THE AUDITORY NERVE. 



821 



The SACCULE (fig. 305, a) is a smaller and rounder cyst than the 

 utricle, in front of which it is placed, in the hollow of the fovea 

 hemispherica. It communicates with the utricle through the ductus 

 endolymphaticus, and is continuous below by a short and small 

 passage (canal i* /v///m//x; c] with the canal of the cochlea. Like 

 the other sac, it possesses a macula (6) and otoliths where the 

 nerves enter. 



The ductus endolymphaticus is a fine tubular offset of the mem- 

 branous labyrinth, which occupies the aqueduct of the vestibule, 

 and ends in a dilated blind extremity (saccus endolymphaticus), 

 embedded in the dura mater 

 on the posterior surface of the 

 petrous portion of the temporal 

 bone. In the vestibule the duct 

 divides into two small branches, 

 one of which joins the saccule, 

 and the other the utricle. 



For an account of the minute 

 structure of the membranous 

 labyrinth, reference must be 

 made to a work on microscopic 

 anatomy. 



NERVE OF THE LABYRINTH. A 

 special nerve, the eighth cranial 

 or auditory, is distributed to the 

 labyrinth. Entering the internal 

 auditory meatus with the facial 

 nerve, it divides into an upper 

 smaller, and a lower larger piece, 

 each of which again subdivides 

 into three branches. At the 

 bottom of the meatus, the upper 

 piece is marked by a ganglionic 

 swelling the intumescentia gan- 

 glioformis of Scarpa. 



The upper division of the nerve 

 sends its branches to the macula 

 of the utricle (fig. 306, c), to the 



ampulla of the superior vertical semicircular canal, and to the 

 ampulla of the external semicircular canal. 



From the lower division of the nerve proceed an offset to the 

 saccule (a) and a slender branch to the ampulla of the posterior 

 vertical semicircular canal (6) ; but the greater part is destined for 

 the cochlea. 



Each of the branches of the auditory nerve breaks up into a 

 bundle of filaments, which pass through minute apertures of the 

 lamina cribrosa, to reach their special part of the membranous 

 labyrinth. The nerves of the semicircular canals enter the ampullae 

 on their outer flattened side, and end in the crista acustica ; while 

 those of the sacs end in the respective maculae. 



Saccule 

 has com- 

 munications 

 with utricle 

 and cochlea; 

 macula and 

 otoliths. 



Endo- 

 lymphatic 

 duct 

 and sac ; 



FIG. 306. DISTRIBUTION OF NERVES 

 TO THE MEMBRANOUS LABYRINTH 

 (BRESCHET). 



ft. Nerve to the saccnle. 



b. Nerve entering the arapullary 

 enlargement on the posterior serai- 

 circular canal. 



c. Nerve entering the utricle. The 

 nerve to the cochlea is not repre- 

 sented. 



upper has 

 ganglion, 



and supplies 

 utricle, 

 superior and 

 external 

 canals : 



lower gives 

 branches to 

 saccule, 

 posterior 

 canal, and 

 cochlea ; 

 ending of 

 vestibular 

 branches ; 



