The precorneal tear film is essential to maintain the normal transparent state of the cornea. The tear film consists of a superficial oily layer, a central aqueous (serous) layer, and a thin mucous (glycoproteinaceous) layer covering the cornea. The oily layer, produced by the modified sebaceous glands of the lid margin (tarsal or Meibomian glands), provides lubrication, prevents overflow of tears from the lid margins, and retards evaporation of the underlying aqueous layer. The thickness of this lipid layer varies from 0.013 to 0.586 µm in the normal dog. Expressed as equivalents of lauryl laurate, lipid levels in canine tears ranged between 3.51 (±0.8) and 3.41 (±0.68) g/mm2 eyelid margin surface. The aqueous layer is the major component of the precorneal tear film and is understood to mix readily with subjacent mucous later. It is produced by the lacrimal gland and the superficial gland of the third eyelid and contains a variety of factors necessary for maintaining corneal health. The aqueous layer also contains antimicrobial compounds such as transferrin and the immunoglobulin (Ig) A (the predominant immunoglobulin), IgG, and IgM. Lysozyme, another nonspecific antimicrobial compound, is absent in the dog.
Nerve growth factor is found in normal dog tears, corneal epithelium, third eyelid gland, and lacrimal gland at concentrations of 15.4 (±4.6) ng/mL, 33.5 (±12.3) ng/mL, 52.4 (±17.4) ng/mL, and 48.8 (±9.4) ng/mL, respectively. Although the concentration of nerve growth factor increases following corneal injury, it does not appear to facilitate wound healing in the dog. Additionally, the neuropeptide substance P has been measured in canine tears and has been implicated in the development of spontaneous chronic corneal epithelial defects in dogs.
The deepest layer of the precorneal tear film is the glycoproteinaceous or mucous layer, which is intimately associated with the surface of the superficial squamous cells of the anterior corneal epithelium. This layer, a product of the conjunctival goblet cells, is thought to assist in adherence of the precorneal tear film to the corneal surface by decreasing the surface tension of the tears. A deficiency in the mucous layer will result in a decreased tear breakup time. The glycoprotein associated with mucus contains high levels of sialic acids, which mediate a variety of events including those associated with immunity. Normal canine ocular mucin contains neutral O-linked glycans consisting mainly of di-, tri-, or tetrasaccharides terminating in α 1-2 fucose or α 1-3 N-acetylgalactosamines. Mucins with different subunit structures, one of which may be complexed with lipid, and two membrane-bound mucins have also been described.
The lacrimal apparatus includes those structures responsible for the production, dispersal, and disposal of the tears. The lacrimal gland, tarsal glands, conjunctival goblet cells, and the superficial gland of the third eyelid contribute to the tear film. Lacrimal fluid flows across the cornea, aided by blinking, to the lacrimal puncta at the medial commissure of the lids and thence through the lacrimal canaliculi and nasolacrimal duct to the nasal vestibule.
A deficiency in the aqueous layer of the tear film resulting from a defect in the lacrimal apparatus may result in corneal lesions or chronic conditions such as keratoconjunctivitis sicca (dry eye).
When exposed to a natural photoperiod, dogs exhibit significant circadian rhythms of tear production, which peaks during nocturnal hours. This circadian rhythm disappears during periods of constant light but not during constant darkness. Changes in the protein composition of the tear film may indicate the presence of systemic disease such as cancer.
The lacrimal gland (glandula lacrimalis) is a pink, oval, lobated gland that lies deep to the periorbita on the superiolateral aspect of the eyeball. The gland is flattened between the eyeball and the orbital ligament and zygomatic process of the frontal bone. A shallow fossa for the lacrimal gland may be present in the orbital face of the frontal bone. A thin sheet of fascia separates the gland from the underlying dorsal and lateral rectus muscles. The seromucous secretion of the gland empties into the dorsolateral conjunctival fornix through three to five microscopic secretory ducts. Removal of the lacrimal gland results in only a minor decrease in tear production, probably as a result of a compensatory increase in production by the superficial gland of the third eyelid.
