Essentials of Veterinary Ophthalmology. Kirk N. Gelatt

These muscle fibers apically (i.e., posteriorly) contain pigment around their nuclei and are innervated by sympathetic nerve fibers. The basal regions of each cell, which contain the myofilaments, overlap one another in a shingle‐like fashion. The basal aspect of the posterior epithelium of the iris faces the posterior chamber and has numerous surface projections.

      The apical portion of the cells of the anterior epithelium (iris dilator muscle) contains the nucleus and is located adjacent to the apical portion of the posterior epithelium. Melanin granules are predominately present in the apical portion of the cell. The myoepithelial (basal) portion has scattered melanin granules, forms irregular projections into the stroma, and is covered by a basement membrane.

In avian species and other lower vertebrates, the iris muscles are striated. In addition to controlling the amount of light that enters the back of the eye, the iris of birds is thought to contribute to lenticular accommodation. Changes in the pupil diameters of chickens and pigeons result in changes in the positioning of their lenses.

Figure 1.31 Sphincter muscle (SM) location in the dog (a) and in the horse (b). The sphincter muscle in the horse is capped by the granula iridica (GI), which is a proliferation of the posterior epithelium (PE). (Original magnification, 200×.)

Figure 1.32 (a) Iris sphincter muscles that create a slit pupil when the pupil is constricted as found in domestic cats, bobcats, and lynx. (b) The circular iris sphincter muscle as found in primates, birds, dogs, and pigs. (c) Iris sphincter muscle in an ungulate with a horizontal pupil.

      The iris contains numerous myelinated and nonmyelinated nerves for autonomic innervation. The myelinated fibers do not specifically follow the iris vessels, but they have a similar pattern as they follow the collagen fibers of the stroma. Upon entering the iris, each long ciliary nerve forms a dorsal and a ventral branch, to form a circular nerve in the ciliary zone and also to meet their counterparts from the opposite side dorsally and ventrally. The belief that reflex constriction of the mammalian pupil in response to light depends exclusively on neural pathways between the eye and central nervous system may not be true.

      Ciliary Body

The ciliary body is a heavily pigmented structure that provides nourishment and removes wastes for the cornea and lens, and participates in lens accommodation. The ciliary body is divided into the anterior pars plicata (corona ciliaris) and the posterior pars plana. The pars plicata consists of a ring of 70–100 ciliary processes, depending on the species, with intervening valleys (Figure 1.33). The processes are generally more prominent and numerous in animals with larger anterior chambers (the cow and horse have 100 and 102 processes, respectively) than in animals with smaller anterior chambers (carnivores and primates have 74–76 processes). Ciliary body processes are often absent in lower vertebrates (most fish, lizards, and snakes). In anurans, birds, and some reptiles, the ciliary body processes are attached to the lens and participate directly in accommodation. In mammals, the ciliary body processes are attached to the lenticular zonules, which connect to the lens equator (Figure 1.34). The appearance of individual ciliary body processes varies among species (Figure 1.35). In carnivores, the processes are thin and bladelike, with rounded tips that are invested with zonular fibers.

Figure 1.33 Inner surface of the ciliary body of a dog treated with α‐chymotrypsin to remove the lenticular zonules. Note the thin ciliary processes (CP), which posteriorly give rise to smaller secondary folds (small arrows). These folds flatten and disappear in the region called the pars plana (PP), which ends posteriorly at the adjoining retina, forming a line known as the ora ciliaris retinae (arrowheads). (Original magnification, 18×.)

Figure 1.34 SEM (sagittal view) of the inner ciliary body of a dog reveals numerous zonular fibers attached along the epithelial surface. (Original magnification, 130×.)

Each ciliary process consists of a central core of stroma and blood vessels covered by a double layer of epithelium: an inner, nonpigmented, cuboidal epithelium and an outer, pigmented, cuboidal epithelium (Figure 1.36a and b). In ungulates, the double‐layered epithelium is more columnar than cuboidal. The nonpigmented ciliary body epithelium is confluent posteriorly with the neurosensory retina at the ora ciliaris retinae and anteriorly with the posterior pigmented epithelium of the iris. The nonpigmented epithelium most likely produces the GAGs of the vitreous humor. The enzyme carbonic anhydrase has been cytochemically localized at or in the nonpigmented epithelium. The types of cellular junctions between the nonpigmented and pigmented epithelia of the ciliary processes consist of many gap junctions interspersed with desmosomes and unusual junctions termed puncta adherentes. The apical ends possess gap junctions, zonula adherens, and zonula occludens, which represent the anatomical site of the blood–aqueous barrier (Figure 1.37). There are also dilated portions of the apical intercellular spaces with villous cytoplasmic processes protruding into them. These dilations are termed ciliary channels, and they are usually near the apical termination of two adjacent cells.

Figure 1.35 SEM of the ciliary processes and zonular fibers in a horse. Ciliary process (A). Arrows point in the direction of the lens equator as well as to the horizontal fiber network joining adjacent process (B). Zonular fibers in valleys between processes (C). Note also the zonular fiber ensheathment of the ciliary processes (black arrows). (Original magnification, 41×.)

Figure 1.36 The bilayered ciliary epithelium that lines the ciliary processes and intervening valleys. The outer layer is pigmented; the inner layer is nonpigmented. (a) Feline ciliary processes. Inset: Cross section of ciliary processes. The bilayered epithelium, which is cuboidal, lines blood vessels (BV), which together form a blood–aqueous barrier. (b) Longitudinal section of an equine ciliary epithelium at the base of a process. Both layers are considerably more columnar than those in the dog and cat. (Original magnification, 400×).

The ciliary process pigmented epithelium is confluent with the retinal