108 



THE EAR. 



the middle ; and beyond each rounded end of the crista is a crescent-shaped 

 edge (covered by columnar epithelium) which has been termed planum semi- 

 hmatum (fig. 119, C. D.). 



Branches of the eighth nerve. Within the internal auditory meatus the 

 eighth nerve divides into two branches, which, broken up into minute filaments, 

 pass through perforations in the plate of bone 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 superior 

 division (fig. 118, 8, 9, 10), which is at first also anterior in position (ramus anterior 

 of Retzius), and is separated by a crest of bone from the other division below it, gives 

 off three branches, which proceed respectively to the utricle and the ampullae of the 

 superior and external semicircular canals, entering the vestibular cavity in a group 



falciform orifice for orifices for vestibular division 

 ridge facial nerve of eighth nerve 



\^$$/fi* 



i^/turfc 



apex of petrous 



cent. orif. foram. vest, scala fossa 



canal for sing, 

 foram. of mo- nerve of 

 diolus saccule 



tymp. ovalis 



Fig. 120. THE PETROSAL SAWN THROUGH IN TWO PLANES, so AS TO SHO\V THE EXTREMITY OF THE 



INTERNAL AUDITORY MEATUS AND THE VESTIBULE. (Testllt. ) 



along the crista vestibuli. This superior division has a ganglion upon it whilst still 

 within the internal auditory meatus (ganglion of Scarpa) and the ganglion- cells 

 also extend for some distance along the branches within the canals in the bone. 

 The inferior division on the other hand (ramus posterior of Retzius) (Q, which is at 

 first behind as well as below the vestibular division, gives off, besides the cochlear 

 branch which enters the cochlea by the tractus foraminulentus, a branch for the 

 saccule which enters the vestibule by a small group of foramina, opening at the 

 bottom of the fovea hemispherica and a branch for the posterior semicircular 

 canal ; this is long and slender, and traverses a small passage in the bone (foramen 

 singulars} behind the foramina for the nerve of the saccule. The part of the 

 inferior division which gives the branches to the saccule and posterior canal is 

 distinguished by Schwalbe as the ramus medius. The branch to the saccule also 

 has a ganglion upon it within the meatus and that to the posterior semicircular 

 canal a ganglion near its ending in the ampulla (ganglion of Corti), whereas the 



