![]() In saggital section the ciliary body is triangular in shape with its base at the iris and the apex posteriorly at the ora ciliaris retinae. It is divided into 2 parts, the anterior pars plicata and the posterior pars plana. The ciliary body, like the iris, contains both neurectodermal and mesodermal tissue. These appear as 3-4 pigmented, irregular masses. This an extension of the posterior epithelium into the pupil both dorsally and ventrally. In herbivores a structure termed the corpora nigra or granula iridica is found. The dilator muscle is a radially-arranged, smooth muscle and is innervated primarily by sympathetic fibers. The anterior epithelial layer consists of a pigmented apical portion and a myoepithelial basal portion which forms the dilator muscle of the pupil. Adjacent, posterior epithelial cells are attached by desmosomes and terminal bars to each other. It is separated from the posterior chamber by a basal lamina. The posterior most cell layer is pigmented and is continuous with the non-pigmented layer of the ciliary body and ultimately with the neural retina. Because of their origin from the invaginating optic cup these cells are situated apex to apex and remnants of the optic vesicle may occasionally persist between these cell layers. The posterior epithelium of the iris is actually 2 cell layers, both of neuroectodermal origin. The sphincter muscle is innervated by parasympathetic fibers from the Edinger-Westphal nucleus, the fibers of which travel with the third cranial nerve (oculomotor). Contraction of the sphincter muscle results in a decrease in the pupil diameter, termed miosis. It is situated in the pupillary zone and varies in shape depending on the shape of the pupil. As stated previously, it has its origin from neural ectoderm. It is smooth muscle in mammals and striated in birds and lower vertebrates. The sphincter muscle is found within the iris stroma. ![]() The capillary endothelium is not fenestrated and is part of the blood-eye-barrier. The blood vessels are endothelial lined structures with a basement membrane, smooth muscle, and pericytes. The iris stroma is a loose tissue comprised of collagen bundles, blood vessels, nerves, melanocytes, fibroblasts, and the sphincter muscle of the iris. This angle formed by the cornea and the root of the iris is termed the iridocorneal angle and is the entrance to the trabecular meshwork, the site of outflow of the aqueous humor. ![]() The peripheral portion of the iris inserts as the pectinate ligaments in the area where Descemet’s membrane ends. ![]() The anterior border is formed by fibroblasts and melanocytes. The anterior border of the iris is often discussed as possessing an epi- or endothelial cell layer. In animals with blue eyes the pigment is absent in the stroma, while in the albino it is lacking in both the stroma and the posterior epithelium. The color of the iris depends on the pigmentation of both the stromal melanocytes and the posterior epithelium. The color of the iris can vary with species, age, and sex, and can even differ between eyes or within eyes in the same animal. This central portion of the iris normally rest against the anterior lens surface and in the absence of this support, the iris will tremble. The pupil varies in shape between species and can be circular (dog, rabbit), oval (horse, cow), or vertical (cat). Embryologically, the iris is comprised of components that originate from neural ectoderm, the sphincter and dilator muscles and the 2-layered posterior epithelium, and mesodermal components that form the iris stroma. The iris is the anterior most portion of the vascular tunic and functions as a moveable diaphragm between the anterior and posterior chambers.
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