SENSE OF SIGHT. 213 



Structure. The eyeball is spheroid in shape and measures about T 9 ^ 

 of an inch in its anteroposterior diameter, and a little less in its transverse 

 diameter. When viewed in profile, it is seen to consist of the segments of two 

 spheres, of which the posterior is the larger, occupying five sixths, and the 

 anterior the smaller, occupying one sixth, of the ball. 



The eye is made up of several membranes, concentrically arranged, within 

 which are inclosed the refracting media essential to vision. These membranes 

 enumerated from without inward, are 



1. The sclerotic and cornea. 



2. The choroid and iris. 



3. The retina. 



The refracting media are the aqueous humor, the crystalline lens, and the 

 vitreous humor. 



The Sclerotic and Cornea. The sclerotic is the opaque fibrous mem- 

 brane covering the posterior five sixths of the ball. It is composed of con- 

 nective tissue arranged in layers, which run both transversely and longitudin- 

 ally; it is pierced posteriorly by the optic nerve about T ^ of an inch internal 

 to the optic axis. The sclerotic, by its density, gives form to the eye and 

 protects the delicate structures within it, and serves for the attachment of the 

 muscles by which the ball is moved. 



The cornea is a transparent non-vascular membrane covering the anterior 

 one sixth of the eyeball. It is nearly circular in shape and is continuous at 

 the circumference with the sclerotic, from which it cannot be separated. 

 The substance of the cornea is made up of thin layers of delicate, transparent 

 fibrils of connective tissue, more or less united; between these layers are 

 found a number of intercommunicating lymph-spaces, lined by endothelium, 

 which are in connection with lymphatics. Leukocytes or lymph-corpuscles 

 are often found in these spaces. The anterior surface of the cornea is 

 covered by several layers of nucleated epithelium, which rest upon a struc- 

 tureless membrane known as the anterior elastic lamina. The posterior 

 surface is covered by a similar membrane, the membrane of Descemet, 

 which at its periphery becomes continuous with the iris; it is also covered by 

 a layer of epithelial cells. At the junction of the cornea and sclerotic is 

 found a circular groove, the canal of Schlemm. 



The choroid, the iris, the ciliary muscle, and the ciliary processes 

 together constitute the second or middle coat of the eyeball. 



The choroid is a dark brown membrane which extends forward nearly to 

 the cornea, where it terminates in a series of folds, the ciliary processes. In 

 its structure the choroid is highly vascular, consisting of both arteries and 

 veins. Externally it is connected with the sclerotic by connective tissue ; 



