430 AUDITORY ORGAN OF THE MAMMALIA. 



original otic vesicle, into which all the above parts open, the utri- 

 culus. 



Soon after the formation of the sacculus hemisphericus, the coch- 

 lear canal and the semicircular canals become invested with cartilage. 

 The recessus labyrinthi remains however still enclosed in undifferen- 

 tiated mesoblast. 



Between the cartilage and the parts which it surrounds there 

 remains a certain amount of inditferent connective tissue, which 

 is more abundant around the cochlear canal than around the semi- 

 circular canals. 



As soon as they have acquired a distinct connective-tissue coat, 

 the semicircular canals begin to be dilated at one of their termina- 

 tions 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 hemisphericus ; and the relations 

 of the parts become so altered that communication between the sac- 

 culus 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 subse- 

 quently developed. Above the ridge there appears a delicate cuticu- 

 lar membrane, the membrane of Corti or membrana 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 esta- 

 blished, 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 

 into the scala tympani. The latter ends blindly at the fenestra 

 rotunda. 



The fluid contained in the two scalae, and in the remaining lym- 

 phatic cavities of the auditory labyrinth, is known as perilymph. 



The cavities just spoken of are formed by an absorption of parts 

 of the embryonic mucous tissue between the perichondrium and the 

 walls of the membranous labyrinth. 



