Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 33

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.