THE EAR. 



1483 



optic vesicle, about the fourth or fifth month, in consequence of which folds in the membrane 

 arise, into which blood-vessels and other mesodermic elements extend. The corneal stroma 

 becomes blended with the sclera, thenceforth the two forming a continuous tunic. 



Development of the Vitreous Body. As already stated, the vitreous body is at present re- 

 garded as developing, at least in part, from the cells of the inner wall of the optic vesicle, 

 especially from its anterior or ciliary portion. The suspensory ligament of the lens is derived 

 from the same source. The cells develop into the fibres which form the fine net-work of the 

 vitreous body; at the periphery these become condensed and form the boundary layer or 

 hyaloid membrane. The vitreous is supplied with blood by branches of the hyaloid artery, 

 which springs from the head of the optic nerve. An especially complete net-work is found at 

 the periphery of the vitreous and these vessels pass forward to the equator of the lens and 

 assist in forming the tunica vasculosa lentis. The retinal vessels are formed later as branches of 

 the central artery, the vitreous vessels usually undergoing complete absorption before birth. 



The development of the eyelids begins with the production of folds of integument, which 

 appear above and below the cornea during the second fcetal month. The folds approach each 

 other and the epidermal cells fuse about the third month, the eyelids remaining united until 

 shortly before birth. The Meibomian and other glands of the lids are produced by ingrowths 

 of the surface ectoblast. The lachrymal gland arises during the third month as a solid ingrowth 

 from the conjunctival epithelium close to the upper lid. The lachrymal canal begins as a solid 

 process of epithelial cells from the lid, which dips inward along the lachrymal furrow, between 

 the superior maxillary and nasal processes. This cord of cells becomes isolated from the sur- 

 face, and later acquires a lumen, connecting by means of the canaliculi with the conjunctival sac 

 above. The duct establishes communication with the nasal fossa just before birth. 



THE EAR. 



The ear (organon auditus) may be conveniently studied under its three natural 

 subdivisions, which are conventionally described as the external, middle and the 

 internal ear structures lodged entirely or in part within the temporal bone. The 



FIG. 1242. 



Bone 

 \ Malleus 



Incus 



Stapes 



Inner ear 



Semicircular canal 



Internal auditory canal 

 Auditory nerve 



Endolymphatic sac 



Cartilage 



Diagram showing relations of three subdivisions of ear. {Modified from Schwalbe.) 



external ear includes the auricle and the external auditory canal ; the middle ear 

 the tympanum, the Eustachian tube and the mastoid cells ; and the internal ear 

 the labyrinth, with the peripheral ramifications of the auditory nerve. 



Such division, moreover, is justified by the developmental history of the organ, 

 since the internal ear is developed essentially from the highly differentiated otic vesicle 

 which gives rise to the complicated membranous labyrinth ; the middle ear largely 

 from the first pharyngeal pouch ; whilst the external ear represents the deepened 

 and modified boundaries of the first external visceral furrow. 



