result in profuse hemorrhage!
Along the outer portion of the anterior scleral stroma is an interconnecting network of veins, the intrascleral plexus, which receives aqueous humor from the veins that drain the angular aqueous plexus (AAP) (Figure 1.26). In domestic animals, the intrascleral plexus is variably connected with the choroidal venous system, the vortex system. The intrascleral plexus is variable in size and depth within the sclera. In rabbits and primates, the plexus is formed on the outer side of circumferentially coursing canals, and it is composed of its small vessels deep in the sclera. In carnivores, the intrascleral plexus is prominent and composed of two to four large, anastomosing vessels in the mid‐sclera. The intrascleral plexus also receives afferent channels superficially via the episcleral network at the limbus. In the horse, the plexus, which is less prominent, collateralizes entirely with the anterior vortex system, because it is oriented radially to facilitate unidirectional flow outward from the angle region toward the vortex veins (posterior or uveoscleral aqueous humor outflow).
Figure 1.25 Photomicrographs of canine limbus. (a) The irregular connective tissue of the sclera (S) merges with the highly organized connective tissue of the cornea (C). (b) Close‐up of the outer limbus reveals an anterior epithelium that is markedly thickened, and contains small blood vessels (BV) and melanocytes. (Original magnification, 250×.)
The color of the sclera depends on the thickness of its stroma, appearing blue when thin (less than 0.2 mm) or yellow with increased fat content (carotenoids). The inner surface, which is referred to as the “lamina fusca,” is brown because of the adherent suprachoroidal pigment. The sclera contains elastic fibers that are interlaced among the collagen fibers, as are melanocytes (anteriorly) and fibrocytes (the sclera is probably more elastic than the cornea). The collagen fibers, fibrocytes, and occasional melanocytes are arranged meridionally, obliquely, and radially in an irregular fashion. The most notable emissaria accommodate the optic nerve, long and short ciliary nerves, long posterior ciliary arteries, vortex veins, and anterior ciliary vessels and represent weak areas in the sclera and can enlarge and protrude (called staphylomas) when IOP is chronically elevated.
Figure 1.26 The intrascleral plexus (ISP) of a dog is located within the mid‐sclera (S), and is interconnected to the AAP by aqueous veins (AV). ESV, episcleral veins. (Original magnification, 125×.)
The episclera is a collagenous, vascular, and elastic tissue that is between the sclera and the conjunctiva and attaches to Tenon's capsule. Tenon's capsule consists of small, compact bundles of collagen that lie parallel to the surface of the episclera.
Figure 1.27 Scleral ossicles (SO) in birds vary in size and shape. (a) Screech owl with large intraosseous spaces. (Original magnification, 40×.) (b) Chicken with smaller scleral ossicles and considerable overlap between adjacent ossicles. (Original magnification, 100×.) CM, ciliary body musculature (Crampton's muscle); TM, trabecular meshwork.
Besides dense connective tissue, the sclera can be largely composed of cartilage in some species, as in fish, lizards, chelonians, some amphibians, and birds. When cartilage is found in the sclera, it usually forms a complete cup that extends to the margin of the cornea or, in birds and lizards, to a ring of bony plates or ossicles. Scleral ossicles are located external to the ciliary body (Figure 1.27a and b). Ossicles are believed to have evolved for retaining ocular rigidity. The number of ossicles that comprise a ring can vary within the same species; in individual eyes with fewer ossicles, the single ossicle area increases, resulting in a constant scleral ring area, which must be avoided when this area is incised.
Between the inner sclera and outer uveal tissues is the sclerociliary/sclerochoroidal space, which is the exit pathway for the posterior uveoscleral aqueous humor drainage; this extends all the way to the optic nerve head.
Uvea
The iris, ciliary body, and choroid form the uvea. Unlike the fibrous coat, the uveal coat is highly vascular and usually pigmented. The ciliary body and choroid are loosely attached to the internal surface of the sclera (Figure 1.28). The iris originates from the anterior portion of the ciliary body, and it extends centrally to form a diaphragm (the pupil) anterior to the lens. The iris and ciliary body are collectively the anterior uvea, and the choroid is the posterior uvea.
Figure 1.28 SEM of the canine anterior uvea. Cornea (C), ciliary processes (CP), ciliary body musculature (CM), iris (I), and sclera (S). (Original magnification, 25×.)
Iris
The iris is a diaphragm that extends centrally from the ciliary body to cover the anterior surface of the lens, except for a central opening, the pupil. It divides the anterior ocular compartment into anterior and posterior chambers, which communicate through the pupil. The shape of the pupil varies widely among species. Among mammals, it is round in primates, canines, most large felines (cougar, leopard, lion, and tiger), and pigs; it is vertical when constricted in the smaller felines (bobcat, lynx, and domestic cat); and it is oval in a horizontal plane in herbivores (horses, cattle, sheep, and goats). In herbivores, along the upper and lower margins of the pupil are several round dark brown “masses” referred to as granula iridica (corpora nigra; Figure 1.29). The camelid species have a prominent pupillary ruff along the dorsal and ventral pupillary margins. These pigmented masses are extensions of the posterior pigmented epithelium that augment the effectiveness of horizontal pupillary constriction. Eyes of animals with pupils that constrict to a slit are believed, in most instances, to be more sensitive to light than those with circular pupils.
The iris has a central pupillary zone (the most active with pupillary changes) and a peripheral ciliary zone. The demarcation between these two zones is the collarette, which is best demonstrated clinically with moderate pupillary constriction. The portion of the pupillary zone adjacent to the pupil is sometimes more pigmented than the rest of the iris.
The function of the iris is to control the quantity of light entering the posterior segment through a central pupil. Constriction of the pupil reduces the amount of light entering the eye. Narrowing the pupil also eliminates the peripheral portion of the refractive system, which diminishes lenticular spherical and chromatic aberrations. During periods of reduced light, the pupil dilates allowing maximal stimulation of photoreceptor cells.
The iris is composed of an anterior border layer, stroma and sphincter muscle, and posterior epithelial layers. The anterior border layer
