THE MEMBRANOUS LABYEINTH. 



647 



462). At the ampullas they are thicker and less translucent than in the 

 rest of their extent, and nearly fill their bony cases. That part of each 

 which is towards tlie concavity of the semicircle of the canal is free ; 

 whilst the opposite portion is fixed to the wall of the bony canal ; and 

 in the ampulla this part is flattened, receives branches of nerves and blood- 

 vessels, and presents on its inner surface a transverse projection, spptum 

 transversiim or crista acuslka, which partly divides the cavity into two. 

 Auditory nerve : vestibular division. — At the bottom of the 

 meatus auditorius internus the auditory nerve divides into an anterior 

 and a posterior branch, which, broken up into minute filaments, pass 

 through the perforations of the cribriform ])late which separates the 

 meatus from the internal ear, and are distributed respectively to the 

 cochlea and vestibule. In both branches, as well as in the trunk, there 

 are numerous nerve-cells. The vestibular nerve (fig. 45!)) divides into 

 five branches, which proceed respectively to the utricle, the saccule, and 

 the three ampullar of the semicircular canals ; those for the utricle and 

 the superior and external semicircular canals enter the cavity in a group 

 along the crista vestibuli ; the fibrils for the sacculus {q) enter the ves- 

 tibule by a smaller group of foramina, which are situated below those 

 just described, and open at the bottom of the fovea hemispherica ; the 

 branch for the posterior semicircular canal is long and slender, and 

 traverses a small passage in the bone behind the foramina for the nerve 



Fis. 461. 



Fig. 461.— Section op one of the Human Semicircular Canals (Riidinger). 



Magnified. 

 1, Osseous wall ; 2, fibrous bands with included blood-vessels, united at 3 with the 

 periosteum ; 4, membranous canal with its three layers ; 5, short fibrous bands (wiUi 

 intervening spaces) uniting tlie membranous canal firmly to the periosteum ; 6, union of 

 its outermost layer with the periosteum. 



