THE EAR. 271 



of the cochlea is a lamina, or shelf, of bone — tei'med the osseous spiral 

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

 respectively the scala timpani and the scala vestibuli. The separation 

 between these two passages is rendered moi-e complete, and a third 

 passage is marked off, by certain membranous structures. These are 

 the basilar membr^ane and Beissner'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 Sjyiral ligament. Reissnerh Membrane is much more delicate, 

 and stretches from the adsta spiralis at the free edge of the osseous 

 spiral lamina, obliquely upwards and outwards to the wall of the tube. 



Fig. 35. 

 Transverse Section through the Tube of the Cochlea. 

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

 cochlearis ; »)iR. Membrane of Reissner ; bm. Basilar membrane ; sc. Crista spiralis ; si. Spiral liga- 

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



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 

 rotunda, by which, in the dried bone, it communicates Avith the 



