680 HANDBOOK OF PHYSIOLOGY. 



ing filled by the base of the stapes; in its posterior and superior 

 walls, five openings by which the semicircular canals communicate with 

 it: in its anterior wall, an opening leading into i\\Q cochlea. The hinder 

 part of the inner wall of the vestibule also presents an opening, the 

 orifice of the aquceductus vestibuli, a canal leading to the posterior mar- 

 gin of the petrous bone, with uncertain contents and unknown purpose. 



The semicircular canals (figs. 400,401) are three arched cylindriform 

 bony canals, set in the substance of the petrous bone. They all open at 

 both ends into the vestibule (two of them first coalescing). The ends of 

 each are dilated just before opening into the vestibule; and one end 

 being more dilated than the other is called an ampulla. Two of the 

 canals form nearly vertical arches; of these the superior is also anterior; 

 the posterior is inferior; the third canal is horizontal, and lower and 

 shorter than the others. 



The cochlea (6, 7, 8, figs. 400 and 401), a small organ, shaped like a 

 common snail-shell, is situated in front of the vestibule, its base resting 

 on the bottom of the internal meatus, where some apertures transmit 

 to it the cochlear filaments of the auditory nerve. In its axis, the 

 cochlea is traversed by a conical column, the modiolus, round which a 

 spiral canal winds with about two turns and a half from the base to the 

 apex. At the apex of the cochlea the canal is closed; at the base it 

 presents three openings, of which one, already mentioned, communicates 

 with the vestibule; another called fenestra rotunda, is separated by a 

 membrane from the cavity of the tympanum ; the third is the orifice of 

 the aquceductus cochlea?, a canal leading to the jugular fossa of the 

 petrous bone, and corresponding, at least in obscurity of purpose and 

 origin, to the aquseductus vestibuli. The spiral canal is divided into two 

 passages, or scala3, by a partition of bone and membrane, the lamina 

 spiralis. The osseous part or zone of this lamina is connected with the 

 modiolus. 



The Membranous Labyrinth. The membranous labyrinth corre- 

 sponds generally with the form of the osseous labyrinth, so far as regards 

 the vestibule and semicircular canals, but is separated from the walls of 

 these parts by perilymph, except where the nerves enter into connection 

 within it. The labyrinth is a closed membrane containing endolymph, 

 which is of much the same composition as perilymph, but contains less 

 solid matter. It is somewhat viscid, as is the perilymph, and it is 

 secreted by the epithelium lining its cavity; all the sonorous vibrations 

 impressing the auditory nerves in these parts of the internal ear, aro 

 conducted through fluid to a membrane suspended in and contain in ir 

 fluid. In the cochlea, the membranous labyrinth completes the septum 

 between the two scalce, and incloses a spiral canal, previously mentioned, 

 called canalis membranaceus or canalis cochlea (fig. 403). The fluid in 



