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The pecten or pecten oculi (Latin for "comb of the eye") is a comb-like structure of belonging to the choroid in the eye of a bird, and no other species. It is a non-Sensory system, structure that projects freely into the vitreous humor from the point where the optic nerve enters the eyeball, and undulates with movements of the vitreous humor. It almost entirely covers up the optic disc.
Histology, it contains 3 types of tissues: a plexus of modified blood vessels, darkly pigmented cells interdigitated between the blood vessels, and supporting tissue. The supporting tissue is and syncytium, and derived from the optic disc. There are no muscle, nerve fibers, or sensory tissue. The arterial blood is supplied by a branch of the hyaloid artery emerging from the optic disc entirely separate from the Choroidal artery. The artery runs along the base of the pecten and sends ascending branches to each of the folds. Melanin is abundant at the apical and peripheral pecten, produced by pleomorphic Melanocyte that form incomplete sheaths along the plexus of capillaries.
More than 30 functions have been proposed for the pectan, which are reviewed in and. The most commonly accepted theory is that it provides nutrition to the retina, and control the pH of the vitreous body. High levels of alkaline phosphatase and carbonic anhydrase activity in the pecten oculi have been linked to the transport of nutrient molecules from the highly vascularized choroid into vitreous and retinal cells, thus nourishing the eye. Saccadic eye movements caused the pecten to oscillate, fanning the liquid in the vitreous. This suggests that saccade and the pecten co-evolved to diffuse metabolites such as oxygen and glucose out from the pecten.
In the vertebrate eye, there are blood vessels in front of the retina, partially obscuring the image. In most avians, the retina is completely free of blood vessels and leading to the extremely sharp eyesight of birds such as . The retina is supplied instead by the choroid and the pecten. The pigmentation of the pecten is believed to protect the blood vessels against damage from ultraviolet light. Stray light absorption by melanin granules of pecten oculi is also considered to give rise to small increments in temperature of pecten and eye; this may offer increased metabolic rate to optimize eye physiology in low temperatures at high-altitude flights.
There was also an anatomical description of the eye of an eagle in 1681, which was quoted from a description by Ole Borch in 1674. Early alternative names included the pecten plicatum and the marsupium, mentioned in a paper by Crampton, who argued that the pecten could not have been used in visual accommodation, because it has no muscles connecting to the lens. Instead, it was due to a ciliary muscle (which he discovered) that attaches to the inner lamella of the cornea and the scleral ring.
For the detailed history, see.
The conical type looks similar to the conus papillaris, and is a simple cone rising up from the base on a circular optic disc. It has no folds. It is trumpet-shaped and heavily brown-black in color. It almost touches the lens.
The vaned type looks like a thin sheet rising up from the base over an oval-shaped optic disc. There are 25-30 thin folds extending out from the sheet. The folds are roughly trapezoidal, short on the top and long on the base. See Figure 507 for an example.
The pleated type looks like an accordion. The base is longer than the top. There is usually a ridge at the top called the "bridge", which keeps the accordion shorter at the top. If the pectan is cut off from the retina at the base, then its bridge is cut off, then it can be flattened to a flat sheet. Owls, Podargus, and Haliaeetus albicilla do not have the bridge. In the Alcedo atthis japonica, the crest of each pleat contains 1 to 3 membranous extensions that resemble the vaned type.
The pectans tend to be larger and have more folds in Diurnality birds than Nocturnality birds. The number of pleats varies between 5 and 30. In predators the folds are thicker but fewer (13 to 17). Sea-birds and shore-birds tend to have fewer pleats (≤ 12).
In the owl Bubo virginianus, it projects out into the vitreous cavity 5–6 mm, whereas in the dove Wonga pigeon, it reaches almost as far anteriorly as the equatorial lens.Ringvold, Amund. "The function of pecten oculi. Conus papillaris in reptiles and its analogue pecten oculi in birds evolved in tandem with increasing uric acid in serum." Int J Pure Appl Zool 10 (2022): 136-149.
Most of the primates also possess a small bump on the optic disc, which is termed the papilla nervi optici. Amongst mammals, vestiges of a structure similar to the conic pecten oculi can occasionally be observed in marsupials.
Some mammals, such as and brushtail possum, have nearly avascular retina. Since they also do not have the pecten, the retina is supplied entirely from the choroid. This limits their retinas to be significantly thinner than in mammals with vascular retina.
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