Page 1152 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

CHAPTER 11

EYES
VetBooks.ir Claudia Hartley and David Donaldson 1127

The horse has the largest eye of any land 11.1
mammal. The lateral globe placement and
horizontal ovoid pupil allow a total visual field
of 350°. Narrow blind spots exist immediately in
front of the nose and directly behind the hind-
quarters. Both eyes are used to look at a distant
object until it comes within 1.0–1.3 metres
(3–4 feet), which forces the horse to turn the
head and look with only one eye. The mean
refractive error is –1.0 dioptres (D).

ANATOMY OF THE EQUINE EYE
Fig. 11.1 The granula iridica (arrow) is most
The eyes of horses have a number of unique ana- prominent dorsally.
tomical differences when compared with those of
the dog and cat. These include a complete bony The horse has a paurangiotic fundus that con-
orbit, which gives added protection to the eye, and tains 40–60 small retinal vessels that radiate from
well- developed extraocular muscles, which make the edge of the optic nerve head (ONH) and extend
manipulation of the eyelids and globe challenging. only 1.0–2.5 disc diameters from the optic disc.
The globe is slightly deviated medially and ventrally The variations in the normal equine fundus are
(mild medioventral strabismus) in the neonatal foal; numerous and are primarily related to coat and eye
however, it attains the normal adult position by colour. The optic disc appears as a horizontal, oval to
1 month of age. The iridocorneal angle (ICA) in the round, salmon pink structure that is located slightly
horse is easily visible temporally and nasally without temporally in the non-tapetal region (Fig. 11.2). It is
the use of a goniolens. The equine pupil is bordered approximately 5–7 mm horizontally and 3.5–5 mm
by a granula iridica (corpora nigra), an exaggerated vertically and is used to estimate the size of fundic
prominence of the posterior pigmented epithelium lesions in terms of optic disc diameters. A physiolog-
layers of the iris (Fig. 11.1). The granula iridica is ical cup is generally not apparent and the ONH is
more prominent on the dorsal pupillary margin than only rarely myelinated. When present, myelin may
ventrally. This structure is believed to play a role in appear as white to grey streaks radiating from the
the filtration of light through the pupil. The normal ONH and following the course of the retinal vessels
equine lens has prominent Y sutures and needs to nasal and temporal to the disc. The normal tapetum
accommodate less than 2 D to maintain a focused may be yellow, green or blue, with small reddish-
image on the retina. Persistent hyaloid artery rem- brown dots that represent end-on views of choroi-
nants are common in the equine neonate and may dal capillaries, or ‘stars of Winslow’ (Fig. 11.3). The
contain blood in the first few hours after birth; how- non-tapetal fundus is typically heavily pigmented,
ever, they usually disappear by 3–4 months of age. appearing dark brown or chocolate. The fundus may