768 THE EAR. 



bably venous. On the outer wall of the membranous canal there is a 

 specially vascular strip which has received the name of stria vascularis. 



Besides the foregoing vessel, which is the chief artery of the internal ear, 

 the stylo-mast oid branch of the posterior auricular, and occasionally the occi- 

 pital artery (Jones), send twigs to the vestibule and the posterior semicircular 

 canal. 



Veins. The veins of the cochlea issue from the grooves of the cochlear 

 axis and join the veins of the vestibule and semicircular canals : these accom- 

 pany the arterial branches, and, uniting with those of the cochlea at the base 

 of the modiolus, pour their contents into the superior petrosal sinus. 



DEVELOPMENT OF THE EAR. 



In the very young embryo the first rudiment of the ear is seen in the form of a 

 small vesicle the primary auditory vesicle lying at the side of the third primary 

 cerebral vesicle. It has to a certain extent an appearance similar to that of the 

 primary optic vesicle situated further forwards, and was long very naturally supposed 

 to be formed like it by a protrusion of the wall of the primary medullary cavity of 

 the brain ; but it has latterly been established by various observers that it is pro- 

 duced solely by invagination of the integument, and has no original connection 

 with the brain. During the third day of incubation it can be seen in the chick, still 

 open to the outside, above and behind the second branchial lappet. It soon becomes 

 completely closed, and is afterwards developed into the membranous labyrinth. 

 The first complication which the vesicle exhibits is by the extension of a process 

 upwards and backwards, which remains permanent in the lower vertebrata, but in 

 mammals is obliterated, its vestiges remaining in the aqueduct of the vestibule. The 

 semicircular canals next appear as elongated elevations of the surface of the primary 

 vesicle : the middle portion of each elevation becomes separated from the rest of the 



Fig. 524. OUTLINES SHOWING THE FORMATION OP THE EXTERNAL EAR IN THE FCETUS. 



A, head and upper part of the body of a human foetus of about four weeks (from 

 nature). ^ Four branchial plates (the first, forming the lower jaw, is marked 1), and 

 four clefts are shown ; the auditory vesicle (a), though closed, is visible from the tran- 

 sparency of the parts, and is placed behind the second branchial plate. 



B, the same parts in a human foetus of about six weeks (from Kcker). f The third 

 and fourth plates have nearly disappeared, and the third and fourth clefts are closed ; 

 the second is nearly closed ; but the first (!') is somewhat widened posteriorly in con- 

 nection with the formation of the mealus externus. 



C, human foetus of about nine weeks (from nature), f The first branchial cleft is 

 more dilated, and has altered its form along with the integument behind it in connection 

 with the formation of the meatus externus and the auricle. 



