160 TOPOGRAPHICAL ANATOMY OF THE 



internal wall of the tympanum, and crosses the roof of this cavity. The 

 main structure contained in the canal is the seventh cerebral or facial 

 nerve. In addition there is the small auricular branch (ramus auri- 

 cularis) of the vagus nerve, which, arising from the jugular ganglion, 

 gains the canal, gives filaments to the facial nerve, along with which it 

 leaves the canal by the stylomastoid foramen to ascend behind the 

 external acoustic meatus and end in the skin within the meatus. 



The internal ear (Auris interna). — The internal ear consists of 

 spaces and canals tunnelled out of the temporal bone, to which the 

 collective name of osseous labyrinth is applied, and in which mem- 

 branous sacs and tubes — the membranous labyrinth — are contained. 



The Osseous Labyrinth (labyrinthus osseus). — The central space of 

 the osseous labyrinth, the vestibule (vestibulum), has a lateral wall 

 formed by part of the medial wall of the tympanum, and in the dried 

 skull communicates with this cavity by the fenestra vestibuli. In the 

 fresh state and in the living animal, as has been stated, the fenestra is 

 occluded by the base of the stapes. The medial wall of the vestibule 

 corresponds to the depths of the internal acoustic meatus, and is crossed 

 by an oblique crista vestibuli. In front of the crista there is a 

 depression, the recessus sphcericus, for the lodgment of the saccule of 

 the membranous labyrinth, to which filaments of the vestibular nerve 

 gain access by a number (12-15) of small foramina in the lower part of 

 the recess. A larger oval recessus ellipticus, for the utricle of the 

 membranous labyrinth, occurs behind the crista vestibuli. In this 

 recess, and in the upper part of the crista, are numerous foramina for 

 the transmission of filaments of the vestibular nerve to the utricle and 

 the ampullar of the superior and lateral semicircular ducts. The 

 posterior-inferior end of the crista splits to enclose the recessus 

 cochlearis, which is perforated by small foramina for nerve filaments 

 that pass to the cochlear duct. Below the recessus ellipticus is the 

 small, slit-like entrance to the aqucecluctus vestibuli, which passes 

 through the temporal bone to open on its cerebral surface, and transmits 

 the endolymphatic duct and minute vessels. 



In the posterior part of the vestibule are openings leading into the 

 three osseous semicircular canals — superior, posterior and lateral ; 

 while in the lower and anterior part of the cavity is an opening into the 

 spiral canal of the cochlea. Each of the semicircular canals (canales 

 semicirculares ossei) forms about two-thirds of a circle, and is enlarged 

 at one end into an ampulla} The superior canal is vertical, with its 

 ampulla anterior and lateral. The more posterior non-ampullated end 



^ Ampulla [L.], a globular vessel for holding liquids, a flask. 



