446 NERVOUS SYSTEM. 



into the vestibulum, from the semicircular canals, to five instead 



of six. 



The Cochlea forms the fore part of the labyrinth, and resem- 

 bles very strongly the shell of the common snail. Its base is 

 the bottom of the meatus auditorius internus, and its apex is 

 directed towards the cavity of the tympanum, so that the 

 axis of the cochlea is turned downwards and outwards. It 

 consists in a conoidal tube ivound spirally twice and a half 

 around a column of bone termed the Modiolus. The tube then 

 of course diminishes in size from the base to the apex of the 

 cochlea. 



This conical tube is divided in its length by a plate called 

 Lamina Spiralis. Of the two compartments thus formed, one 

 is above the other*. The inferior is the larger, and communi- 

 cates at its base, through the foramen rotundum, with the tym- 

 panum; it is, therefore, called Scala Tympani. The other com- 

 partment communicates at its base with the vestibulum, and is, 

 therefore, called Scala Vestibuli. 



The Modiolus is of a conical shape, and cribriform: one 

 canal, larger than the others, runs from its base to its summit. 

 This canal is surrounded by many others, which diminish suc- 

 cessively as they advance towards the apex, and terminate in 

 orifices upon the lamina spiralis. This cribriform arrange- 

 ment of the modiolus is the Tractus Spiralis Foraminulosus. 

 The base of the modiolus is towards the meatus auditorius in- 

 ternus, and its. point does not go to the apex of the cochlea, 

 but stops short of it, and is expanded into a cavity called the 

 Infundibulum, the base of which is towards the apex of the 

 cochlea. That portion of the apex of the cochlea which covers 

 over the infundibulum, is the Cupola. 



It was just mentioned that the lamina spiralis divides the 

 cochlea into two tubes; the septum thus formed, does not, how- 

 ever, run their whole length, for it ceases in the infundibulum 

 by a small crooked process of bone, called the Hamulus Coch- 

 lea?. The lamina, when examined by strong glasses, is seen 

 to consist of four distinct structures called its Zones. 1. The 

 Zona Ossea is next to the modiolus, and is composed of two 

 bony laminje, with an intermediate diploic structure, in which 



