Tag: introduction to sense organs

The plica semilunaris in humans is a small fold of tissue on the inside corner of the eye. It is the vestigial remnant of the nictitating membrane, an organ that is fully functional in some other species of mammals.

At extremely low intensities of stimuli, when only rods are stimulated, the retina shows a variable sensitivity to light according to its wavelength, being most sensitive at about 5000 angstroms, the absorption maximum of the rod visual pigment, rhodopsin. There is a characteristic shift in the maximum sensitivity from 5000 angstroms for scotopic (night) vision to 5550 angstroms for photopic (day) vision, called Purkinje shift.

Six extraocular muscles facilitate eye movement. These muscles arise from the common tendinous ring in the orbit, the eye cavity and attach to the eyeball. The six muscles are the lateral, medial, inferior and superior rectus muscles and the inferior and superior oblique muscles. The muscles, when contracting, cause movement of the eyeball, by pulling the eyeball towards the muscle.

Neuroectoderm gives rise to the following compartments of the eye: retina, epithelial lining of ciliary body and iris, optic nerves. Surface ectoderm produces the following parts: lens, corneal epithelium, skin of eyelid.

The harderian gland is a gland found within the eye's orbit, which occurs in tetrapods (reptiles, amphibians, birds and mammals) that possess a nictitating membrane. The gland can be compound tubular or compound tubuloalveolar, and the fluid it secretes (mucous, serous or lipid) varies between different groups of animals. In some animals it acts as an accessory to the lacrimal gland, secreting fluid that eases movement of the nictitating membrane. 

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.