214 HUMAN PHYSIOLOGY. 



internally it is lined by a layer of hexagonal pigment cells, which, though 

 usually classed as belonging to the choroid, is now known to belong, embryo- 

 logically and physiologically, to the retina. From without inward may be 

 distinguished the following layers: 



1. The lamina suprachoroidea. 



2. The elastic layer of Sattler, consisting of two endothelial layers. 



3. The chorio-capillaris, choroid proper, or membrane of Ruysch a thick 

 elastic network of arterioles and capillaries lying within the outer layer of 

 veins and arteries called the venae vorticosae. 



4. The lamina vitrea, or internal limiting membrane. 



FIG. 32. SCLEROTIC COAT REMOVED TO SHOW CHOROID CILIARY MUSCLE, AND 



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



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

 choroid. e. Ciliary muscle. /. Iris. 



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 circum- 

 ferential border is connected with the cornea, 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 



