696 TEXT-BOOK OF PHYSIOLOGY 



though 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- 

 tained 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- 



FIG. 284. SECTION THROUGH THE CILIARY REGION OF THE HUMAN EYE. a. Radiating 

 bundles of the ciliary muscle, b. Deeper bundles, c. Circular network, d. Annular muscle of 

 Miiller. e. Tendon of ciliary muscle. /. Muscle-fibers on posterior side of the iris. g. Muscles 

 on the ciliary border of the same. h. Ligamentum pectinatum. (After Iwanoff.) 



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

 pigment is wanting. In gray, brown, and black eyes the pigment is present 

 in progressively increasing amounts. The blood-vessels are connected with 

 those of the chorioid coat. 



The muscle-fibers are of the non-striated variety and arranged in two sets, 

 one circularly, the other radially, disposed. 



The circular fibers are found close to the pupil near the posterior surface 

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

 the size of the pupil. This muscle is known as the sphincter pupilla or 

 sphincter iridis. 



The radial fibers form a more or less continuous layer in the posterior 

 part of the iris, extending from the margin of the pupil, where they blend 

 with the circular fibers, to the outer border. Contraction of the fibers in- 

 creases the size of the pupil. The muscle is known as the dilatator pupilla. 



The nerves exciting the sphincter pupilla to action are the ciliary nerves, 

 axons of nerve-cells located in the ciliary or ophthalmic ganglion. Stimula- 

 tion of these fibers gives rise to contraction of the sphincter and diminution in 

 the size of the pupil. The nerves exciting the dilatator pupilla to action are 



