206 STEUCTUEE OF CEISTA ACUSTICA. [BOOK in. 



(cf. Fig. 166) which corresponds to the base of the cochlea, and 

 which overhangs the depression leading to the fenestra rotunda. 

 From this base the cochlea placed nearly horizontally, and thus 

 nearly at right angles to the median wall of the tympanum, runs 

 forwards inclining to the median side, the apex abutting on the 

 bony wall of the Eustachian tube. Above the promontory the 

 fenestra ovalis marks on the tympanic wall the position of the 

 projecting portion of the vestibule ; from this the semicircular 

 canals project backwards and laterally, in their several planes, the 

 ampulla of the superior and external canal forming a projection on 

 the wall of the epitympanic cavity above the fenestra ovalis. The 

 auditory nerve entering, in company with the facial nerve, on the 

 median side in the open angle between the base of the cochlea 

 and the vestibule, is distributed, as we have seen, to both these 

 structures. 



The Vestibular Labyrinth. 



825. Little need be said concerning the minute structure of 

 those parts of the vestibular division of the labyrinth with which 

 the auditory nerve makes no connections. The epithelium consists 

 of a single layer of cells which are for the most part flat and poly- 

 hedral, though they differ somewhat in form and in other features 

 in different regions. The epithelium rests on a thin connective 

 tissue basis known as the " tunica propria ; " this is hyaline, and 

 except for some fibrillation or striation more obvious in some parts 

 than in others, appears to be structureless ; nuclei are absent from 

 it and blood vessels do not pass into it. This tunic, which in the 

 semicircular canals, in man, is with its epithelium frequently 

 raised into irregular papillae or warts, rests on an outer coat of 

 vascular connective tissue, in some places continuous with, in 

 others connected by bridles with the periosteum of the bony 

 envelope. The spaces between the bundles of connective tissue 

 of this coat gradually open out into the general perilymph 

 cavity ; it and they are lined with lymphatic epithelioid plates. 

 The fluid (perilymph) contained in them, though it is not ordinary 

 lymph, being viscid through the presence of mucin, finds access 

 along the sheath of the auditory nerve into the subdural and 

 subarachnoid spaces of the brain. 



826. The three cristae acusticae are as we have said ridges 

 projecting crosswise into the cavities of the ampullae to which 

 they respectively belong, each ridge being formed partly by a 

 development of the tunica propria and underlying connective 

 tissue into a thick cushion of somewhat peculiar nature, and 

 partly by an increase in the epithelium which, thick on the top 

 of the ridge, gradually thins away at the sides. 



Immediately below the epithelium the connective tissue 

 cushion, especially at the top of the ridge, consists of a hyaline 



