424 



CHORDATE ANATOMY 



tion from the utriculus is initiated by a horizontal constriction, which 

 finally reduces the connexion to the slender utriculo-saccular duct. By a 

 similar constriction, the connexion between the cochlear duct and the 

 sacculus becomes a ductus reuniens. 



From the beginning, the otic vesicle is directly attached to that part 

 of the neural crest which forms the ganghon of the facial nerve. This 

 connexion is retained by the auditory nerve, which arises as a branch 

 of the facial, although the roots of the two subsequently become separated. 

 Throughout the changes which convert the otic vesicle into the membra- 

 nous labyrinth, the connexions of the auditory nerve are retained. Two 

 main branches are differentiated, the vestibular, connected with utriculus 



ABC D 



Fig. 372. — Lateral or external surfaces of models of the membranous portion of the 

 left internal ear from human embryos. Different enlargements. A, from an embryo 

 of 6.9 mm.; B, 10.2 mm.; C, 13.5; and D, 22 mm.; am., ampulla; c.v., cecum vestibulare 

 of d.c, cochlear duct; d.e., endolymphatic duct; d.s.L, d.s.p., and d.s.s., lateral, posterior, 

 and superior semicircular ducts; sac, sacculus; ut., utriculus. (After His, Jr.) (From 

 Bremer's "Text Book of Histology.") 



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 

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



