1560 A MANUAL OF ANATOMY 



divides at its deep end into two branches — vestibular and cochlear. 

 The vestibular artery supplies the utricle, saccule, and semicircular 

 canals, and the cochlear artery supplies the cochlea. 



The veins of the labyrinth ultimately join, for the most part, to 

 form one vessel, called the internal auditory vein, which opens into 

 the inferior petrosal sinus. The aqueductus cochleae and the aque- 

 ductus vestibuli «ach transmit a vein, that which passes through 

 the former terminating in the inferior petrosal sinus or in the bulb 

 of the internal jugular vein, and that which passes through the latter 

 terminating in the superior petrosal sinus. 



Development of the Ear. 



Internal Ear — Membranous Labyrinth. — The membranous labyrinth is 

 developed from the surface-ectoderm in a manner similar to the development 

 of the crystalline lens. Over a circumscribed area, corresponding to the upper 

 end of the first visceral cleft, and upon the side of the hind-brain, the ecto- 

 derm becomes thickened and invaginated. A depression is thus formed, 

 which is called the auditory pit. This pit becomes deepened, its mouth 

 becomes constricted, and its lips, coming together, unite. The auditory pit 

 then becomes transformed into a closed sac, called the auditory, or otic, 

 vesicle, or otocyst. The auditory vesicle now becomes isolated from the 

 surface-ectoderm, and sinks into the adjacent mesoderm, taking up a position 

 close to the side of the hind-brain. 



The auditory vesicle, which is at first almost spherical, soon becomes pyri- 

 form, this being due to the formation of a process, called the recess of the- 

 labyrinth or vestibule, which is prolonged from its dorsal wall. As this process 

 lengthens it gives rise to the ductus endolymphaticns, which occupies the 

 aqueductus vestibuli of the petrous portion of the temporal bone. The 

 expanded terminal part of the ductus endolymphatious is called the saccus 

 endolymphaticns, and it lies underneath the dura mater. The ventral aspect 

 of the vesicle gives off a tapering diverticulum, which gives rise to the ductus 

 cochlearis, or scala media, of the cochlea. This duct describes a bend, within 

 which lies the cochlear ganglion of the cochlear division of the auditory nerve. 

 As the duct elongates, i't continues to bend in a spiral manner, and so the 

 ductus cochlearis is formed. The cochlear ganglion elongates, and follows 

 the spiral turns of the duct, from which circumstance the ganglion is known 

 as the ganglion spirale. 



The three semicircular canals are developed from tne upper or cephalic 

 part of the auditory vesicle, this part representing the future utricle. This 

 portion of the vesicle presents two folds — vertical and horizontal. From the 

 vertical fold the superior and posterior semicircular canals are formed, whilst 

 the horizontal fold gives rise to the external semicircular canal. 



The auditory vesicle now becomes transversely constricted, and is divided 

 into two sacs — large and small. The large sac is called the utricle, from which 

 the semicircular canals have just been developed. The small sac is called 

 the saccule, from which the ductus cochlearis has been developed. The 

 portion of this duct which communicates with the saccule becomes constricted, 

 and forms the canalis reuniens ol Hensen. 



The constriction which completely divides the auditory vesicle into utricle 

 and saccule also involves the vesicular end of the ductus endolymphatious, 

 and divides it into two ducts, called the ductus utriculi and ductus sacculi. 

 This arrangement constitutes the only communication which now exists 

 between the utricle and the saccule. 



As stated, the cochlear ganglion, or ganglion spirale, of the cochlear division 

 of the auditory nerve Ues witliin the spiral turns of the ductus cochlearis. 

 As the osseous cochlea becomes formed, the ganglion spirale comes to occupy 

 the spiral canal of the modiolus. The vestibular ganglion of the vestibular 



