THE SENSE OF SIGHT 681 



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iinough usually described as a part of the chorioid, are now known to belong, 

 embryologically and physiologically, to the retina. Lying within the outer 

 layer of arteries and veins there is a thick layer of small arterioles and 

 capillaries, known as the chorio-capillaris. The chorioid with its con- 

 i rained blood-vessels bears an important relation to the nutrition and function 

 of the eye. It provides a free supply of lymph and presents a uniform tem- 

 perature to the retina in contact with it. 



The Iris. The iris is the circular, variously colored membrane in the 

 anterior part of the eye just behind the cornea. It presents a little to the 

 nasal side of the center a circular opening, the pupil. The outer or circum- 

 ferential border is united by connective tissue to the cornea, sclera, and ciliary 

 muscle ; the inner border forms the boundary of the pupil. The iris consists 

 of a framework of connective tissue supporting blood-vessels, muscle-fibers, 

 and pigmented connective-tissue cells. The anterior surface is covered by a 

 layer of cells continuous with those covering the posterior surface of the cor- 

 nea The posterior surface is formed by a thin structureless membrane sup- 

 porting a layer of pigment cells continuous with those. lining the chorioid. 

 The color which the iris presents in different individuals depends on the rela- 



on the ciliary border of the same. h. Ligamentum pectmatum.-(4/to 



tive amount of pigment in the connective-tissue corpuscles. In blue eyes the 



Thl ^sctnltsrof the non-striated variety and arranged in two sets, 



of the iris Contraction of this band of fibers diminishes, relaxation increases, 

 the size of the pupil. This muscle is known as the sphmcter pup** < 

 sphincter - more or less continuous layer in the posterior 





