THE VASCULAR TUNIC. 



345 



The blood-vessels of the ciliary body, from the anterior and the long 

 ciliary arteries, form a ring around the root of the iris, the circulus arteriosus 

 iridis major, from which vessels are sent inwards to supply the iris, ciliary 

 muscle and ciliary processes. The veins 

 from the ciliary muscle empty chiefly into 

 the anterior ciliary veins; those from the 

 ciliary processes and a few from the ciliary 

 muscle pass backwards and become tribu- 

 tary to the vena vorticosce. The nerves 

 of the ciliary body form an annular plexus 

 within the ciliary muscle and include 

 sensory and sympathetic fibres, the latter 

 being distributed to the walls of the blood- 

 vessels and to the involuntary muscle. 



The Iris. The iris forms the ante- 

 rior segment of the vascular tunic and is 

 visible through the cornea. Slightly to 

 the inner side of its centre is placed an approximately circular opening, the 

 pupil. The periphery of the iris is attached to the ciliary body behind and 

 receives fibres from the pectinate ligament in front. The color of the iris 

 varies in different individuals and gives the "color of the eye." It is 

 dependent partly upon the amount of pigment within the iris-stroma, and 

 partly upon the density of the pigmentation of the cells on its posterior surface. 

 In light blue eyes, the stroma contains almost no pigment and the posterior 

 pigment layer, seen through it, gives a bluish tint; whereas in brown eyes 

 the stroma contains so much pigment that the effect of the posterior pigment 

 layer is augmented and the iris appears brown. 



The stroma of the iris encloses numerous thick-walled blood-vessels, 

 radiating from the ciliary border towards the pupil. They are supported by 

 a delicate connective tissue framework, which contains irregularly shaped, 



'Stroma 



Pigmented 

 cells 



Clear cells 



Blood-vessels 

 in processes 



FIG. 393. Section of ciliary processes, show- 

 ing layers of ciliary part of nervous tunic. 

 X 80. 



Pupillary margin 



Anterior endothelium Stroma of iris 





Sphincter muscle Pigmented retinal layer 



FIG. 394. Section of pupillary end of iris. X 210. 



branching pigmented cells, many nerves and lymph-spaces. The anterior 

 surface is covered with a single layer of polygonal cndothelial cells, con- 

 tinuous with those lining the cornea. Beneath these cells is a condensation 

 of the connective tissue stroma, the anterior boundary layer,\x\ which 

 the cells are closely placed. Minute clefts in the tissue are said to form 

 communications between the anterior chamber and the interfascicular lymph- 

 clefts. In very dark irides pigment is found not only within the branched 

 cells, but heaped in irregular masses within the stroma.. The muscular 

 tissue of the iris includes two distinct masses, the sphincter and dilatator 

 of the pupil. 



