846 



THE OEGANS OF SENSE. 



base of the modiolus. It is divided by a transverse ridge, the crista transversa, 

 into two parts an upper or fossula superior and a lower or fossula inferior. The 

 anterior part of the fossula superior is termed the area n. facialis and exhibits a 

 single large opening, the commencement of the facial canal, for the transmission of 

 the facial nerve. Its posterior part is named the area vestibularis superior, and is 

 perforated by the nerves for the utricle and the ampullae of the superior and lateral 

 semicircular ducts. The anterior part of the fossula inferior is termed the area 

 cochleae, and consists of the canalis centralis and the surrounding tractus spiralis 

 foraminosus, for the passage of the nerves to the cochlea. Behind the area cochleae, 

 and separated from it by a ridge, is the area vestibularis inferior, which is pierced 

 by the nerves to the saccule ; at the posterior part of the fossula inferior is the 

 foramen singulare, which gives passage to the nerves for the ampulla of the posterior 

 semicircular duct. 



Recessus utriculi 



Saccule 



semicircular duct 

 Ampulla of lateral 

 duct 



Ductus 



cochlearis 



Ductus reunions 



Ductus 



endolymphaticus 



Ampulla of posterior duct 



Saccus endolymphaticus 



FIG. 720. DIAGRAMMATIC REPRESENTATION OF THE DIFFERENT 

 PARTS OF THE MEMBRANOUS LABYRINTH. 



Crus commune 



Ductus 



utriculosaccularis 

 Sinus inferior 



LABYEINTHUS MEMBRANACEUS. 



The membranous labyrinth (Fig. 720) is contained within the bony labyrinth, 

 but does not nearly fill it. It contains a fluid termed endolymph, while the interval 

 between it and the bony labyrinth is named the perilymphatic space, and is occupied 

 by a fluid termed perilymph. The perilymphatic space in the vestibule is continuous 

 behind with the perilymphatic space of the semicircular canals, and opens in front 

 into the scala vestibuli. At the apex of the cochlea it is continuous, through the 



helicotrema, with the scala 

 tympani, which is shut off 

 of superior from the tympanic cavity 

 by the secondary tympanic 

 membrane. The perilym- 

 phatic space is prolonged 

 into the aquseductus coch- 

 leae, at the extremity of 

 which it communicates 

 with the subarachnoid 

 cavity. The ductus semi- 

 circulares and the ductus 

 cochlearis follow the course 



and lie along the inner surface of the outer walls of the corresponding bony 

 tubes. The bony vestibule, on the other hand, contains two chief membranous 

 structures, the utricle and saccule. The former receives the extremities of the 

 ductus semicirculares, whilst the latter communicates with the ductus cochlearis. 

 Moreover, the cavities of the utricle and saccule are indirectly connected, and 

 thus all parts of the membranous labyrinth communicate with each other, and 

 the endolymph is free to move from one portion to another. The vestibule 

 contains also the ductus endolymphaticus and the commencement of the due tut 

 cochlearis. 



Utriculus. The utricle, the larger of the two sacs (Fig. 720), occupies tht 

 postero-superior portion of the vestibule. Its highest part, or recessus utriculi, lie* 

 in the recessus ellipticus and receives the ampullae of the superior and latera 

 semicircular ducts. Its central part receives on its lateral aspect the non 

 ampullated end of the lateral semicircular duct, and is prolonged upwards anc 

 backwards as the sinus superior, into which the crus commune of the superior anc 

 posterior semicircular ducts open. The ampulla of the posterior semicircular due' 

 opens into the lower and medial part, or sinus inferior. The floor and anterior wal 

 of the recessus utriculi are thickened to form the macula acustica utriculi, to whicl 

 the utricular fibres of the acoustic nerve are distributed. Whitish in colour, am 

 of an oval or nearly rhombic shape, this macula measures 3 mm. in length am 

 2-3 mm. in its greatest breadth. 



Sacculus. The saccule occupies the recessus sphaericus, in the lower and anterio 

 part of the vestibule (Fig. 717). Smaller than the utricle, it is of an oval shape an< 

 measures 3 mm. in its longest, and about 2 mm. in its shortest diameter. It present 



