Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 34

An adequate precorneal tear film (PTF) is necessary for optical integrity, maintenance of the cornea, and normal ocular function. The PTF serves several functions, including

maintenance of an optically uniform corneal surface, removal of foreign material and debris from the cornea and conjunctival sac, an oxygen source to the outer avascular cornea, and lastly presence of antimicrobial substances (see Chapter 2, Figure 2.1).

The PTF is trilaminar, although all three layers are intricately mingled, and can be visualized clinically with slit lamp biomicroscopy. The outer, thin, oily layer is produced by the meibomian glands and sebaceous glands of Zeis. This layer reduces evaporation of the underlying aqueous layer and forms a barrier along the lid margins that prevents tear overflow. The middle layer is the aqueous layer and is secreted by the orbital lacrimal gland (61.7%), the accessory glands (3.1%), and the gland of the NM (35.2%). This layer delivers oxygen and other nutrients to the avascular cornea and provides a volume of fluid to “flush” the ocular surface and remove debris. The innermost layer is the mucin layer and is produced predominately by the conjunctival goblet cells. The glycocalyx, produced by the corneal epithelial cells, also contributes to the mucin layer. This layer provides a hydrophilic surface over which the aqueous tear fluid spreads evenly and lubricates the corneal and conjunctival surfaces.

Excess lacrimal fluid collects by gravity in the lower conjunctival sac and is mechanically “pumped” through the upper and lower lacrimal puncta located approximately 1–2 mm inside the margin of the medial eyelid (Figure 1.16). Each lacrimal punctum is surrounded by smooth muscle that works in coordination with eyelid blinking to remove excess lacrimal fluid and prevent its backflow. These puncta continue as the upper and lower canaliculi, which pass slightly vertically away from the eyelid margins and turn toward the medial canthus, pass through the periorbita, and meet at a dilation, the lacrimal sac, located in the lacrimal fossa of the lacrimal bone. This sac empties into the nasolacrimal duct, which passes through a short, bony canal (hence, its smallest diameter and the frequent site of obstructions) and opens into the nasal cavity, where it continues as a duct until it reaches an opening at the floor of the nostril approximately 1 cm from the end of the nares. Approximately 40% of dogs have an accessory opening in the canal as it passes by the root of the upper canine tooth.

The lacrimal gland is a diamond‐shaped structure in the dorsolateral aspect of the orbit underneath the orbital ligament. The mean length, width, thickness, and weight of the relatively flat lacrimal gland in three different breeds of dogs were ~17 ± 0.7 mm, ~13 ± 0.4 mm, ~3 ± 0.1 mm, and ~316 ± 21 mg, respectively. Fifteen to twenty small ductules drain into the superior conjunctival fornix. Histologically, the gland is a tubuloalveolar type. The innervation to the lacrimal gland is not fully understood, but the lacrimal branch of cranial nerve V, and sympathetic and parasympathetic nerves are all involved in its function. Clinically, certain cholinergic drugs (e.g., pilocarpine) stimulate tear secretion, whereas other drugs (i.e., anticholinergics) decrease tear secretion.

Figure 1.16 The nasolacrimal system: lacrimal puncta, canaliculi, lacrimal sac, nasolacrimal duct, lacrimal gland, and lacrimal ducts.

Figure 1.17 Diagram of the three tunics that comprise the mammalian globe. Outermost fibrous tunic (light and dark purple), consisting of the cornea and sclera; the middle tunic called the uvea (light orange), consisting of the iris, ciliary body, and choroid; and the nervous tunic (dark orange) consisting of the retina and optic nerve.