156 HUMAN PHYSIOLOGY. 



to the cornea, where it terminates in a series of folds, the ciliary processes. 

 In its structure the choroid is highly vascular, consisting.of both arteries and 

 veins. Externally it is connected with the sclerotic by connective tissue ; 

 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 supra-choroidea. 



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



3. The chorio-capilfaris, 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 vena vorticosse. 



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, for drainage from 

 the body of the eye, and presents an uniform and high temperature to the 

 retina. 



The Iris is the circular variously-colored membrane placed in the an- 

 terior portion of the eye just behind the cornea. It is perforated a little to 

 the nasal side of the centre by a circular opening, the pupil. The outer 

 or circumferential 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 com- 

 posed of connective tissue, blood vessels, muscular fibres and pigmented 

 connective-tissue corpuscles. The anterior surface is covered with a layer 

 of epithelial cells continuous 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 pigmentary granules. 



The muscular fibres of the iris, which are of the non-striated variety, are 

 arranged in two sets, the sphincter and dilator. 



The sphincter pupillce is a circular flat band of muscular fibres surround- 

 ing the pupil close to its posterior surface ; by its contraction and relaxa- 

 tion, the pupil is diminished or increased in size. The dilator pupilla 

 consists of a thin layer of fibres arranged in a radiate manner ; at the mar- 

 gin of the pupil they blend with those of the sphincter muscle, while at the 

 outer border they arch to form a circular muscular layer. 



T\\z ciliary muscle is a gray circular band consisting of unstriped muscu- 

 lar fibres about one-tenth of an inch long running from before backward. 



