Tag: biology

The meibomian glands (or tarsal glands) are a special kind of sebaceous gland at the rim of the eyelids inside the tarsal plate, responsible for the supply of meibum, an oily substance, that prevents evaporation of the eye's tear film. Meibum prevents tear spillage onto the cheek, trapping tears between the oiled edge and the eyeball, and makes the closed lids airtight. There are approximately 50 glands on the upper eyelids and 25 glands on the lower eyelids. Dysfunctional meibomian glands often cause dry eyes.

Retina is the light-sensitive tissue lining the back of our eye. It is the black pigment in the eye, which reduces the internal reflection. Thereby, it controls the impact of the bright light entering our eye. Function of retina is to receive light that the lens has focused, convert the light into neural signals, and send these signals on to the brain for visual recognition. Hence option A is correct.

The almond-shaped glands in the eyes present above each eyeball are called lacrimal glands. Thes glands are called tear gland because they secrete the fluid called tears. These glands have their openings between the eyelashes. Water, salts and bactericidal protein such as lysozyme are components of tear. The secretion of this glands is involved in the clearance of dust, grit and microorganisms in the cornea. They are also involved in the lubrication of eyes and eyelashes. Thus the correct answer is option C.

Glands of Zeis are modified sebaceous gland located on the margin of eyelids and produces oil into the follicles of eye lashes. 

Rods and cones are photoreceptors in the human retina. Rods are responsible for vision at low light levels (scotopic vision). They do not mediate colour vision and have a low spatial acuity. Cones are active at higher light levels (photopic vision), are capable of colour vision and are responsible for high spatial acuity. So, the correct answer is 'Rods are responsible for scotopic vision and Cones are responsible for photopic vision'.

Rods and cones are used to process light, in a human eye, their number is estimated to be 5-7 million cones and 110-130 million rods. Cones have iodopsin and are found mainly in the central area of the retina (fovea), while rods containing rhodopsin are found in the peripheral retina. Cones sense colour and need more light than rods to work well. Cones are most helpful in the normal or bright light. There are 3 types of cones - red, green, and blue - to help you see different ranges of colour. Together, these cones sense combinations of light waves, that enable our eyes to see millions of colours. The light-sensitive photopigments are made of the protein opsin and the chromophore retinene. Bleaching of iodopsin in cones is similar to rhodopsin in rods, only rods contain a higher concentration of visual pigment than cones, so more light is needed to cause an action potential to be fired in cones ( a threshold for cones is higher than rods). In other words, rods are mainly used for dim light vision, cones for bright light vision.

Rods and cones are the photoreceptors present in retina of the eye. They have their own bipolar neurons and photo-sensitive pigments.

The fovea centralis is the central portion of the human retina, that has the most acute vision and the best color perception. The cones have 3 types of specific photopigments - red, green and blue, or RGB. Most of the color sensitive cones are packed in the fovea centralis.
The colored part of the eye is called as the iris. It controls light levels inside the eye similar to the aperture on a camera. The round opening in the center of the iris is called as the pupil. The iris is embedded with tiny muscles, that dilate (widen) and constrict (narrow) the pupil size.
The lens, by changing shape, functions to change the focal distance of the eye, so that it can focus on objects at various distances, thus allowing a sharp real image of the object of interest to be formed on the retina, which is then transmitted to the brain by the optical nerve.

Vultures are highly reliant on their sensory systems for the rapid detection and localization of carrion before other scavengers can exploit the resource. Vulture has acute vision. Retina has a comb like pectin in birds.

The human eye is sensitive to violet (400 nm) to red (700 nm ) light only that can generate photometric effect i.e. visual sensation on an eye. So, the correct answer is option B.