754 PHYSIOLOGY. 



The vestibule is an oval, irregular cavity, lying between the tym- 

 panum and the bottom of the internal auditory meatus. The semi- 

 circular canals open from it posteriorly and the cochlea opens from 

 it anteriorly. Through its outer wall it communicates with the tym- 

 panum by the oval window. The fovea hemispherica and fovea hemi- 

 elliptica are two depressions upon the inner and superior walls of 

 the vestibule, respectively. They are pierced by numerous fora- 

 mina; through the former pass the filaments of the cochlear branch 

 of the auditory nerve ; through the latter foramina pass the branches 

 of the vestibular branch. Through the latter also pass small veins 

 which communicate with the inferior petrosal sinus. 



The semicircular canals are three in number. They are located 

 above the inner and back part of the tympanum. From their loca- 

 tion they are named superior, posterior, and external. The canals lie 

 in three distinct planes: the first two are vertical, but nearly at 

 right angles to one another; the last is horizontal. 



Each canal is rather more than half of a circle, and forms at 

 one extremity a dilatation called the ampulla. The canals communi- 

 cate with the vestibule by five openings, one of which belongs to both 

 the superior and posterior canal. 



The interior of the vestibule and semicircular canals is lined 

 with a delicate membrane. The cavity formed by this membrane 

 contains a fluid of serous nature. It is known" as the perilymph, by 

 reason of its surrounding a secondary structure, the labyrinth. This 

 last structure consists of a pair of saccules in the vestibule, and three 

 semicircular saccules whose form is the same as the osseous canals 

 containing them. This membranous labyrinth comprising the sac- 

 cules just mentioned itself contains a serous fluid, the endolymph. 



The inner portion of the bony labyrinth is the cochlea : so named 

 from its resemblance to a shell. Its base is attached to the internal 

 auditory meatus, while its apex is directed forward and outward. 

 The axis of the cochlea is nearly at right angles to that of the 

 petrous portion of the temporal bone in which it lies. The cochlea 

 is a tube of bone wound around a central axis, each turn successively 

 rising. This bony tube is about one and one-half inches long. Its 

 beginning is connected with the fore part of the vestibule to produce 

 the promontory of the tympanum; it ends in a closed extremity 

 called the infundibulum. The central axis just spoken of is termed 

 the modiolus. The apex of the cochlea is often called the cupola. 



The bony canal is divided into two passages, or scales, by a 

 septum known as the lamina spiralis } which projects from the 



