SENSE ORGANS. 83 



man the nictitating membrane is reduced to a vestigial fold, the 

 plica semilunaris, visible at the inner angle of the eye. 



The free surface of the eye is covered by the conjunctiva 

 already mentioned ; and beneath this is a thicker, dense trans- 

 parent layer, the cornea, composed of connective tissue fibres 

 produced from mesenchyme cells, which penetrate between the 

 conjunctiva and the lens. Laterally the cornea is continuous 

 with a hard white capsule, the sclerotic coat, which envelops 

 the whole eyeball, its anterior portion being the well-known 

 ' white of the eye.' This sclerotic is usually cartilaginous, and 

 in the sauropsida and in monotremes, bony structures, sclerotic 

 bones, may be developed in it. This sclerotic forms a sense 

 capsule, comparable in a way to those enclosing the ears 

 and olfactory organs, but never, like them, uniting with the 

 skull. 



Between the cornea and the lens is the anterior chamber of 

 the eye, filled with a watery fluid, the aqueous humor, less dense 

 than the vitreous humor already mentioned. 



Inside of the sclerotic is a highly vascular layer, the choroid, 

 which carries numerous blood-vessels to nourish the eye. The 

 choroid extends forward nearly to the edge of the optic cup ; but 

 beyond this point it becomes muscular, a portion of it forming 

 the contractile portion of a circular curtain, which extends from 

 the edge of the optic cup into the anterior chamber. This 

 curtain, known as the iris, is opaque, and is usually colored by 

 pigment derived from the edge of the optic cup. The opening 

 in the centre of the iris, the pupil, can be enlarged or contracted 

 by means of the muscles already referred to, and thus the amount 

 of light admitted to the retina can be regulated. 



Just inside the iris the inner wall of the optic cup becomes 

 developed into a strong ridge, the ciliary process, which ex- 

 tends inwards towards the lens to which it is attached by a 

 feriestrated, suspensory ligament (zonula Zinnii), thus partially 

 separating the anterior from the posterior chambers. Close to 

 this region the choroid develops a layer of ciliary muscles, 

 which by their action can move the lens nearer to or farther 

 from the retina, and at the same time, by stress conveyed by 

 the suspensory ligament, can slightly alter its shape. This 



