XII.] THE MEMBRANOUS LABYEINTH. 395 



semiciroular canal ; 6. canalis reiiniens ; a. constriction by 

 means of which the saccnliis hemispherious S.B. is formed ; 

 /. narrowed opening between sacoulus hemispherious and 

 utriculus ; C.G. cochlea ; C.C^. lumen of cochlea ; K.K. 

 cartilaginous capsule of cochlea ; K.B. basilar plate ; Ch. 

 notochord. 



connective tissue, which is more abundant around the 

 cochlear canal than around the semicircular canals. 



As soon as they have acquired a distinct connective- 

 tissue coat, the semicircular canals begin to be dilated 

 at one of their terminations to form the ampullae. At 

 about the same time a constriction appears opposite the 

 mouth of the recessus labyrinthi, which causes its open- 

 ing to be divided into two branches — one towards the 

 utriculus and the other towards the sacculus hemispheri- 

 cus ; and the relations of the parts become so altered 

 that communication between the sacculus and utriculus 

 can only take place through the mouth of the recessus 

 labyrinthi (Fig. 132). 



When the cochlear canal has come to consist of two 

 and a half coils, the thickened epithelium which lines 

 the lower surface of the canal forms a double ridge 

 from which the organ of Corti is subsequently de- 

 veloped. Above the ridge there appears a delicate 

 cuticular membrane, the membrane of Corti or mem- 

 brana tectoria. 



The epithelial walls of the utricle, the saccule, the 

 recessus labyrinthi, the semicircular canals, and the 

 cochlear canal constitute together the highly complicated 

 product of the original auditory vesicle. The whole 

 structure forms a closed cavity, the various parts of 

 which are in free communication. In the adult the 



