6o4 



COMPARATIVE ANATOMY 



and semicircular canals, and the cochlear, which innervates the hair-cells 

 of Corti's organ. 



Most of the membranous labyrinth retains throughout life its primitive 

 epithelial character. In local patches, however, it is converted into 

 sensory columnar epithelium, the cells of which are connected with 

 branches of the auditory nerve. Each ampulla develops a ridge-like 

 crista composed of hair cells. The utriculus and sacculus develop larger 

 cushion-like maculae. One side of the cochlear duct is modified as the 

 spiral organ of Corti. 



The development of the tectorial membrane, like its function, has 

 long been a controversial question. Primarily, this membrane is closely 



Fig. 500. — Diagram of developing human labyrinth from 6 to 30 mm. long, am, 

 ampulla; c, cochlear region and cochlea; au, ampullo-utricular region; d, endolymph 

 duct; e, endolymph region; sc, semicircular canal; 5^, endolymph sac; 5, sacculus; u, 

 utriculus; us, utriculo-saccular canal; v, vestibule. (From Kingsley's "Comparative 

 Anatomy of Vertebrates," after Streeter.) 



attached to the columnar epithelium from which the organ of Corti arises. 

 The intimacy of the connexion is gradually reduced during ontogenesis, 

 so that eventually the membrane loses its connexion with the cells of 

 the spiral organ and floats above them in the endolymphatic fluid. 



The differentiation of the elements of the spiral organ is not completed 

 at the time of birth, so that an infant is stone deaf until it is some weeks 

 old. 



The mesenchyma surrounding the membranous labyrinth becomes 

 differentiated into an inner connective tissue membrane which lies in 

 contact with the epithelium of the sac, and an outer cartilaginous capsule 

 which encloses the membranous ear. Later, the portion of the cartilage 

 near the membranous sac disappears and is replaced by liquid perilymph 

 around the sac. As the cochlear duct develops, it becomes triangular in 

 cross section. 



