628 THE EYE 



tiva. Where the sclera passes into the cornea,, the conjunctival epithe- 

 lium becomes continuous with the anterior corneal epithelium, the tunica 

 propria blending with the corneal stroma. 



The globe of the eye or eyeball is a spheroidal body whose surface 

 consists of three coats, an outer, middle, and inner, and whose contents 

 are the vitreous and aqueous humors and the crystalline lens. 



The eyeball is not a true sphere, but may be said to comprise segments 

 of two spheres, the smaller of which is inserted into the anterior surface 

 of the larger. The anterior or smaller segment consists chiefly of trans- 

 parent tissues which permit the entrance of light. Its border nearly 

 corresponds to -the posterior margin of the ciliary body, and it may be 

 approximately indicated by a parallel circle midway between the margin 

 of the cornea and the equator of the eyeball. The anterior segment con- 

 tains the cornea, the sclerocorneal junction, the anterior and posterior 

 chambers, the aqueous humor, the iris, and the ciliary body. The pos- 

 terior segment comprises the posterior two-thirds of the eyeball and in- 

 cludes the sclera, choroid, retina, and, within these coats, the vitreous 

 humor. The crystalline lens with its suspensory ligament forms, as it 

 were, a partition separating the two segments. 



The optical or visual axis of the eye is a horizontal, anteroposterior, 

 imaginary line, about an inch in length, which extends from the center 

 of the cornea through the anterior chamber, the center of the pupillary 

 opening of the iris, the center of the crystalline lens, and the center of 

 the vitreous humor, and reaches the fovea centralis which lies in the 

 middle of a thickened portion of the retina, the macula lutea. The ver- 

 tical and transverse axes measure about 1 mm. less than the anteropos- 

 terior axis. Toward the inner side, at a distance of 3.5 mm., and about 

 1 mm. below the center of the fovea centralis, is the entrance of the optic 

 nerve. This nerve pierces the coats of the eye, its fibers spreading out 

 in a radial manner, upon the inner surface of the retina. 



The extremities of the visual axis mark the two poles of the ocular 

 globe; the anterior extremity, lying in the center of the cornea, is in 

 the anterior or smaller spheroidal segment, the posterior extremity, in 

 the fovea centralis, lies in the posterior segment of the eye. 



THE EXTERNAL COAT THE FIBROUS TUNIC 



The outer tunic of the eyeball includes the cornea, the sclera, and the 

 sclerocorneal junction. 



