THE EYEBALL. 321 



The Globe of the Eye is on the whole spheroidal, but 

 consists of segments of two spheres (see Fig. 88), a portion 

 of a sphere of smaller radius forming its anterior transpar- 

 ent part, and being set on to the front of its posterior 

 segment, which is part of a larger sphere. In general 

 terms it may be described as consisting of three coats and 

 three refracting media. 



The outer coat 1 and 3, Fig. 88, consists of the sclerotic 

 and the cornea, the latter being transparent and situated in 

 front ; the former is opaque and white and covers the back 

 and sides of the globe and part ol the front, where it is 

 seen between the eyelids as the white of the eye. Both are 

 tough and strong, being composed of dense connective tissue. 



The second coat consists of the choroid, 9, 10, and the 

 iris, 14. The choroid consists mainly of blood-vessels sup- 

 ported by loose connective tissue, which in its inner layers 

 contains many dark brown or black pigment granules.* 

 Towards the front of the eyeball, where it begins to dimin- 

 ish in diameter, the choroid separates from the sclerotic 

 and turns in to form the iris, or that colored part of the 

 eye which is seen through the cornea ; in the centre of the 

 iris is a circular aperture, the pupil, through which light 

 reaches the interior of the eyeball. 



The third or innermost coat of the eye, the retina, 15, 

 is its essential portion, being the part in which he light 

 produces those changes that give rise to nervous impulses 

 in the optic nerve. It lines the posterior half of the eyeball. 



The Microscopic Structure of the Retina is very com- 



What is the form of the globe of the eye? Of what does it consist? 

 Describe the outer coat. The second coat. What is the retina? 



* In pink-eyed rabbits and in the pink-eyed ladies of " dime museums" this 

 pigment is absent. 



