THE SENSE OF SIGHT. 155 



situated as to permit of an extensive range of vision. The eyeball is loosely 

 held in position by a fibrous membrane, the capsule of Tenon, which is 

 attached on the one hand to the eyeball itself and on the other to the walls 

 of the cavity. Thus suspended, the eyeball is capable of being moved in 

 any direction by the contraction of the muscles attached to it. 



Structure. The eyeball is spheroidal in shape and measures about nine- 

 tenths of an inch in its antero-posterior 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 enclosed the refracting media essential to vision. These mem- 

 branes enumerated from without inwards, are : 1st, the sclerotic and cornea ; 

 2d, the choroid and iris; 3d, the retina; the refracting media are the 

 aqueous humor, the crystalline lens and 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 longitudi- 

 nally; it is pierced posteriorly by the optic nerve about one-tenth 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 trans- 

 parent fibrils of connective tissue more or less united together; between 

 these layers are found a number of inter-communicating lymph spaces lined 

 by endothelium, which are in connection with lymphatics. Leucocytes 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 structureless membrane known as the anterior elastic lamina. The 

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

 Decemet, which becomes continuous at its periphery 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 Ciliary processes, 

 together constitute the second or middle coat of the eyeball. 



The choroid is a dark brown membrane which extends forward nearly 