The lacrimal gland is innervated by the lacrimal nerve, a small branch of the ophthalmic nerve from the trigeminal nerve. Parasympathetic postganglionic axons from the pterygopalatine ganglion enter the periorbita in orbital rami and are distributed to the gland with the lacrimal or zygomaticotemporal nerves. Parasympathomimetic drugs increase the rate of glandular secretion. Tear production can also be stimulated by the topical application of nerve growth factor. The blood supply is derived from the muscular branches of the external ophthalmic artery. The lacrimal vein drains the gland into the dorsal external ophthalmic vein.
Superficial Gland of the Third Eyelid
The superficial gland of the third eyelid (glandula superficialis plica semilunaris conjunctivae) in the dog is an accessory lacrimal gland that normally produces a significant proportion of the aqueous component of the tear film. The gland surrounds the column of the cartilage of the third eyelid in the inferiomedial aspect of the eyeball. The gland is mixed in the dog; both mucous and serous acini are seen microscopically. The secretions of this gland flow through numerous microscopic ductules into the conjunctival sac. There is no deep, lipid-producing (Harder) gland of the third eyelid in the dog.
The glycoprotein component of the tear film is produced mostly by the goblet cells of the conjunctivae. These cells are especially numerous in the regions of the fornices and contribute significantly to the tears. The dog does not have isolated accessory lacrimal glands throughout the conjunctiva corresponding to the several types bearing eponyms in humans. The tarsal glands (see section on eyelids) produce the oily superficial layer of the tear film. Tears accumulate in the lacrimal lake (lacus lacrimalis), which is the shallow cleft between the third eyelid and inferior palpebral conjunctiva just lateral to the medial commissure. Blinking both facilitates tear secretion and spreads the tear film over the cornea. This action is essential to its health. Inability to close the lids, as a result of facial nerve lesions, allows the cornea to dry and results in marked pathologic alterations (neuroparalytic keratitis). The lacrimal fluid produced in excess of that which evaporates from the surface of the cornea drains into the nasal cavity through the nasolacrimal duct system.
Nasolacrimal Duct System
The outflow pathway of excess lacrimation includes the puncta, the lacrimal canaliculi, the lacrimal sac, and the nasolacrimal duct. The lacrimal puncta (puncta lacrimalia) are located on the deep surface of the superior and inferior lid margins 2 to 5 mm from the medial commissure. The puncta are the oval openings of the lacrimal canaliculi (canaliculi lacrimale) and measure approximately 0.7 mm by 0.3 mm. The long axis of each punctum is parallel to the lid margin. The puncta are distinguished from the openings of the tarsal glands by their larger size and greater distance from the lid margin. In some dogs, one or both puncta may be smaller than normal or absent. Spilling of tears onto the face (epiphora) results.
The superior lacrimal canaliculus runs medially parallel to the lid margin for 3 to 7 mm from the superior punctum, and then turns ventrally medial to the commissure of the lids to enter the lacrimal sac (saccus lacrimalis). The inferior lacrimal canaliculus arcs inferiomedially from its punctum to join the superior duct at the lacrimal sac. The canaliculi are easily cannulated if the instrument is directed medially, parallel to the lid margin. Radiopaque contrast material can be introduced into the nasolacrimal duct system through the canaliculi to outline the duct system radiographically.
The lacrimal sac is the dilated origin of the nasolacrimal duct. It is not as prominent in the dog as in humans, leading some authors to conclude that it is not present. The sac occupies a depression (fossa sacci lacrimalis) in the center of the orbital surface of the lacrimal bone, medial and ventral to the medial commissure.
The nasolacrimal duct (ductus nasolacrimalis) forms an arch, which is concave dorsally as it passes rostrally from the lacrimal sac through the lacrimal canal of the lacrimal bone and maxilla. Rostral to the conchal crest, the duct is no longer covered by bone but continues rostrally deep to the nasal mucosa on the nasal face of the maxilla. In approximately 50% of dogs, the nasolacrimal duct has two openings. At the level of the root of the canine tooth, there is an inconstant communication of the duct with the nasal cavity ventral to the ventral nasal concha. Rostrally, the duct passes medial to the ventral lateral nasal cartilage and ends by opening onto the ventrolateral floor of the nasal vestibule ventral to the alar fold. The rostral opening cannot be visualized without a speculum in the living dog.
The nasolacrimal duct is supplied by a small branch of the malar artery.