542 



THE HUMAN EMBEYO. 



already separated from the vesicle, except at their ends, while 

 the horizontal canal, EH, is still a wide, flattened, pouch-like 

 outgrowth from the vesicle. From the sacculus, the cochlea, 

 EL, arises as a short, blunt, anteriorly directed outgrowth. 

 The recessus labyrinthi, ER, is much larger than before, and is 

 divided at its lower part by a partition into two tubes, of which 

 one opens into the sacculus and the other into the utriculus ; 

 these tubes affording the sole communication between the two 

 chambers, sacculus and utriculus, of the auditory vesicle. 



EV EH 



Fig. 229. 



Fig. 230. 



Fig. 229. — The left auditory vesicle of a Human Bmhryo four weeks old, seen 

 from the outer surface icf. Fig. 227). From W. His, jun. x 35. 



Fig. 230. — The left auditory vesicle of a Human Embryo five weeks old, seen 

 from the outer surface. From W. His, jun. x 35. 



EA, anterior vertical semicircitlar canal. ED, common stem of tlie two verticnl 

 semicircular canals. EH, horizontal or external semicircular canal. EL, cocUea. 

 EP, posterior vertical semicircular canal. EB, recessus labyi'iutlii. EV, auditoiy 

 vesicle. S, sacculus. UT, utriculus. 



By the eighth week, the shape and proportions of the internal 

 ear are as shown in Fig. 231. The sacculus, S, and utriculus, 

 UT, are now comparatively small parts of the internal ear. 

 The semicircular canals have greatly increased in length ; and 

 the cochlea, EL, has grown enormously, and is rolled up spirally 

 on itself. 



The auditory nerve, as already noticed (p. 536), very early 

 becomes continuous with the auditory epithelium : it soon 



