Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 28

The orbit is the bony fossa that surrounds and protects the eye while separating it from the cranial cavity. Through numerous foramina, the orbit also provides pathways for various blood vessels and nerves involved in the function of the eye and nearby structures. The size, shape, and position of the orbit differ by species and are closely associated with time of visual activity and feeding behavior (Table 1.5). In domestic carnivores such as the cat and dog, the orbital axes are set rostrolaterally, approximately 10° and 20° from midline, respectively, to enhance binocular vision and predatory feeding behavior.

In horses and ruminants, the orbits are positioned more laterally than carnivores, being approximately 40° (i.e., horses) and 50° (i.e., cattle) from midline. Monocular vision in these and other ungulate species is enhanced, providing a strong panoramic line of vision, which allows for scanning the horizon to search for potential predators.

In the rabbit, the axis of each eye extends as much as 85° from the midline; this orbit placement also occurs among the majority of lizards, some snakes, and in certain fish. In these latter instances where binocular vision has become greatly reduced, there is a tendency for the eyes to protrude so that the visual axis of the eye can expand what the optic axis of the skull provides.

All vertebrate orbits are one of two kinds: (i) the enclosed orbit, which is completely encompassed by bone; or (ii) the open or incomplete orbit, which is only partially surrounded by bone (Figure 1.5a and b). Among domestic animals, horses, sheep, cattle, and goats have enclosed orbits. Pigs and carnivores (i.e., dogs and cats) have open orbits. The enclosed orbit of large herbivorous prey species is theorized to be essential for protection (and sometimes horns), whereas the open orbit gives carnivores the ability to open their jaws widely during consumption of prey.

The bony orbit typically consists of five to seven bones, depending on the species (see Table 1.4). The canine orbit is composed of five, and sometimes six, bones, the supraorbital ligament that extends from the frontal to the zygomatic bone, and the periosteum (Figure 1.5a). The orbital rim is formed by the frontal, lacrimal, and zygomatic bones. Laterally, the orbit is formed by the supraorbital ligament that is contiguous with a fibroelastic connective tissue sheath for much of the floor of the orbit. The orbital floor is incomplete, being partially formed by the sphenoid and palatine bones. Therefore, surgical entry into the deeper orbit is from the dorsal but primarily the lateral wall.

Table 1.4 Orbital dimensions.

Dimension	Feline (mm)	Canine (mm)	Bovine (mm)	Equine (mm)
Width	24	29	65	62
Height	26	28	64	59
Depth	—	49	120	98
Distance between orbits	23	36	151	173

In the feline orbit, the processes of the frontal and zygomatic bones extend a great deal more toward one another, resulting in a shortened supraorbital ligament (Figure 1.5b). There is limited orbital space in cats. In animals with enclosed orbits, closure of the temporal side of the orbit is accomplished by union of the zygomatic process of the frontal bone with the frontal process of the zygomatic bone. In the horse, the zygomatic process of the temporal bone intervenes between these two and completes the orbital rim (Figure 1.6).

Within the orbit, various foramina and fissures provide osseous pathways for blood vessels and nerves to pass from the cranial cavity and alar canal into the orbital region (Table 1.5). Those foramina of rather constant position in domestic animals are the rostral alar, ethmoidal, lacrimal, orbital, ovale, optic, rotundum, and supraorbital. Other foramina closely related to the orbital structures are within the pterygopalatine region, and these are the maxillary, caudal palatine, and sphenopalatine. The orbital foramen is elongated in most domestic animals, except the horse; therefore, it is referred to as the orbital fissure. In cattle, the orbital fissure and foramen rotundum are typically fused to form the foramen orbitorotundum.