THE INTERNAL EAR. 



861 



apex, and of a delicate lamina (the lamina spiralis ossea} -which projects from the 

 modiolus, and, following the windings of the canal, partially subdivides into two. 

 In the recent state certain membranous layers are attached to the free border of 

 this lamina, which project into the canal and completely separate it into two 

 passages, which, however, communicate with each other at the apex of the modiolus 

 by a small opening, named the helicotrema. 



The modiolus or columella is the central axis or pillar of the cochlea. It is 

 conical in form, and extends from the base to the apex of the cochlea. Its base 

 is broad, and appears at the bottom of the internal auditory meatus, where it 

 corresponds with the area cochleae ; it is perforated by numerous orifices, which 

 transmit filaments of the cochlear division of the auditory nerve, the nerves for 

 the first turn and a half being transmitted through the foramina of the tractus 

 spiralis foraminosus ; the fibres for the apical turn passing up through the foramen 

 centrale. The foramina of the tractus spiralis foraminosus pass up through the 

 modiolus and successively bend outward to reach the attached margin of the lamina 

 spiralis ossea. Here they become enlarged, and by their apposition form a spiral 

 canal (canalis spiralis modioli), which follows the course of the attached margin 

 of the lamina spiralis ossea and lodges the ganglion spirale. The foramen centrale 

 is continued as a canal up the middle of the modiolus to its apex. The axis 

 diminishes rapidly in size in the second and succeeding coil. 



The bony canal of the cochlea (Fig. 461) takes two turns and three-quarters 

 round the modiolus. It is a little over an inch in length (about 30 mm.), and 



FIG. 45!. The cochlea laid open. (Enlarged.) 



diminishes gradually in size from the base to the summit, where it terminates in 

 a cul-de-sac, the cupola, which forms the apex of the cochlea. The commence- 

 ment of this canal is about the tenth of an inch in diameter ; it diverges from the 

 modiolus toward the tympanum and vestibule, and presents three openings. One, 

 the fenestra rotunda, communicates with the tympanum ; in the recent state this 

 aperture is closed by a membrane, the membrana tympani secundaria. Another 

 aperture, of an elliptical form, enters the vestibule. The third is the aperture of 

 the aquseductus cochleae, leading to a minute funnel-shaped canal, which opens on 

 the basilar surface of the petrous bone and transmits a small vein, and also forms 

 a communication between the subarachnoidean space of the skull and the 

 perilymph contained in the seal a 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 tAvo 

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

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

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

 the scala tympani. Near the summit of the cochlea the lamina terminates in a 

 hook-shaped process, the hamulus, which assists to form the boundary of a small 

 opening, the helicotrema, by which the two scalse 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 



