522 AUDITORY ORGANS OF THE MAMMALIA. 



cochlear canal and the semicircular canals become invested with 

 cartilage. The recessus labyrinthi remains however still enclosed 

 in undifferentiated mesoblast 



Between the cartilage and the parts which it surrounds there 

 remains a certain amount of indifferent 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 opening to be divided into two branches one 

 towards the utriculus and the other towards the sacculus hemi- 

 sphericus ; 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. 305). 



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 developed. Above the ridge there appears a 

 delicate cuticular membrane, the membrane of Corti or mem- 

 brana tectoria. 



The epithelial walls of the utricle, 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 fluid present in this 

 cavity is known as the endolymph. 



In the mesoblast lying between these parts and the cartilage, 

 which at this period envelopes them, lymphatic spaces become 

 established, which are partially developed in the Sauropsida, but 

 become in Mammals very important structures. 



They consist in Mammals partly of a space surrounding the 

 utricle and semicircular canals, and partly of two very definite 

 channels, which largely embrace between them the cochlear canal. 

 The latter channels form the scala vestibuli on the upper side 

 of the cochlear canal and the scala tympani on the lower. The 

 scala vestibuli is in free communication with the lymphatic cavity 

 surrounding the vestibule, and opens at the apex of the cochlea 



