Tapetum
These neural crest‐derived tissues are all induced by the outer layer of the optic cup (future RPE). Normal RPE differentiation is a prerequisite for normal development of the sclera and choroid. The choroid and sclera are relatively differentiated at birth, but the tapetum in dogs and cats continues to develop and mature during the first four months postnatally.
Vitreous
The primary vitreous forms posteriorly, between the primitive lens and the inner layer of the optic cup. In addition to the vessels of the hyaloid system, the primary vitreous also contains mesenchymal cells, collagenous fibrillar material, and macrophages. Primitive hyalocytes produce collagen fibrils that expand the volume of the secondary vitreous.
The tertiary vitreous forms as a thick accumulation of collagen fibers between the lens equator and the optic cup. These fibers are called the marginal bundle of Drualt, or Drualt's bundle. Drualt's bundle has a strong attachment to the inner layer of the optic cup, and it is the precursor to the vitreous base and lens zonules. The early lens zonular fibers appear to be continuous with the inner, limiting membrane of the nonpigmented epithelial layer covering the ciliary muscle. Atrophy of the primary vitreous and hyaloid leaves a clear, narrow central zone, which is called Cloquet's canal.
Optic Nerve
Axons from the developing ganglion cells pass through vacuolated cells from the inner wall of the optic stalk. A glial sheath forms around the hyaloid artery. As the hyaloid artery regresses, the space between the hyaloid artery and the glial sheath enlarges. Bergmeister's papilla represents a remnant of these glial cells around the hyaloid artery. Glial cells migrate into the optic nerve and form the primitive optic disc. The glial cells around the optic nerve and the glial part of the lamina cribrosa come from the inner layer of the optic stalk, which is of neural ectoderm origin. Later, a mesenchymal (neural crest origin) portion of the lamina cribrosa develops. Myelinization of the optic nerve begins at the chiasm, progresses toward the eye, and reaches the optic disc after birth.
Eyelids
The eyelids develop from surface ectoderm, which gives rise to the epidermis, cilia, and conjunctival epithelium. Neural crest mesenchyme gives rise to deeper structures, including the dermis and tarsus. The eyelid muscles (i.e., orbicularis and levator) are derived from craniofacial condensations of mesoderm called somitomeres. The upper eyelid develops from the frontonasal process; the lower eyelid develops from the maxillary process. The lid folds grow together and elongate to cover the developing eye. The upper and lower lids fuse on day 32 of gestation in the dog. Separation occurs two weeks postnatally.
Extraocular Muscles
The extraocular muscles (EOM) arise from mesoderm in somitomeres (i.e., preoptic mesodermal condensations). Spatial organization of developing eye muscles is initiated before they interact with the neural crest mesenchyme. From studies of chick embryos, it has been shown that the oculomotor‐innervated muscles originate from the first and second somitomeres, the superior oblique muscle from the third somitomere, and the lateral rectus muscle from the fourth somitomere. The entire length of these muscles appears to develop spontaneously rather than from the orbital apex anteriorly.
Section II: Morphology of the Eye and Adnexa
Introduction
A thorough understanding of normal ophthalmic anatomy is an integral part of the foundational knowledge for diagnosis and treatment of ophthalmic diseases as the majority of the ocular tissues can be visualized directly. The veterinarian can examine eyes from a wide variety of animal species, and fortunately the eye has largely retained the same basic components, but important and clinically relevant differences do exist. This chapter will primarily present the ophthalmic anatomy of dogs, cats, horses, livestock species, and occasional birds, and relates information important to the clinician. More detailed anatomical information is available in the standard veterinary and comparative anatomy texts.
Orbit
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 |
