THE SENSE ORGANS 



603 



By the elongation and spiral twisting of the ventral part of the vesicle, the 

 cochlear duct is formed. 



The dorsal portion of the vesicle, following the outgrowth of the 

 endolymphatic duct, becomes the utriculus. The two vertical canals 

 develop from a single dorsal hollow outpocketing of the utriculus; but 

 the lateral hollow outgrowth from which the horizontal semicircular duct 

 develops arises later, as might be expected from its phylogenetic history. 

 Each semicircular duct is formed by a partial fusion of the lateral walls 

 of its hollow anlage, by which the cavity is obliterated except in its 

 periphery, where it persists as the cavity of the definitive duct connected 



ABC D 



Fig. 499. — 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 vestib- 

 ulare 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.") 



with the utriculus at each end. At one of these ends the duct swells 

 out into an ampulla. 



That portion of the membranous sac which is intermediate between 

 the utriculus and the cochlea is converted into the sacculus. Its separa- 

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

 finally reduces the connexion to the slender utriculosaccular 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 ganglion 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 



