INTERNAL EAR 



147 



come together and coalesce, except near the circumference of the semicircle, which 

 now forms a tube connected at its ends with the vesicle. Subsequently, by 

 absorption of the coalesced lamellae, the tube is converted into a free loop. 

 One of the ends becomes dilated into an ampulla and connected with a branch 

 of the auditory nerve. In consequence of the manner in which the two 

 vertical canals arise from the upper vertical fold they are at first in a line with 

 on> another, but as they take form they come by differential growth to be 



p.s.c. 



FIG. 189. MODEL OP THE MEMBBANOUS LABYBINTH OF A HUMAN EMBRYO OF 80 MM. 



(After Streeter.) 



u, utricle ; s, saccule ; cc, cochlea ; r.l., recessus labyrinth! (aqueductus vestibuli) ; crus, common 

 opening of superior and posterior semicircular canals; sin, sinus utriculi lateralis; a.s.c. superior 

 semicircular canal: ani]).a, its ampulla; p.s.c., posterior semicircular canal: amp.p, its ampulla; 

 l.s.c. horizontal semicircular canal; vr, vestibular division; cr, cochlear division of auditory nerve'; 

 br, branch from vestibular division of nerve to ampulla of the posterior semicircular canal. 



placed at right-angles to one another, the ampullary end of the superior 

 retaining its original position. While the semicircular canals are forming, 

 the ventral cochlear portion begins to grow out and become curved on itself, 

 while the atrium becomes subdivided by a fold into an upper and posterior 

 chamber connected with the semicircular canals, the utricle, and a ventral and 

 anterior connected with the cochlea, the saccule. This fold extends into the 

 attachment of the recess of the labyrinth and separates it longitudinally for a 

 short distance into two tubes, one of which opens into the utricle and the other 



L2 



