THE EAR. 



279 



of the cochlea is a lamina, or shelf, of bone — termed the osseous spiral 

 lamina. The tube is thus imperfectly divided into two passages, 

 termed respectively the scala tympani and the scala vestibuli. The 

 separation between these two passages is rendered more complete, and 

 a third passage is marked off, by certain membranous structures. 

 These are the basilar membrane and Reissner's membrane. The basilar 

 membrane stretches from the free edge of the osseous spiral lamina to 

 the outer wall of the tube, where it joins a thickening of the lining of 

 the tube, termed the spiral ligament. Reissner's membrane is much 



Fig. 39. 



Transverse Section through the Tube of the Cochlea. 



m. Modiolus; O. Outer wall of cochlea ; SV. Scala vestibuli ; ST. Scala tympani ; DC. Ductus 

 cochlearis ; mR. Membrane of Reissner ; bm. Basilar membrane ; cs. Critsa spiralis ; si. Spiral liga- 

 ment ; sg. Spiral ganglion of auditory nerve ; oc. Organ of Corti (Turner). 



more delicate, and stretches from the cristi spiralis at the free edge of 

 the osseous spiral lamina, obliquely upwards and outwards to the wall 

 of the tube. 



The tube is thus divided into three passages, viz., the scala tympani, 

 the scala vestibuli. and the scala intermedia. 



The Scala Tympani is the largest of the three passages, and is 

 separated from the other two by the osseous spiral lamina and the 

 basilar membrane. At the base of the cochlea it begins at the fenestra 



