Nictitating membrane
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The nictitating membrane ( Plica semilunaris conjunctivae , Membrana nicitans ) or "third eyelid " ( Palpebra tertia ) is an additional fold of the conjunctiva in the nasal corner of the eye. In humans and most primates - the common bear lemur is an exception - it is only rudimentary . In many other vertebrates, it is transparent and can be folded over the eye like protective goggles .
construction
The nictitating membrane consists of its own layer in which the T-shaped cartilage ( Cartilago palpebrae III ) is embedded. The self-layer is covered by a multilayered epithelium with goblet cells . There are numerous lymph follicles in the mucous membrane of the nictitating membrane .
Are in the haw different tierartlich, glands into storage, the additional tear ducts represent. The superficial nictitating gland ( glandula palpebrae tertiae superficialis ) lies on the nasal crossbar of the blink cartilage. The deep nictitating gland ( glandula palpebrae tertiae profunda ) - also known as Harder's gland - is not found in all animals. It is pronounced, for example, in rodents, cattle and pigs. The nictitating glands, together with the lacrimal gland, produce the middle layer of the tear film .
In many vertebrates , the nictitating membrane has embedded muscle fibers that can actively move the third eyelid horizontally over the cornea. In most mammals the nictitating membrane muscles are rudimentary, cats are an exception. In animals without nictitating membrane muscles, retraction of the eyeball ( enophthalmus ) leads to a passive forward displacement of the nictitating membrane. This fact is also used when examining the nictitating membrane: The nictitating membrane is moved forward by gently pressing the eyeball through the eyelids.
The blood supply takes place through the arteria palpebrae tertiae from the arteria malaris .
function
Like protective goggles, the nictitating membrane protects the cornea from mechanical influences and can be used like a windshield wiper to remove foreign bodies. In some aquatic animals ( beavers , manatees ) it is shifted in front of the cornea during diving. With other aquatic mammals such as the ear seals , however, it is only used when staying on land and prevents the penetration of dust grains. In polar bears , the nictitating membrane acts like snow goggles and protects against snow blindness .
In birds, two muscles are embedded in the nictitating membrane, the quadratus membranae nicitantis muscle and the pyramidalis membranae nicitantis muscle . They enable the nictitating membrane to blink actively , which in birds plays a greater role in the distribution of tear fluid than the lids themselves. In domestic fowls, the nictitating membrane makes about 35 blinkers per minute.
Diseases
A pathological prolapse of the nictitating membrane is observed in enophthalmos , such as is observed in tetanus , for example . Poisoning, Horner's Syndrome , Feline Dysautonomia , Cachexia , Phthisis Bulbi , Conjunctivitis and Haw's Syndrome can also lead to a nictitating membrane prolapse.
A hypertrophy of the lymphoid follicles ( conjunctivitis follicularis ) on the cornea side surface of the third eyelid comparatively frequently in young dogs. The enlarged lymph follicles then act like a rubbing plate and lead to mechanical irritation of the cornea.
The blink cartilage version is a kink of the blink cartilage. When the nictitating gland prolapses, the nictitating gland prolapses. Both diseases are mostly treated surgically.
Web links
literature
- Paul Simoens: organ of vision , organum visus. In: Franz-Viktor Salomon, Hans Geyer, Uwe Gille (Ed.): Anatomy for veterinary medicine. Enke, Stuttgart et al. 2004, ISBN 3-8304-1007-7 , pp. 578-611.
- Uwe Gränitz, Vera Schmidt: Nictitating membrane prolapse. In: Marian C. Horzinek, Vera Schmidt, Hans Lutz (eds.): Diseases of the cat. 4th, edited Ed. Enke, Stuttgart 2005, ISBN 3-8304-1049-2 , pp. 492-493.
Individual evidence
- ^ Franz-Viktor Salomon, Maria-Elisabeth Krautwald-Junghanns: Anatomy of the birds. In: Franz-Viktor Salomon, Hans Geyer, Uwe Gille (Ed.): Anatomy for veterinary medicine. 2nd, revised and expanded edition. Enke, Stuttgart et al. 2008, ISBN 978-3-8304-1075-1 , p. 806.