412 



CHORDATE ANATOMY 



A vascular chorioid layer surrounds retina and pigment epithelium. 

 Besides many pigment cells, this layer contains many blood-vessels, 

 branches of the central retinal and of the anterior and posterior ciliary 

 arteries, and veins. The retina, however, is supplied by its central 

 vessels. Most of the blood leaves the eye-ball by way of four to six 

 vorticose veins. 



The ganglion cells of the retina give off neurites which converge towards 

 the "blind spot," where the optic nerve fibers leave the retina and rods 

 and cones are absent. Of the fibers of the optic nerve, half pass to the 

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

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

 The outer layer of the eye-ball 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 epithe- 

 lium 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 a 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 eye-ball to the lacrimal 

 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. 



Fig. 362. — Stereogram of de- 

 veloping eye. (/, chorioid fissure; 

 fb, cut wall of fore-brain; I, anlage 

 of lens; oc, optic cup; os, optic 

 stalk; p, layer for pigmented epi- 

 thelium; r, retinal layer. (From 

 Kingsley's " Comparative Anatomy 

 of Vertebrates.") 



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 

 ectoderm. The lens and conjunctiva are also ectodermal. On the other 

 hand, the sclerotic, the cornea, the aqueous humor, possibly the vitreous 

 humor, the chorioid, the ciliary body, the iris, the eye muscles, and the 



