Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 61

Classically, variations in the retinal vasculature have been categorized into four basic patterns: holangiotic, merangiotic, paurangiotic, and anangiotic. Most mammals possess the holangiotic pattern, in which the majority of the neurosensory retina receives a direct blood supply. The merangiotic pattern consists of blood vessels localized to a region of the retina medial and lateral to the optic disc. Examples of animals with this retinal vascular pattern are lagomorphs (rabbits and pika). In the paurangiotic pattern, blood vessels within the retina occur only circumferentially near the optic disc (peripapillarily). This pattern is seen in certain ungulates, such as horses, elephants, and rhinoceroses, and in some marsupials such as kangaroos. The anangiotic pattern is characterized by an absence of any vasculature within the neurosensory retina, and it occurs in sugar gliders, guinea pigs, chinchillas, and nonmammalian species such as birds (Figure 1.56a and b). In birds, a structure called pectin (pectin oculi) lies vitread to the optic nerve head. It is pigmented and pleated structure, and contains a rich plexus of blood vessels. In general, the retinal arterial supply in domestic animals comes from the short posterior ciliary arteries, which are termed cilioretinal arteries, rather than via a central retinal artery origin as in higher primates, rats, and mice.

Figure 1.56 (a) The avian pecten, as seen here in the chicken, consists of a pleated vascular plexus that lies vitread atop the optic nerve head (ON). (Original magnification, 50×.) (b) Close‐up of the base of the pecten as it internally lines the nerve fibers (NF) that form the optic nerve head. BV, blood vessels of the pecten. (Original magnification, 250×.)