380 StiwSlwr'e of lie internal Ear. [Book IX, 



the os petrofum, between the tympanum, the femi- 

 circular canals, and the cochlea. It is open on the 

 fide of the tympanum by means of the feneftra ovalis, 

 dhd communicates with the upper portion of the 

 cochlea by an oblong foramen, which is under the 

 feneftra ovalis, from which it is feparated only by a 

 very thin partition. 



The femicircular canals in the infant are formed 

 of a diftindt bony fhell, but in the adult coalefce with, 

 the firm os petrofum, and are three in number. 

 They form rather more than femicircles, and open 

 at both ends into the veftibule. Only five openings, 

 however, are obferved, fince two of the canals are 

 united at one termination. 



The cochlea, fo called from its refemblance to the 

 Ihell of a fnail, is formed, by a conical nucleus and 

 circumvolutions of thin bony lamellae, which perform 

 two complete circles and an half before they termi- 

 nate at the apex. The canal of the cochlea is divided 

 by a feptum into two parts, which are called the 

 fcalae ; of thcfe one begins from the feneftra rotunda, 

 and is called the fcala tympani, the other from the 

 bule, and is called the fcala veftibuli. The fep- 

 tum, which divides the fcalae from each other, is partly 

 bony and partly membranous; it is deficient at the 

 apex of the cochlea, where the cavities of the fcalas 

 communicate. The bony lamella which feparates the 

 two canals is exceedingly thin, and fills about two-thirds 

 of the diameter of the canal. The reft of the fep- 

 tum is compofed of a moft delicate membrane, which 

 lines the whole internal furface of the cochlea. The 

 portio mollis of the feventh pair of nerves, furnifhes 

 a film of medullary matter to the whole internal fur- 

 face of the veftibule, the femicircular canals, and the 

 cochlea. Every part of the labyrinth is alfo fuppliecj 



with 



