THE INTERNAL EAR. 



371 



municates with the vestibule, whilst the lower division, the scaJa tympani, 

 would open into the tympanic cavity, were it not separated from that space 

 by the secondary tympanic membrane. The latter is a thin nbro-elastic 



Scala vestibuli 

 Scala tympani 



Apex of modiolus 



Lamina spiralis 

 ossea 



Canalis spiralis 

 tnodioli 



Modiolus 



Internal auditory 

 canal 



FIG. 419. Cochlea and bottom of internal auditory canal exposed by vertical section ; cochlea rests with 

 its base downwards and apex pointing upwards. X 5. 



sheet, covered internally with endothelium and externally with epithelium. 

 The scalse communicate with each other through an opening, the helico- 

 trema, at the apex of the cochlea. 



THE MEMBRANOUS LABYRINTH. 



The membranous labyrinth lies within the bony labyrinth, which it 

 resembles in general form. This agreement is least marked within the vesti- 

 bule, since here the single division of the bony capsule is occupied by two 

 compartments of the membranous sac, the utricle and the saccule. The 

 membranous labyrinth comprises: (i) the utricle and the saccule, which, 

 with the ductus endolymphaticus, lie within the vestibule; (2) the three 

 membranous semicircular canals lodged within the bony semicircular canals; 

 and (3) the membranous cochlea enclosed within the bony cochlea. The 

 membranous labyrinth is attached, especially in certain places, by connective 

 tissue to the inner wall of the bony capsule. The interval between the 

 membranous and bony labyrinths, largest in the scalae tympani and vestibuli 

 of the cochlea and in the vestibule, constitutes the perilymphatic space and 

 contains a modified lymphatic fluid, the perilymph. The fluid within the 

 membranous labyrinth, the endolymph, can pass from one part of the labyrinth 

 to another, although the saccule and utricle are only indirectly connected 

 through a narrow channel, the ductus endolymphaticus \ and the saccule and 

 cochlear duct communicate by means of a small tube, the canalis reuniens. 



Structure of the Utricle, Saccule and Semicircular Canals. 

 The walls of these subdivisions of the membranous labyrinth are made up 

 of (a) an outer fibrous connective tissue lamella and () an inner epithelial 

 lining, the latter consisting throughout the greater part of its extent, of a 

 single layer of thin flattened polyhedral cells. Beneath the epithelium, espe- 

 cially in the region of the maculae, is (c) a thin almost homogeneous hyaline 

 membrane, with few cells. This middle layer presents in places on its inner 

 surface small papillary elevations covered by epithelium. On the concave 



