Fig. 1.5 Monkey iris injected with horseradish peroxidase and then prepared as a whole mount. The collarette (**) of the iris lies near the pupillary opening (P) (From Wolfe [45], p. 322, with permission). Inset: Ultrastructure of a small iris arteriole. The clear perivascular space denoted by the arrows contains a dense ground substance that is associated with the endothelial basement membrane. There is a dense circular collagenous zone (a) outside the clear zone (From Hogan [19], p. 241, with permission)

and inferior anterior ciliary occlusion following ligation of the respective muscles.

Although the blood vessels of iris are designated artery, vein, and capillary, the assignments of type of blood vessel are based mostly on the

size of the blood vessel. Freddo and Raviola found that the iridial blood vessels are homogenous in structure regardless of diameter [11]. They possess no artery/capillary/vein units (ACV). There is a continuous layer of endothelial cells sitting on slender basal lamina and a thick, dense collagenous adventitia (Fig. 1.5, inset). The vessels have been described as a tube within a tube [45]. There is no traditional smooth muscle layer, but instead, there are fibroblasts, melanocytes, and an occasional macrophage arranged in one or two layers (Fig. 1.5, inset). The artery and vein differ only in the lack of muscularis in the vein. This unusual arrangement of the blood vessel wall is thought to adapt to the iris movement [11].

1.2.2Ciliary Body and Processes

The ciliary body is the circumferential tissue posterior to the iris composed of the ciliary muscle and ciliary processes (Fig. 1.6). It is part of the uveal tract—the layer of tissue, which provides most of the nutrients in the eye. It extends from