OSSEOUS LABYRINTH. 



845 



varying distance, they bend outwards to reach the attached edge of the lamina 

 spiralis ossea, where they expand and form by their apposition a spiral canal, the 

 canalis spiralis cochleae, which lodges the ganglion spirale cochleae. From this spiral 

 canal numerous small foramina, for the transmission of vessels and nerves, pass 

 outwards to the free edge of the lamina spiralis ossea. The lamina spiralis ossea, 

 a thin, flat shelf of bone, winds round the modiolus like the thread of a screw, and, 

 projecting about half-way into the cochlear tube, incompletely divides it into two 

 passages an upper is named the scala vestibuli ; a lower, the scala tympani. The 

 lamina spiralis ossea begins at the floor of the vestibule, near the fenestra cochleae, 

 and ends close to the apex of the cochlea in a sickle-shaped process, the hamulus 

 laminae spiralis, which assists to bound an aperture named the helicotrema. In the 



Section through promontory 



Lamina spiralis 

 ossea secundaria 

 Fissura vestibuli 

 Lamina spiralis ossea 



Canalis centralis 

 Canalis spiralis cochleae 



Modiolus 

 Scala vestibuli 



Lamina spiralis ossea 

 Scala tympani 



Tractus spiralis 

 foraminosus 



Recessus cochlearis of vestibule Internal acoustic meatus 



FIG. 718. SECTION OF BONY COCHLEA. 



basal coil the upper surface of the lamina spiralis ossea forms almost a right angle 

 with the modiolus, but the angle becomes more and more acute on ascending the 

 tube. In the lower half of the basal coil a second smaller bony plate, the lamina 

 spiralis secundaria, projects from the outer wall of the cochlea towards the lamina 

 spiralis ossea, without, however, reaching it. If viewed from the vestibule the 

 slit-like fissura vestibuli, already referred to (p. 844), is seen between the two 

 laminae. A membrane, the membrana basilaris, stretches from the free edge of the 

 lamina spiralis ossea to the outer wall of the cochlea, and completes the septum 

 between the scala vestibuli and scala tympani, but the two scales communicate 

 with each other through the opening of the helicotrema at the apex of the cochlea. 

 The scala tympani begins at the fenestra cochleae, which is closed by the secondary 

 tympanic membrane (vide p. 833). At the commencement of the scala tympani 

 a crest, termed the crista semilunaris, stretches 

 from the attached margin of the lamina Area vestibuiaris superior 



spiralis ossea towards the orifice of the fenestra 

 cochleae. Close to this crest is seen the inner 

 orifice of the aquaeductus cochleae, a canal 

 measuring from 10 to 12 mm. in length, and 

 opening on the under surface of the petrous 

 part of the temporal bone medial to the 

 fossa jugularis. Through it a communica- 

 tion is established between the scala tympani 

 and the subarachnoid cavity, and through 

 it, also, a small vein passes to join the inferior 

 petrosal sinus. The scala vestibuli, the higher 



Area n. facialis 



Foramen singulare 



Tractus spiralis 



foraminosus 

 Foramen centrale 



of the two passages, begins in the vestibule ; 



its diameter in the basal coil is less than that Area vestibularis infcrior 



of the scala tympani, but in the upper coils 



it exceeds that of the scala tympani. 



Meatus Acusticus Internus. It is 

 convenient, at this stage, to study the fundus of the internal acoustic meatus, 

 which has been referred to as forming the medial wall of the vestibule and the 



AREAS BY THE CRISTA TRANSVERSA. 



