THE MEMBRANOUS LABYRINTH 



883 



2. The saccule (Sacculus) is situated in the recessus sphsericus of the vestibule. 

 From its lower part the ductus reuniens proceeds to open into the ductus cochlearis, 

 a little in front of the blind end of the latter. A second narrow tube, the ductus 

 endolymphaticus, passes from the posterior part of the saccule, and is joined by the 

 ductus utriculo-saccularis; it then traverses the aquseductus vestibuli, and ter- 

 minates under the dura mater on the posterior part of the medial surface of the 

 petrous temporal bone in a dilated blind end, the saccus endolymphaticus. 



3. The semicircixlar ducts (Ductus semicirculares)^ correspond in general to 

 the osseous canals already described, but it may be noted that while the ampullae of 

 the ducts nearly fill those of the osseous canals, the other parts of the ducts only 

 occupy about one-fourth of the bony cavities. 



4. The cochlear duct (Ductus cochlearis) is a spiral tube situated within the 

 cochlea. It begins by a blind end (Caecum vestibulare) in the cochlear recess of 

 the vestibule, and ends by a second blind end (Csecum cupulare), which is attached 

 to the cupola of the cochlea. The vestibular part is connected with the saccule 

 by the ductus reuniens. The duct is triangular in cross-section, and it is usual to 

 regard it as having three walls. The vestibular wall or roof, which separates the 

 cochlear duct from the scala vestibuli, is formed by the very delicate membrana 

 vestibularis (of Reissner), which extends obliquely from the lamina spiralis ossea 



Fig. 707. — Left Membraxous Labtrixth (Ex- 

 larged). 

 i, Cochlea; ;?, fenestra vestibuli; 3, fenestra coch- 

 Ie£e; 4, ductus endolymphaticus; 5, dorsal, 6, lateral, 7, 

 ventral, duct. (After EUenberger, in Leisering's At- 

 las.) 



Fig. 70S. — Schematic Sectional View of Labyrixth 

 (Enlarged). 

 1, 2, 3, Dorsal, lateral, and ventral ducts; 4i 

 utricle; 5, saccule; 6, cochlea; 7, acoustic nerve. 

 (After EUenberger, in Leisering's Atlas.) 



to the outer wall of the cochlea. The tympanic wall or floor intervenes between 

 the cochlear duct and the scala tympani; it is formed by the periosteum of the mar- 

 ginal part of the lamina spiralis and the membrana basilaris, which stretches be- 

 tween the free edge of the lamina and the outer wall of the cochlea. The outer 

 wall consists of the fibrous lining of the cochlea, which is greatly thickened to form 

 the ligamentum spirale cochleae. 



Structure.- — The membranous labyrinth consists in general of an outer thin 

 fibrous layer, a middle transparent tunic, and an internal epithelium, composed of 

 flattened cells. But in certain situations special and remarkable structures occur, 

 among which are the foUowing: (1) The maculae acousticae appear as small whitish 

 thickenings of the inner walls of the saccule and utricle. The epithelium here 

 consists of two kinds of cells, viz., supporting cells and hair cells. The latter are 

 flask-shaped and are surrounded by the fusiform supporting cells. The free end 

 of each hair cell bears a stiff, hair-like process composed of a bundle of cilia. Fibers 

 of the saccular and utricular branches of the vestibular nerve form arborizations 

 about the basal parts of the hair cells. Adherent to the surface of the maculae 



1 Also termed the membranous semicircular canals. 



2 For the finer structure reference is to be made to the histological hterature. 



