the spiral tube and the spiral lamina. It is much thickest 

 within the first turn of the cochlea, and rapidly diminishes in size in the 

 succeeding parts. The outer surface is dense, being, iu fact, composed of the 

 walls of the spiral tube ; but the centre is soft and spongy as far as the last 

 half coil, and is pierced by many small canals, for the passage of the nerves 

 and vessels to the lamina spiralis : one of these canals, larger than the rest 

 (canalis centralis modioli), runs from the base through the centre of the 

 modiolus. 



The lamina spiralis ossea is a thin, flat plate, growing from and winding 

 round the modiolus, and projecting into the spiral tube, so as to divide it 

 partly into two. Its free margin, which gives attachment in the recent 

 state to the membranous septum, or zone, does not reach farther than about 

 half of the distance between the modiolus and the outer wall of the spiral 

 tube. The osseous lamina terminates close to the apex of the cochlea in a 

 hook-like process (hamulus), which partly bounds the helicotrema. 



The lamina is thin and dense towards its free margin ; but near the 

 modiolus it is composed of two dense outer plates enclosing a more open 

 and spongy structure, in which are numerous small canals, continuous, but 

 running at right angles with the canals in the centre of the modiolus. In 

 these the nerves and vessels are lodged : they terminate on the inferior or 

 tympanic aspect of the lamina, and the line of their orifices forms the tractus 

 foraminosus spiralis. Winding round the modiolus, close to the lamina 

 spiralis, is a small canal, named by Rosenthal the canalis spiralis modioli. 



The scalce in the osseous cochlea are two in number, distinguished as the 

 scala tympani and scala vestibuli. 



