THE CORNEA 565 



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

 middle of a thickened portion of the retina, the macula lutea. 



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 fibres 

 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 pos- 

 terior extremity, in the fovea centralis, lies in the posterior seg- 

 ment of the eye. 



THE EXTERNAL COAT 



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

 and the sclero-corneal junction. 



THE CORNEA. The cornea is a concavo-convex, trans-parent, 

 colorless disk of approximately equal thickness throughout all its 

 portions. It is nearly circular in outline, its horizontal exceeding 

 its vertical diameter by only 0.5 mm. ; its external surface is con- 

 vex, its internal surface concave. The cornea forms the anterior 

 one-fourth of the tunica externa, and represents a spheroidal seg- 

 ment whose radius is somewhat shorter than that of the posterior 

 segment of the eyeball. It is inserted into the anterior margin of 

 the sclera much after the manner in which a watch-glass is set in 

 its rim ; hence the inner surface of the cornea possesses a slightly 

 greater diameter than the outer. 



The cornea may be said to consist of Jive layers : 1, the anterior 

 epithelium; 2, the anterior homogeneous membrane; 3, the cor- 

 neal substance ; 4, the posterior homogeneous membrane ; 5, the 

 posterior epithelium. 



The anterior epithelium (corneal epithelium, cornea! conjunc- 

 tiva) at the margin of the cornea is continuous with the scleral 

 portion of the conjunctiva. It consists of a relatively thin layer 

 six to eight cells deep of stratified squamous epithelium, the deep- 

 est cells of which are elongated or columnar, the middle cells poly- 

 hedral, and the superficial cells somewhat flattened. The cells at 

 all levels are nucleated and, like the other corneal tissues, perfectly 

 transparent, The columnar cells are often slender and much elon- 

 gated, their pointed apices extending well toward the surface of the 

 epithelial layer. 



