SENSES
Being a prey species, the rabbit has acutely developed sense organs. Like rodents, rabbits have acute hearing and are very sensitive to high frequency sounds (Batchelor 1999). The ears are long and funnel shaped to gather sound, and the eyes are laterally located, offering binocular vision.
Eyes
The orbits are laterally situated with prominent globes and the rabbit can obtain almost 360 degree vision by tilting the head and moving the eyes. The upper lid is shorter and thicker than the lower lid with more numerous cilia. Rabbits blink only 10 to 12 times per hour, which makes them predisposed to corneal ulceration and dry eye and they have been used - and abused - as models for opthalmological research (Andrew 2002; Peiffer et al. 1994). The third eyelid does not visibly nictitate but can be retracted by applying pressure to the globe. It does not move by more than two-thirds across the globe (Peiffer et al. 1994).
The orbit is circular and made of bone, except at the lower rim where it is walled by the muscles of mastication. The rabbit has one more extraocular muscle than most mammals, making a total of nine. This is because the globe extends beyond the lower rim so the depressor palpebrae muscle is necessary to depress the lower lid (Peiffer et al. 1994).
Blood supply
The internal maxillary artery enters the orbit where it gives rise to the external ophthalmic artery. This anastomoses with the internal to supply the extraocular muscles. An extensive postorbital venous sinus provides venous drainage and must be avoided if operating to enucleate the eye (Peiffer et al. 1994).
Lacrimal system
The aqueous tear film is produced by three glands which empty via the lacrimal puncta and canaliculi into the nasolacrimal duct. Normal Schirmer tear tests are 5 (2.96) mm/ minute (Peiffer et al. 1994).
The lacrimal gland
The lacrimal gland is large, bilobed, pale red in color and lies beside the lower rim with a bulbous enlargement at the medial canthus.
It plays a lesser role in tear secretion so portions can be removed without adverse long-term effects to tear production (Peiffer et al. 1994).Harderian gland
This is also called the “deep gland of the third eyelid” and is attached to the rostromedial wall of the orbit. The gland is large, encapsulated, and measures about 15?5 mm. It is surrounded by the orbital venous sinus. It is kidney shaped and has two distinct lobes. The excretory ducts from both lobes converge into one duct which opens behind the third eyelid. The gland is larger in males than females and is at its largest during the breeding season (Peiffer et al. 1994).
Third eyelid gland
This is also called the “superficial gland of the third eyelid.” This gland is similar to the Harderian gland and surrounds the shaft of the third eyelid.
Nasolacrimal duct
The rabbit is unique in having only one lacrimal puncta located on the lower eyelid 3 mm from the eyelid margins and medial canthus. A short canaliculus (2 mm) leads into the funnel shaped lacrimal sac. The duct then passes through the lacrimal bone on the medial orbit and along the maxilla where it is encased in bone. It can be divided roughly into four portions (Burling et al. 1991; Peiffer et al. 1994):
1. Nasal puncta to proximal maxillary curve where it narrows (1 mm)
2. Tubular portion along lacrimal canal to base of incisor root (2 mm)
3. ‘S'-shaped bend around incisor root (1 mm)
4. Rostral duct running through nasal cartilage to exit at tiny nasopunctum just caudal to the mucocutaneous junction
Along its length the duct is lined by a highly folded epithelium with a rich vascular and lymphatic supply. This undulating epithelium, along with the narrow, sharp bends and small nasal opening, means it can easily accumulate inflammatory debris that cause obstruction of the tear duct (Burling et al. 1991).
CLINICAL NOTE
The tortuous and narrow nasolacrimal duct with two sigmoid curves means that pet rabbits are very vulnerable to duct obstruction and dacryocystitis. Treatment can only be helped by flushing the duct and treating any underlying dental disease (Burling et al. 1991; Harcourt-Brown 1997).
Globe
The cornea is large and occupies 30% of the globe (Andrew 2002; Donnelly 1997). The rabbit pupil is ovoid vertically but becomes circular when widely dilated. The lens is large and has a poorly developed ciliary body, giving the rabbit poor accommodation. The rabbit retinal vascular pattern is merangiotic, which is characterized by a limited area of blood vessels. Albino rabbits have a non-pigmented retina. The optic nerve is located above the midline so retinal examination involves looking up into the eye. There is no tapetum lucidum and rabbits, like dogs, have a depression in the optic disk (Andrew 2002; Peiffer et al. 1994).