THE INTERNAL EAR OR LABYRINTH 1051 



I to reach the attached margin of the lamina spiralis ossea. Here they become 

 enlarged, and by their apposition form the spiral canal of the modiolus, which 

 follows the course of the attached margin of the osseous spiral lamina and lodges 

 the spiral ganglion (ganglion of Corti). The foramen centrale is continued into 

 a canal which runs up the middle of the modiolus to its apex. The modiolus 

 diminishes rapidly in size in the second and succeeding coil. 



The bony canal of the cochlea takes two turns and three-quarters around the 

 modiolus. It is about 30 mm. in length, and diminishes gradually in diameter 

 from the base to the summit, where it terminates in the cupula, which forms the 

 apex of the cochlea. The beginning of this canal is about 3 mm. in diameter; 

 it diverges from the modiolus toward the tympanic cavity and vestibule, and 

 presents three openings. One, the fenestra cochleae, communicates with the tym- 

 panic cavity in the fresh state this aperture is closed by the secondary tympanic 

 membrane; another, of an elliptical form, opens into the vestibule. The third is the 

 aperture of the aquseductus cochlese, leading to a minute funnel-shaped canal, 

 which opens on the inferior surface of the petrous part of the temporal bone 

 and transmits a small vein, and also forms a communication between the 

 subarachnoid cavity and the scala tympani. 



The osseous spiral lamina (lamina spiralis ossea) is a bony shelf or ledge which pro- 

 jects from the modiolus into the interior of the canal, and, like the canal, takes two- 

 and three-quarter turns around the modiolus. It reaches about half-way toward 

 the outer wall of the tube, and partially divides its cavity into two passages or 

 scalse, of which the upper is named the scala vestibuli, while the lower is termed 

 the scala tympani. Near the summit of the cochlea the lamina ends in a hook- 

 shaped process, the hamulus laminae spiralis; this assists in forming the boundary 

 of a small opening, the helicotrema, through which the two scalse communicate 

 with each other. From the spiral canal of the modiolus numerous canals pass out- 

 ward through the osseous spiral lamina as far as its free edge. In the lower part 

 of the first turn a second bony lamina, the secondary spiral lamina, projects inward 

 from the outer wall of the bony tube; it does not, however, reach the primary 

 osseous spiral lamina, so that if viewed from the vestibule a narrow fissure, the 

 vestibule fissure, is seen between them. 



The osseous labyrinth is lined by an exceedingly thin fibro-serous membrane; 

 its attached surface is rough and fibrous, and closely adherent to the bone; its 

 free surface is smooth and pale, covered with a layer of epithelium, and secretes 

 a thin, limpid fluid, the perilymph. A delicate tubular process of this membrane 

 is prolonged along the aqueduct of the cochlea to the inner surface of the dura 

 mater. 



The Membranous Labyrinth (labyrinthus membranaceus) (Figs. 924, 925, 926). 

 The membranous labyrinth is lodged within the bony cavities just described, 

 and has the same general form as these; it is, however, considerably smaller, and 

 is partly separated from the bony walls by a quantity of fluid, the perilymph. In 

 certain places it is fixed to the walls of the cavity. The membranous labyrinth 

 contains fluid, the endolymph, and on its walls the ramifications of the acoustic 

 nerve are distributed. 



Within the osseous vestibule the membranous labyrinth does not quite preserve 

 the form of the bony cavity, but consists of two membranous sacs, the utricle, 

 and the saccule. 



The Utricle (utriculus) . The utricle, the larger of the two, is of an oblong form, 

 compressed transversely, and occupies the upper and back part of the vestibule, 

 lying in contact with the recessus ellipticus and the part below it. That portion 

 which is lodged in the recess forms a sort of pouch or cul-de-sac, the floor and ante- 

 rior wall of which are thickened, and form the macula acustica utriculi, which receives 

 the utricular filaments of the acoustic nerve. The cavity of the utricle communi- 



