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Essentials of Veterinary Ophthalmology. Kirk N. Gelatt
Читать онлайн.Название Essentials of Veterinary Ophthalmology
Год выпуска 0
isbn 9781119801351
Автор произведения Kirk N. Gelatt
Жанр Биология
Издательство John Wiley & Sons Limited
Figure 1.13 Bulbar conjunctiva of a porcine eyelid is externally lined by a stratified to pseudostratified columnar epithelium possessing numerous goblet cells (GC) near the fornix.
The lymphatics of the conjunctiva, called the conjunctiva‐associated lymphatic tissue (CALT), are arranged in two plexuses: a superficial and a deep system. CALT is generally diffuse with intermittent nodules or follicles. Often, the diffuse component of CALT infiltrates and is adjacent to tear‐secreting glands, especially those associated with the NM. Variations in the size and distribution of nodules occur between the upper and lower eyelids and are influenced by exposure to various foreign substances, including potentially infectious microorganisms. The conjunctiva at the fornix is very thin and translucent, and it lies loosely on the underlying connective tissue. In the domestic carnivore, approximately 3 mm from the limbus, the bulbar conjunctiva, Tenon's capsule, and sclera become closely united. The connective tissue is much more abundant in this location in the dog than in humans and other species. The primary functions of the conjunctiva are to prevent desiccation of the cornea, to allow mobility of the eyelids and the globe, and to provide a physical and physiological barrier against microorganisms and foreign bodies.
Nictitating Membrane
The NM (membrana nictitans, third eyelid, or plica semilunaris) protrudes from the medial canthus in the ventromedial anterior orbit. It contains a cartilaginous, T‐shaped plate, the horizontal part of which is parallel to the free or leading edge of the membrane (Figures 1.14 and 1.15). In many species, its free edge is pigmented. The stroma consists of loose to dense connective tissue that supports glandular and lymphoid tissue. The distal portion of the anterior (i.e., palpebral) and posterior (i.e., bulbar) surfaces is usually covered with nonkeratinized stratified squamous epithelium. The NM possesses a prominent accessory lacrimal gland often referred to as the NM gland (nictitans gland) or gland of the NM. This gland is serous in horses and cats, mixed (seromucous) in cattle and dogs, and mostly mucous in pigs.
Figure 1.14 Drawing of a histological section of the mammalian NM.
Figure 1.15 NM of the horse contains both glandular (G) and lymphoid (L) tissues, with the latter being superficially located within the stroma next to the bulbar surface (BS). C, cartilage. (Original magnification, 10×.)
The cartilage of the NM is predominately elastic in the horses, cats, and pigs and hyaline in ruminants and dogs. The three‐dimensional shape of the cartilage varies considerably among domestic species. The horizontal portion of the T cartilage appears as a reverse S shape in the cat, a crescent shape in the dog, and a hook shape in the horse. This cartilage is important when placing sutures around it for maximal holding of the nictitans flaps.
The Harderian gland (Harder's gland), when present, is usually located posterior to the NM, and appears grossly and histologically to be an extension of the NM gland. This glandular tissue in some animals can be considerably larger than the NM gland. The anatomical presence of the Harderian gland among mammals has been found mostly in rodents, with only the Mongolian gerbil having the nictitating gland as well. In mammals, the secretory cells of the Harderian glands are columnar and lined by myoepithelium. Most importantly, their secretions contain unusual compounds, including porphyrins and melatonin.
Harderian glands contain autonomically controlled nerves and are also under the control of gonadal, thyroid, and pituitary hormones. The functions of this gland remain speculative, but they may include immunological defense and photoprotection. In most domestic animals, the movement of the NM is indirect, resulting from contraction of the retractor oculi muscle, which retracts the globe into the orbital space and causes passive elevation of the NM, but in the domestic cat, small bundles of smooth muscle have been found in the NM that most likely contribute to its more rapid movements.
Lacrimal and Nasolacrimal System
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