Pupil

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 Pupil.

P.K.Ghatak,MD

No.60



The central aperture of the iris is the pupil. People who like talk and write British English call students as pupils.

The pupils, by varying their sizes, control the amount of light that can enter the eye for us to see clearly. Pupil also serves as a passage for the aqueous humor to circulate from the posterior compartment to anterior compartment. At the periphery of the iris (No.5 in the diagram below) the aqueous humor is absorbed and goes back to the blood, Thus this process maintains the pressure of the eye ball and prevents damage to delicate structures of the retina. Aqueous humor supplies nutrients to the cornea and oxygen and nutrients to the lens.


[ cristalino = lens. cuerpo ciliar = ciliary body. Canal de Schlemm = Schlemm canal, point where aqueous humor is drained from the eye back to the blood.]


When light is shone in one eye, both pupils contact. This reflex action is known as direct contraction for the eye receiving the light and consensual reflex contraction of the other eye.

People living in the tropical zone have a bit larger pupil, 4.7 mm compared with the people living in the arctic zone having  4.2 mm size.

 Humans have round, clear and transparent pupils. It is a portal for examinations of the deeper structures of the eye. When clouding of the lens happens (cataract) the pupil appears milky white.


In the animal world the size, shape and position of pupils are variable.


Is there anyone, who does not remember the poem -

“ Tiger tiger burning bright

 In the forest of the night”    - by William Blake.


Hunters will verify that is an absolute fact. Tigers and all  nocturnal predatory animals have a special membrane behind the retina, which reflects back light and increases their nocturnal visual acuity to 8 times that of humans. When hunters' flash light hits tigers’ eyes glows like a fire ball and an experience of awe and joy which only hunters can describe.


Visitors in Florida and Louisianan are spooked by a floating pair of lights on the water surface following them .These are the eyes of alligators or crocodiles due to the presence of this membrane. Owls' eyes at night impart similar terrors because they not only glow but also move 360 degrees with the movement of the head.


In the animal world, a pair of distinct human-like eyes  should be expected from fish to the upwards in the strata.

Fish have round pupils but the size is fixed.

Molaska.   Cuttlefish have W  shaped pupils. It is an exception.

Insects. Some rare Spiders and Mantis shrimps have eyes with pupils.

Reptiles. Snakes that hunt during daytime have round pupils. Nocturnal poisonous snakes have slits like pupils. 

Crocodiles have elliptical pupils.

Birds. Preditory birds have round pupils,Example - eagles , falcons with excellent vision.

Song birds. Roud pupils

Mammals. Foraging animals have round pupils.

Herbivorous animals that are prayed upon by carnivorous cats have square and rectangular pupils. Example: Goats, sheep, horses

Domestic cats have vertical slits like pupils.

Larger wild cats have round pupils. Example: Tiger, Wolves, Foxes

Dolphines: Crescent shaped pupils.

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Footnote:
Dilated Pupils are seen in emotional stress, fear and surprise, in darks condition, sexusal arousal, cocaine use. Use of eye drops containing Epinephrine, adrenaline, and atropine group of drugs and metamphetamine, The Sympathetic nerve from Superior cervical ganglion innervates the radial muscles of iris causing the pupils to dialate.
Constricted pupils is seen in use of Opium group of drugs, Pilocarpine drops, Medical conditions - Horners syndrome, Brain injury and strokes, injuries of cornea. The nerve supply of sphicter muscles of pupils are supplied by vagus nerve.

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