Tag: introduction to sense organs

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.

The conjunctiva is the thin film composed of non-keratinized stratified columnar epithelium and goblet cells that line the inside of the eyelids and covers the white of the eye called as the sclera. The conjunctiva lubricates the produces mucus and tears in small amounts to lubricate the eye and also prevents the entry of microbes into the eye.
Therefore, the correct answer is option C.

The person wearing spectacles with concave lenses for correcting vision suffers from myopia (near sightedness). In such a person, the image of distant object will be formed in front of retina, if he is not using the glasses.

The structures of the eye that deflect light is called refractive medium. Light is actually refracted through four media in the eye. The first is the cornea (the clear layer on the outside of the eye), then the aqueous humour (liquid), then the lens (which can change shape slightly to allow objects near and far to focus), and then there is the vitreous humour (liquid filling most of the inside of the eye) all of which light needs to pass through before it reaches the retina. Light enters the eye through the pupil, and the iris regulates the amount of light by controlling the size of the pupil. Since, pupil only contacts and dilates, therefore, it is the one not a refractive medium of the eye. So, the correct answer is option D.

The skin is one of the largest organs in the body in surface area and weight. The skin consists of two layers: epidermis and dermis. Beneath the dermis lies the subcutaneous fatty tissue. The skin has three main functions: protection, regulation and sensation.