whic 



DEVELOPMENT OF LABYEINTH. 



853 



which go through the foramina in the recessus cochlearis and are distributed to the 

 hair cells of the vestibular part of the ductus cochlearis. On this vestibular branch, 

 close to its origin from the cochlear nerve, is a minute ganglion (Bcettcher). 



N. Vestibuli. The vestibular nerve is distributed to the utricle, the saccule, and 

 the ampullae of the semicircular ducts. It divides into three branches, superior, 

 inferior, and posterior, and each of these splits into filaments which pass through 

 foramina in the fundus of the internal acoustic meatus. The filaments from the 

 superior branch go through the foramina in the area vestibularis superior and 

 supply the macula of the utricle and the cristae ampullares of the superior and 

 lateral semicircular ducts ; those from the inferior branch run through the fora- 

 mina in the area vestibularis inferior to the macula of the saccule. The posterior 

 branch passes through the foramen singulare, and its filaments, six to eight in 

 number, are distributed to the crista ampullaris of the posterior semicircular duct. 



Ganglion Vestibular e. On the trunk of the vestibular nerve, within the 

 internal acoustic meatus, is a ganglion, the vestibular ganglion, of bipolar nerve cells ; 

 the fibres of the nerve arise from the cells of this ganglion. Sometimes the 

 vestibular nerve divides on the proximal side of the ganglion and the latter is then 

 split into three parts, one on each of the three branches of the nerve. 



Vessels of the Internal Ear. The internal auditory artery, a branch of the basilar, enters the 

 internal acoustic meatus and divides into vestibular and cochlear branches. The vestibular branch 

 supplies the soft tissues in the vestibule and semicircular canals, each canal receiving two arteries, 

 which, starting from opposite extremities of the canal, anastomose on the summit of the arch. 

 The cochlear branch divides into numerous twigs, which enter the foramina in the tractus 

 spiralis foraminosus, and run outwards in the lamina spiralis ossea to reach the soft structures ; 

 the largest of these arteries runs in the canalis centralis. The stylo-mastoid artery also supplies 

 some minute branches to the cochlea. Siebenmann describes the internal auditory artery as 

 dividing into three branches, viz. : (1) anterior vestibular, (2) cochlear proper, and (3) vestibulo- 

 cochlear. The veins from the cochlea and vestibule unite, at the bottom of the meatus, with the 

 veins from the semicircular canals to form the internal auditory vein, which may open either into 

 the posterior part of the inferior petrosal sinus or into the transverse sinus. Small veins also pass 

 through the aquseductus cochleae and aqueeductus vestibuli, the former opening into the inferior 

 petrosal sinus or into the internal jugular vein, the latter into the superior petrosal sinus. 



DEVELOPMENT OF LABYRINTH. 



The epithelial lining of the labyrinth is derived from an invagination of the cephalic 

 ectoderm, termed 

 the auditory pit, 



which appears Auditory pit 



opposite the hind 

 brain immedi- 

 ately above the 

 first visceral cleft. 

 The mouth of the 

 pit is closed by 

 the growing to- 

 gether of its mar- 

 gins, and it then 

 assumes the form 

 of a hollow 

 vesicle, the otic 

 vesicle, which 

 severs its con- 

 nexion with the 

 ectoderm and 

 sinks into the 

 subjacent meso- 

 derm. The vesicle 

 soon becomes 

 pear-shaped; and 

 its dorsal taper- 

 ing part rapidly 

 lengthens into a recess, the recessus labyrinth!, which later forms the ductus and saccus 



Otic vesicle 



Rudiment of ductus cochlearis- 



FIG. 728. SECTIONS THROUGH THE EEGION OF THE HIND BRAIN OF FCETAL BABBITS 

 (to illustrate the development of the labyrinthine epithelium). 



In A the ectoderm is invaginated to form the auditory pit ; in B the auditory pit is closed 

 and detached from the ectoderm, forming the otic vesicle ; while C shows a further 

 stage in the development of the vesicle. 



