588 COMPARATIVE ANATOMY 



thalamus of the opposite side of the brain and half to that of the same side. 

 The sensory centers of vision are located in the occipital lobes. 



The outer layer of the eyeball is the sclerotic or tunica fibrosa, com- 

 posed of interwoven bundles of compact connective tissue fibers. This 

 layer is opaque, except the portion which forms the cornea. A many- 

 layered epithelium or conjunctiva covers that part of the cornea which is 

 exposed to the air, and also lines the eyelids. 



Eyelids occur only in land forms, as an adaptive device for protecting 

 the eyeball. They are folds of skin above and below the eyes, movable 

 except in serpents. They are opened by levator and closed by the 

 orbicularis muscle. The upper lid is more movable than the lower. Along 

 the edges of both lids, a series of sebaceous Meibomian or tarsal glands 

 form a film of oil, which keeps the tears from flowing over the lower lids. 



The lacrimal glands are compound tubular glands, located at the upper 

 border of the upper eyelid. Their secretions are poured into the conjunc- 

 tival sac, and are carried over the surface of the eyeball to the lacrymal 

 ducts, one in each eyelid, near the nose. The two lacrimal ducts of each 

 side unite to form the nasolacrimal duct, which opens into the nasal 

 passage below the inferior concha of the same side. 



Development of the Vertebrate Eye 



The vertebrate eye has a twofold origin. The retina, optic nerve and 



pigment epithelium are derived from the brain, and therefore from the 



^^.^gr232iiEi7>^ ectoderm. The lens and conjunctiva are 



^^^^^^^^^^^^1 j and the connective tissue surrounding the 

 Mm^I^^^^^^^H I eye-ball, all come from the mesenchyma. 



Fig. 485. — Stereogram of de- branchs and amphibia, the anlagen of the 

 ;rc«rw:,rof ftt'lll.'LTi °P''^ resides appear as paired depressions 

 of lens; oc, optic cup; os, optic of the ncural plate. As the optic vesicles 

 •S^l ',!^"Jn",t"er'tFrom continue their lateral expansion, the con- 

 Kingsiey's " Comparative Anatomy nexion with the brain becomes constricted 



of Vertebrates.") ^^ ^^ ^p^.^ ^^^^j^^ ^^^^^ ^^^^^ -^ converted 



into a shallow trough to guide the fibers of the optic nerve as they grow 

 from the retina to the brain wall. 



