644 TEXT-BOOK OF PHYSIOLOGY 



center a circular opening, the pupil. The outer or circumferential 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 frame- 

 work of connective tissue supporting blood-vessels, muscle-fibers, and pig- 

 mented connective-tissue cells. The anterior surface is covered by a layer 

 of cells continuous with those covering the posterior surface of the cornea. 

 The posterior surface is formed by a thin structureless membrane supporting 

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

 color which the iris presents in different individuals depends on the relative 

 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. 



FIG. 303. SECTION THROUGH THE CILIARY REGION OF THE HUMAN EYE. a. Radiating 

 bundles of the ciliary muscle. 6. Deeper bundles, c. Circular network, d. Annular muscle of 

 Muller. 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.) 



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 pupillcB 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 pupill(B to action are 

 axons of nerve-cells located in the superior cervical ganglion. They reach 

 the iris by way of the cervical sympathetic, the ophthalmic division of the 

 fifth, and the long ciliary nerve. Stimulation of these nerves is followed by 

 contraction of the dilatator and an increase in the size of the pupil. Both 

 the ciliary and superior cervical ganglia are in relation with pre-ganglionic 

 fibers coming from the central nerve system. (See pages 582, 618.) 



The ciliary muscle is a gray circular band about two millimeters in 

 width, consisting of non-striated muscle-fibers. The majority of these 



