THE EYE AND VISION 



205 



the choroid membrane and the iris ; the inner is termed the 

 retina. 



The sclerotic coat (Gr. skleros, hard) or tunic of the eye is a 

 strong opaque fibrous membrane which extends over the whole ot 

 the ball with the exception of about one-sixth of its surface 

 in front. This coat is the thickest, and by far the strongest, 

 and is the one on which the maintenance of the form of the eye 

 depends. 



The cornea (Lat. cornu, a horn a transparent horny coat) is 

 continuous with the sclerotic coat, and covers the front of the eye- 



Fig. 189. The Choroid Membrane and Iris, exposed by the removal 

 of the Sclerotic and Cornea. 



a, part of the sclerotic thrown back ; b, ciliary muscle ; c, iris ; e, one of the 

 ciliary nerves ; f, blood-vessel. 



ball. It forms the prominent spherical surface before mentioned, 

 and is clear and transparent. It is not supplied with blood- 

 vessels. 



The choroid coat is a dark-brown membrane lying within and 

 against the sclerotic. It consists of a thickly set network of blood- 

 vessels, supported by connective tissue, and loaded with cells con- 

 taining a dark pigment or colouring matter. One use of this coat, 

 as we shall presently learn, is to darken the chamber of the eye, 

 and thus prevent the reflection of light. Just before the choroid 

 coat reaches the edge of the cornea, it becomes modified, being 



