1142 THE ORGANS OF SPECIAL SENSE 



The bony canal or the spiral canal of the cochlea (canalis spiralis cochleae') (Fig. 

 849) takes two turns and three-quarters around the modiolus. The first turn 

 of the canal is called the basal coil, the second is called the central coil, the third 

 turn is called the apical coil. The promontory on the inner wall of the tympanic 

 cavity is caused by the basal coil. The bonv canal of the cochlea is a little over 

 an inch (about 30 mm.) in length, and diminishes gradually in size from the base 

 to the summit, where it terminates in the cupola (cupula), which forms the apex of 

 the cochlea. The commencement of this canal is about a tenth of an inch (2.5 

 mm.) in diameter: it diverges from the modiolus toward the tympanum and vesti- 

 bule, and presents three openings. One, the fenestra rotunda, communicates with 

 the tympanum; in the recent state this aperture is closed by a membrane, the mem- 

 brana tympani secundaria. Another aperture, of an elliptical form, opens into the 

 vestibule. The third is the aperture of the aquaeductus cochleae, leading to a 

 minute funnel-shaped canal, which opens on the basilar surface of the petrous 

 bone internal to the jugular fossa, and transmits a small vein, and also forms 

 a communication between the subarachnoidean space of the skull and the peri- 

 lymph space in the scala tympani. 



The lamina spiralis ossea is a bony shelf or ledge which projects outward from 

 the modiolus into the interior of the spiral canal, and, like the canal, takes nearly 

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

 toward the outer wall of the spiral canal, and partially divides its cavity into two 

 passages or scalae, 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 terminates 

 in a hook-shaped process, the hamulus (hamulus laminae spiralis), which assists 

 in forming the boundary of a small opening, the helicotrema, by which the two scalae 

 communicate with each other. From the canalis spiralis modioli numerous 

 foramina pass outward through the osseous spiral lamina as far as its outer 

 or free edge. In the lower part of the first turn a second bony lamina (lamina 

 spiralis secundaria) 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 fissura vestibuli, is seen between them. 



The fundus of the internal auditory meatus is described on page 85. 



The Membranous Labyrinth (labyrinthus membranaceus) (Figs. 850 and 851) 

 is contained within the bony cavities just described, having the same general form 

 as the cavities in which it is contained, though considerably smaller, being separated 

 from the bony walls by a quantity of fluid, the perilymph (perilympha). It does 

 not, however, float loosely in this fluid, but in places is fixed to the walls of the 

 cavity. The membranous sac contains fluid, the endolymph (endolympha), and 

 on the sac the ramifications of the auditory nerve are distributed. 



Within the osseous vestibule the membranous labyrinth does not quite preserve 

 the form of the bony cavity, but presents two membranous sacs, the utricle and 

 the saccule. 



The Utricle (utricidus) is the larger of the two, of an oblong form, compressed 

 laterally, and .occupies the upper and back part of the vestibule, lying in contact 

 with the fovea semielliptica and the part below it. The highest portion of the 

 utricle is called the recess (recessus utricuH); it is placed in the elliptical recess, 

 and opening into it are the ampullae of the superior and lateral semicircular 

 canals. The central portion of the recess of the utricle receives upon the side the 

 lateral semicircular canal. This opening has no ampulla. The superior sinus is 

 a prolongation upward and backward from the central portion of the utricle and 

 in the superior sinus the crus commune and the superior and posterior semi- 

 circular canals open. The lower and inner portion of the utricle is the infeiior 

 sinus, and into it the ampulla of the posterior semicircular canal opens. The 

 floor and anterior wall of the recess of the utricle are much thicker than else- 



