THE SENSE OF SIGHT. 



239 



4. The lamina vitrea, or internal limiting membrane. 



The choroid with its contained blood-vessels bears an important 

 relation to the nutrition of the eye ; it provides for the blood-supply 

 and for drainage from the body of the eye, and presents a uniform 

 and high temperature to the retina. 



The iris is the circular variously colored membrane placed in the 

 anterior portion of the eye just behind the cornea. It is perforated 

 a little to the nasal side of the center by a circular opening, the pupil. 

 The outer or circumferential border is connected with the cornea, 



FIG. 30. SCLEROTIC COAT REMOVED TO SHOW CHOROID CILIARY MUSCLE, AND 

 NERVES. (From Holdcn's "Anatomy.") 



i. Sclerotic coat. b. Veins of the choroid. c. Ciliary nerves, d. Veins of 

 the choroid. e. Ciliary muscle, f. Iris. 



ciliary muscle, and ciliary processes ; the free inner edge forms the 

 boundary of the pupil, the size of which is constantly changing. 

 The framework of the iris is composed of connective-tissue blood- 

 vessels, muscle-fibers and pigmented connective-tissue corpuscles. 

 The anterior surface is covered with a layer of epithelial cells con- 

 tinuous with those covering the posterior surface of the cornea ; the 

 posterior surface is lined by a limiting membrane bearing pigment 

 epithelial cells continuous with those of the choroid. The various 

 colors which the iris assumes in different individuals depend upon 

 the quantity and disposition of the pigment granules. 



