THE EYE 375 



optic nerve, and in front it becomes changed into the 

 peculiar tissue that makes up the cornea. 



The cornea forms the transparent covering over the 

 lesser spherical segment of the eyeball, shading into the 

 sclerotic coat at its edges. It has a complex structure, 

 consisting in the main of a transparent form of connective 

 tissue. It serves the purpose of admitting light into the 

 eyeball. 



The Middle Coat consists of three connected portions 

 the choroid coat, the ciliary processes, and the iris. These 

 surround the larger spherical segment. All three parts 

 are rich in blood vessels, containing the blood supply to 

 the greater portion of the eyeball. 



The choroid coat lies immediately beneath the sclerotic 

 coat at all places except a small margin toward the front 

 of the eyeball. It is composed chiefly of blood vessels 

 and a delicate form of connective tissue that holds them in 

 place. It contains numerous pigment cells which give it 

 a dark appearance and serve to absorb surplus light. Near 

 where the sclerotic coat joins the cornea, the choroid coat 

 separates from the outer wall and, by folding, forms many 

 slight projections into the interior space. These are known 

 as the ciliary processes. The effect of these folds is to col- 

 lect a large number of capillaries into a small space and 

 to give this part of the eyeball an extra supply of blood. 

 Between the ciliary processes and the sclerotic coat is a 

 small muscle, containing both circular and longitudinal 

 fibers, called the ciliary muscle. 



The iris is a continuation of the choroid coat across the 

 front of the eyeball. It forms a dividing curtain between 

 the two spherical segments and gives the color to the eye. 

 At its center is a circular opening, called the ptipil, which 

 admits light to the back of the eyeball. By varying the 



