Essentials of Veterinary Ophthalmology. Kirk N. Gelatt

Anteriorly, it continues as the anterior pigmented epithelial layer of the iris, which forms the dilator muscle. The pigmented epithelium is generally cuboidal and heavily laden with round‐to‐oval melanin granules. The basal aspect of the pigmented epithelium faces the ciliary body stroma and is covered by a basement membrane. The nuclei of both pigmented and nonpigmented epithelia are located apically. The cytoplasm of the pigmented epithelium contains melanin granules, rough endoplasmic reticulum, smooth endoplasmic reticulum, free ribosomes (polysomes), and mitochondria.

Figure 1.37 Apical junctions of nonpigmented (NPE) and pigmented (PE) ciliary epithelium in a cat. The nonpigmented epithelial nuclei are located apically; the wide intercellular spaces and villi can be seen in the basilar aspect of the intercellular spaces of the nonpigmented epithelium. The apical aspect of the nonpigmented intercellular space is the anatomical site of the blood–aqueous barrier and contains a fascia occludens (small arrow) and fascia adherens (large arrow). The apical cell surfaces contain a fascia adherens, gap junctions (open arrows), and arch‐shaped gap junctions (curved arrows). The basement membrane (B) of the pigmented epithelium. (Original magnification, 9800×.)

      A thin layer or core of loose connective tissue with blood vessels and nerves lies under the ciliary epithelium, separating the ciliary body epithelium from the underlying ciliary body musculature. The vascular plexus within the stroma of the ciliary process is leaky, being lined with a fenestrated endothelium. Fibrocytes and melanocytes are sparsely populated within the stroma, being more concentrated near the ciliary body muscle.

      The pars plana is the flat, posterior portion of the ciliary body that extends from the posterior termination of the processes to the retina (ora ciliaris retinae) and an important area to access surgically the vitreous space and posterior segment (see Chapter 13, Section III). The width of the pars plana varies because the retina extends more anteriorly in the inferior and medial quadrants in most species, enhancing peripheral vision. Therefore, the pars plana is usually widest superiorly and laterally.

      Ciliary Body Musculature

The ciliary body muscle is comprised of smooth muscle fibers in mammals. Contraction of the ciliary body muscle draws the ciliary processes and body both forward and inward, thus relaxing the lenticular zonules (suspensory ligament of the lens) and altering the shape and refraction of the lens. This muscle is often weakly developed in many nonprimate species and, as a result, offers poor accommodative ability. On the basis of ciliary body musculature development, the placental mammalian ICA has been categorized into three main groups, the herbivorous, the carnivorous, and the anthropoid, and seems based primarily on the lens rather than aqueous humor outflow (Figure 1.38).

The herbivorous type has been characterized as the most common and primitive in orders of mammals up to and including ungulates. This type of angle consists of an inner layer of connective tissue that forms a baseplate of the ciliary body and extends from the root of the iris to the ora ciliaris retinae. It also consists of an outer layer of smooth muscle that presses against the sclera externally and runs meridionally from the corneoscleral junction toward the ora ciliaris retinae. The two layers are often referred to as “leaves” that separate anteriorly forming the ciliary cleft. The ciliary cleft is then a triangular area that varies in both depth (i.e., length) and height, and functionally may be considered a posterolateral extension of the anterior chamber into the ciliary body. Historically, this region was initially called the cilioscleral sinus, but the term cilioscleral sinus has been replaced with ciliary cleft. The ciliary cleft is an area containing wide spaces filled with aqueous humor and interspersed with cell‐lined cords of connective tissue. The spaces between the fibrous cords were initially described in cattle and horses, and they have been often referred to as Fontana's spaces.

Figure 1.38 Degree of development of the ciliary body musculature among mammalian ICAs in the ungulate (a), carnivore (b), and ape (c). The ciliary body musculature is most pronounced in primates and least developed in ungulates. The size of the ICA and its cilioscleral cleft or sinus (CC) is inversely large or most pronounced in the ungulate.

      The carnivorous type possesses a bi‐leaflet configuration as well, but the fibrous inner leaf or layer is usually replaced by meridionally oriented smooth muscle and some radially oriented muscle fibers. In both the herbivorous and carnivorous types, the ciliary cleft offers little support to properly anchor the iris. Compensation for wide and deep ciliary clefts is provided by a series of pectinate ligaments attaching the anterior iridal root and inner ciliary baseplate to the limbal cornea.

      The ciliary body musculature of primates is believed to be the most highly developed among mammals. The muscle, which has three components (i.e., radial, meridional, and circular), forms a large, anterior pyramidal structure that provides a strong baseplate for iridal attachment. The anterior portion of the ciliary body muscle has replaced both the ciliary cleft, which barely exists in the anthropoid angle, and the pectinate ligaments, which vestigially consist of scattered iridal processes in primates, including humans.

      In birds and other nonmammalian species, the ciliary body muscle consists of skeletal muscle cells that are primarily meridional. At least two distinct muscle bundles are located in this region of the avian eye: an anterior bundle, which is known as the muscle of Crampton, arises near the corneal margin; and a posterior bundle, which is known as Brücke's muscle. Contraction of Brücke's muscle causes the ciliary body to push against or compress the lens, thus deforming it, while contraction of Crampton's muscle alters the shape of the cornea by shortening its radius of curvature.

      Ciliary Body Vasculature

The extensive blood supply of the ciliary body is derived from the two long posterior ciliary arteries and the anterior ciliary arteries. As the long posterior arteries pass into the suprachoroidal space equatorially along the lateromedial horizontal plane, they undergo several divisions. These divisions anastomose anteriorly with branches of the anterior ciliary arteries to form the major arterial circle, which is located either in the base of the iris or in the anterior ciliary body. The anterior ciliary arteries, which arise from branches of the ophthalmic artery, typically enter the globe at the attachment sites for the recti muscles and help to supply the ciliary muscles. The major arterial circle is the primary vasculature supply of the ciliary processes.

      Numerous anatomical variations of this vasculature have been found among mammals. The mammalian ciliary body muscle is supplied by parasympathetic fibers from the oculomotor nerve and by sympathetic nerve fibers. The parasympathetic fibers leave the oculomotor nerve, penetrate the ventral oblique muscle, and synapse in the ciliary ganglion. From the ciliary ganglion, short ciliary nerves penetrate the sclera around the optic nerve to pass into the sclera and suprachoroidal space innervating the ciliary muscle and iris muscles. The sympathetic fibers arrive via the long ciliary nerves from the dorsal or superior cervical ganglia in a similar manner.

      Iridocorneal Angle

      Aqueous humor is produced by the ciliary body epithelium and enters the posterior chamber before flowing through the pupil into the anterior chamber. In the conventional outflow pathway, aqueous humor exits the eye primarily through the corneoscleral trabecular (pressure sensitive) meshwork.

      The anatomy of the aqueous humor outflow system has been extensively studied in humans, nonhuman primates, dogs, cats, rabbits, horses, and other ungulate species. This system primarily