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 pigmented epithelium. 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
