ORGAN OP HEARING. 



181 



ditory nerve reach the cochlea. The spiral canal is divided, in its 

 whole length, by a partition, half osseous and half membranous, called 

 lamina spiralis; so that two distinct tubes are thus formed. These are 

 the scalse of the cochlea. At the apex of the cochlea they run into 

 each other, by an opening termed by M. Breschet helicotrema; and at 

 the base, one turns into the vestibule, and is hence called superior or 

 vestibular or internal scala; the other communicates with the cavity 

 of the tympanum by the foramen rotundum, and is called inferior, 

 tympanic, or external scala. At this scala, near the foramen rotun- 

 dum, a bony canal begins, which proceeds towards the posterior sur- 

 face of the pars petrosa, on which it opens. It is aquseductus seu 

 divertieulum cochlese. The cochlea does not exist in all animals that 

 hear. It is not, therefore, of essential importance. It varies, too, 

 greatly, in complication, in different animals. In birds, whose hear- 

 ing is extremely delicate, it merely consists of a short, hollow, bony 

 process, divided into two scalae but without any spiral arrangement. In 

 reptiles, it is still more imperfect; and in many species can scarcely be 

 said to exist. In fishes there is no trace of it. 



Fig. 70. 



Osseous Labyrinth laid open to show especially the Membranous Labyrinth. 



X, Y. Z. Semicircular canals. A, A, A. Ampullae. P. Perilymph between the osseous and mem- 

 branous labyrinth. U. Utricle. S. Sacculus. O, O. Cretaceous bodies. G, N. Auditory nerve. " 

 Branch going to the cochlea. L. Lamina spiralis. M. Apex of modiolus. D. Portio dura. 



K. 



The different cavities of the internal ear are lined by an extremely 

 delicate membrane. In many animals this membrane exists alone, 



