Tag: biology

Vascular tunic is the middle coat of the eyeball consisting of highly vascular and heavily pigmented connective tissue consisting of a posterior choroid layer and an anterior ciliary body and the iris. Choroid is the posterior portion of the vascular middle tunic of the eyeball, lined on the inside by a layer of darkly pigmented cells; it provides the main blood supply to the back of the eye and gives rise to the ciliary body and the iris anteriorly.

Nictitating membrane is also called as the third eye of rabbit and is found in medial canthus (inner corner) of an eye. This is swept closed across the eye in the direction from beak to ear to protect the eye as well as to clean and lubricate it with the moisture of lachrymal gland. 

The pecten or pecten oculi is a comb-like structure of blood vessels, belonging to the choroid in the eye of a bird. It is a non-sensory, pigmented structure that projects into the vitreous body from the point, where the optic nerve enters the eyeball. The pecten nourishes the retina and controls the pH of the vitreous body.

Iris is the part of the eye, which is responsible for one's eye color. It acts like the diaphragm of a camera, dilating and constricting the pupil to allow more or less light into the eye.

Meibomian gland are present in the inner surface of the eyelids. They secrete an oily substance which spreads over the cornea and prevents the cornea from drying. These glands prevent the friction between two eyelids. These glands are present in animals like rabbits. They help in frictionless movement and blinking of the eyelids. 

The retina is the thin light-sensitive membrane lining the inner eyeball towards the back of the eye. When the light goes into the eye it must pass through the lens (a biconvex lens), which inverts the image we are seeing. When we look at an image, light bounces it into our eye. As it enters our eye and passes through the lens the image gets inverted- reversed and flipped the other way- so that the image on our retina looks like an upside down object going from right to left instead of left to right. Once this image is set on the retina, cone cells distinguish the colour and detail, while rod cells distinguish movement and shades of grey. 

The lacrimal gland lies within the orbit on the outer portion of the upper eye. The gland continually secretes tears, which moisten, lubricate, and protect the eye. Meibomian gland is a type of gland in the eyelids, that makes a lubricant called as sebum, which is discharged through tiny openings in the edges of the lids. It is responsible for the supply of meibum, an oily substance that prevents evaporation of the eye's tear film and frictionless blinking.

The large round lens of the frog gives the animal a large field of view. The frog is naturally nearsighted (myopic) to -6 diopters giving it a focus of approximately 6 inches. Frogs and toads can change their focus by moving the lens out towards the cornea. 

Frogs use their regular set of eyelids to keep their eyes moist and clean. They have a bottom lid that stays still and an upper lid that blinks. Frogs also have a third eyelid that serves a different purpose. This one, called as the nictitating membrane, is a semitransparent lid that covers the eye completely, helping the frog see underwater and hide from predators. 

The tapetum lucidum is a layer of tissue in the eye of many vertebrates. Lying immediately behind the retina it reflects visible light back through the retina, increasing the light available to the photoreceptors. The tapetum lucidum contributes to the superior night vision of some animals. Many of these animals are nocturnal, especially carnivores that hunt their prey at night, while others are deep sea animals. Choroidal guanine tapetum is seen in Elasmobranchii (skates, rays, and sharks) and chimaeras. The tapetum is a palisade of cells containing stacks of flat hexagonal crystals of guanine.