368 HUMAN ANATOMY. 



The first turn of the spiral canal presents three openings : 



1. Fenestra rotunda, a circular aperture communicating 

 with the tympanum, and closed by the membrana tympanum 

 secondaria. 



2. Fenestra ovalis, an oval opening, communicating with 

 the vestibule. 



3. Aquceductus cochlearis, a small foramen opening on the 

 inferior surface of the petrous portion of the temporal bone, 

 and transmitting a small vein from the cochlea to the jugular 

 vein. 



Scala vestibuli, begins at the fenestra ovalis, and ascends 

 in a spiral course above the lamina spiralis and membrana 

 basilaris, to communicate with the scala tympani, through the 

 helicotrema within the vestibule. It contains perilymph. 



Scala tympani begins at the fenestra rotunda, and ascends 

 in a spiral course below the scala vestibuli, from which it is 

 separated by the osseo-membranous lamina. It also contains 

 perilymph. 



Helicotrema is an opening within the cupola, by which the 

 two scala communicate. It is formed by a deficiency of the 

 osseous lamina. 



Canalis spiralis modioli, is a small canal which winds around 

 the modiolus at the attachment of the osseous laminae. It con- 

 tains a gangliform swelling ganglion spirale from which 

 nerves pass to the organ of Corti. 



THE MEMBRANOUS LABYRINTH corresponds closely to the 

 osseous, the vestibule consisting of two membranous sacs the 

 utricle and saccule communicating with each other, and the 

 former receiving the five openings of the membranous semicir- 

 cular canals, the latter communicating with the membranous 

 cochlea through the canalis reuniens. The membranous labyrinth 

 is surrounded everywhere with perilymph, filled with endolymph, 

 and is composed of three coats an outer fibrous layer, middle 

 or tunica propria, and inner epithelial layer. 



Utricle,, the larger of the two, is an elliptical sac lodged in 

 the fovea hemi-elliptica. The semicircular canals communicate 

 with it by five openings. Its wall is the thickest near the crista 

 vestibuli, where the filaments of the nerve enter, the macula 

 acoustica. 



Saccule, is a hemispherical sac lodged in the fovea hemi- 

 spherica. It communicates with the cochlea through a small 

 duct the canalis reuniens. 



The otoliths, or otoconia, are small masses of crystals of 

 lime carbonate, contained in the inner wall of the utricle and 

 saccule opposite the distribution of the nerves. 



