THE EYEBALL 399 



the brain, leaves the skull by a hole at the back of the 

 orbit, and enters the back of the globe of the eye, not in 

 the middle, but on the inner, or nasal, side of the centre. 

 Having pierced the wall of the globe, it spreads out into 

 a very delicate membrane, varying in thickness from 

 g^jth of an inch to less than half that amount, which lines 

 the hinder two-thirds of the globe, and is termed the 

 retina. This retina is the only organ connected with 

 sensory nervous fibres which can be afiected, by any 

 agent, in such a manner as to give rise to the sensation 

 of light. 



The eyeball is composed, in the first place, of a tough, 

 firm, spheroidal case consisting of fibrous or connecti'<-e 

 tissue, the greater part of which is white and opaque, and 

 is called the sclerotic (Fig. 126, 2). In front, however, 

 this fibrous capsule of the eye, though it does not change 

 its essential character, becomes transparent, and receives 

 the name of the cornea (Fig. 126, 1). The front surface 

 of the cornea is covered by an epithelium in which the 

 cells are very similar and similarly arranged to those in 

 the epidermis of the skin. The corneal portion of the 

 case of the eyeball is more convex than the sclerotic 

 portion, so that the whole form of the ball is such as 

 would be produced, by cutting off a segment from the 

 front of a spheroid of the diameter of the sclerotic, and 

 replacing this by a segment cut from, a smaller, and con- 

 sequently more convex, spheroid. 



The corneo-sclerotic case of the eye is kept in shape 

 by what are termed the hvjnonrs — watery or semi-fluid 

 substances, one of which, the aqueous humour (Fig. 

 126, 7')j which is hardly more than water holding a few 

 organic and saline substances in solution, distends the 

 corneal chamber of the eye, while the other, the vitreous 

 humour (Fig. 126, 13), which is rather a delicate jelly 

 than a regular fluid, keeps the sclerotic chamber full. 



The two humours are separated by the very beautiful 

 transparent doubly-convex crystalline lens(Fig. 126,12^ 



