Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 42

The sclera comprises the remainder of the fibrous tunic of the globe. Anteriorly, it merges with the peripheral cornea and the bulbar conjunctiva to form a transition zone, the limbus (Figure 1.25a and b). The limbus and now the peripheral cornea are the main entry sites for incisions into the anterior chamber (as for cataract surgery). At the limbus, the sclera is variably pigmented, and the overlying epithelium is thicker, with pigmented epithelial cells (see Figure 1.25b). The stroma loses the regular arrangement characteristic of the cornea and takes on a less organized appearance of irregular, dense connective tissue. Numerous blood vessels (i.e., the anastomosing branches of the anterior ciliary arteries) terminate in the loops of the marginal plexus, and then drain back into the conjunctival venules. Any incision in the sclera will 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.