THE EAR. 543 



divides into two main portions, the vestibular and cochlear 

 ganglia, and from these, by further division, the several nerve 

 endings of the adult ear are derived. 



The epithelial cells of the auditory vesicle, which, it will be 

 remembered, are derived directly from the surface epidermis of 

 the head, become variously modified in different parts of the 

 vesicle. Over the greater part of its surface they remain flat 

 pavement cells, while opposite the nerve endings they become 

 altered into the hair-cells, rods of Oorti, sense cells of the am- 

 pullae, and other specialised structures. 



The mesoblast of the side of the head, in which the auditorv 



Fig. 231.— The left auditory vesicle, or internal ear, of a Human Embryo of 

 the eighth week, seen from the left side. From W. His, jun. x 17. 



EA, anterior vertical semicircular canal. EA', ampulla of anterior vertical semi- 

 circular canal. ED, common stem of the two vertical semicircular canals. EH, horizontal 



semicircular canal. EH', ampulla of horizontal semicircular canal. EL, cochlea. EP, 

 posterior vertical semicircular canal. EP', ampulla of posterior vertical semicircular 

 canal. ER, recessus labyrinthi. S, sacculus. UT, utriculus. 



vesicle is embedded, undergoes important changes. The layer 

 in immediate contact with the epithelial vesicle becomes closely 

 connected with this, and forms the connective tissue wall of the 

 labyrinth ; at a little distance from the labyrinth the mesoblast 

 becomes converted into cartilage, which forms the periotic capsule. 

 Between the cartilaginous capsule and the labyrinth itself, the 

 intervening mesoblast breaks down to form the perilymphatic 

 spaces surrounding the vestibule and the semicircular canals, and 

 the two lymphatic canals — seal a tympani and scala vestibuli — 

 which lie above and below the scala media, or cochlear outgrowth 

 from the labyrinth. 



