DEVELOPMENT OF THE EAR. 



1523 



branch. The cochlear branch is distributed to the lower turn of the cochlea and 

 anastomoses with the cochlear artery proper. The vestibular branch is distributed 

 to the lower part of the vestibule, including the lower part of the saccule and utricle, 

 to the crus commune and part of the semicircular canals, and to the lower end of the 

 cochlea. According to Siebenmann, the macula of the saccule receives its arterial 

 supply from a blood-vessel which usually arises from the common stem of the vestib- 

 ulo-cochlear artery, or, more rarely, runs independently through the whole internal 

 meatus. A similar origin applies to the artery supplying the nerve of the posterior 

 ampulla. In the base of the spiral lamina the arteries are connected by capillary 

 loops especially in the lower turn of the cochlea. As mentioned above, one or more 

 spiral vessels are often seen under the tunnel of Corti within the tympanic covering 

 of the basilar membrane. The region of the stria vascularis and prominentia spiralis 

 are especially well supplied with blood-vessels. Those seen in the scala tympani are 

 principally veins, while a larger number of arteries are found in the scala vestibuli. 

 The blood-supply of the lower turn of the cochlea is much more generous than that 

 of the others. 



The veins by which the blood escapes from the cochlea include : ( i ) the vein 

 of the vestibular aqueduct, which empties into the superior petrosal sinus ; (2) the 

 vein of the cochlear aqueduct, which empties into the internal jugular and (3) the 

 venous plexus of the inner auditory canal, which empties either into the transverse or 

 inferior petrosal sinus. The first of these channels collects the blood from the semi- 

 circular canals; the second from the whole cochlear canal through the anterior, pos- 

 terior and middle spiral veins and from most of the vestibule through the anterior 

 and posterior vestibular veins. The veins of the internal auditory canal form collat- 

 erals to the other veins of the labyrinth and receive the large central cochlear vein 

 (Siebenmann), which leaves the cochlea near the border of the central foramen of the 

 modiolus, as well as tributaries corresponding to the branches of the acoustic nerve. 



FIG. 1276. 



Hind-brain 



THE DEVELOPMENT OF THE EAR. 



The development of the ear includes the formation of two morphologically distinct divis- 

 ions, the membranous labyrinth, the essential auditory structure, and the accessory parts, com- 

 prising the middle ear, with its ossicles and associated cavities, and the external auditory canal 

 and the auricle. The developmental history 

 of the organ of hearing proper in its early 

 stages is largely an account of the growth and 

 differentiation of the ectoblastic otic vesicle, 

 since from this is produced the important 

 membranous tube, the enveloping fibrous 

 and osseous structures being comparatively 

 late contributions from the mesoblast. 



Development of the Labyrinth. The 

 internal ear appears as a thickening and 

 soon after depression of the ectoblast within 

 a small area on either side of the cephalic 

 end of the neural tube, at a level correspond- 

 ing to about the middle of the hind-brain 

 (Fig. 1276). 



This depression, the auditory pit, is 

 widely open for a considerable time and 

 distinguished by the greater thickness of 

 its depressed wall, which contrasts strongly 

 with the adjacent ectoblast. After a time 

 the lips of the pit approximate until, by 

 their final union, the cup-like depression is converted into a closed sac, the otic vesicle. 



This sac, after severing all connection with the ectoblast, gradually recedes from the sur- 

 face in consequence of the growth of the intervening mesoblastic layer ; it next loses its sphe- 

 roidal form and becomes somewhat pear-shaped, with the smaller end directed dorsally. The 

 smaller end rapidly elongates into a club-shaped diverticulum, the recessus endolymphaticus, 

 which later becomes the ductus and the saccus endolymphaticus. The remainder of the otic 

 sac soon exhibits a subdivision into a larger dilatation, the vestibular pouch, and a smaller 

 ventral one, the cochlear pouch (Fig. 1279). 



Auditory pit 



Dorsal aorta 



Oropharynx 



I visceral furrow 



I visceral arch 



Frontal section of early rabbit embryo, showing 

 otic pits. X 40. 



