SKIN It is the most important sense organ which covers the entire body and also acts as excretory organ by regulating the body temperature. It is tough, flexible, water resistant protects the body from damage and invasion of micro organisms. The average surface area of the skin covering of an adult human is about ½ sq km , ½ mm thickness in eyelids and lips ½ cm thickness in soles of feet and palms of hand. It is composed of 3 layers · Epidermis · Dermis · Hypodermis Ø Epidermis It is the most superficial layer of the skin composed of dead cells consisting two layers as follows: · Outer layer: it is also known as corneum layer which consists of dead cells. In this the cells are flat and are non-nucleated with a pigment carotene or keratin (present in nails, hair and thorns) · Inner layer: It is also called as malphigian/germinal layer, cells are nucleated with a pigment melanin (responsible for the colour of skin and hair). If melanin is high, the colour of the skin and hair will be darker and if it is low or not having the colour of the skin will be reddish white with white hair and the pupil is red in colour, these persons are called as albinos. Albinos cannot see the sun light properly and if they expose it leads to burning sensation. Whenever they exposed to sun light, it stimulates the production of melanin pigment. This process is known as tanning. No nerves and blood vessels in this region. 
Ø Dermis It is present below the epidermis consisting of soft connective tissues. It is tough, elasticity which gives strength to the skin. It is composed of · Blood vessels o Number of blood vessels are present which helps in excretion. o These will brings the substance to be excreted through the skin and helps in regulating the body temperature. · Sweat glands o These are widely distributed all over the skin, more in palms and soles. o The tube like structure arises from dermis and extends into epidermis which opens outside as a pore called as sweat pore. o Each gland is supplied by a network of blood capillaries with removes the waste products from the blood in the form of sweat. o When sweat evaporates from the skin the body is cooled from the heat. · Sebaceous glands o These glands open into hair follicle and secrete oily substance called as sebum which keeps the mammalian hair soft and shiny. o In mammals these are modified into mammary glands. · Mammary glands o These glands are well developed in females which secrete a nutritive fluid called as milk useful to feed their young ones. · Hair follicle o It is present in dermis region which produces hair is present all over the body except in palms, soles and lips. · Ridges/grooves o It is useful for finger prints which differ from one person to person even in twins and useful to identify the criminals. o Sensory nerves or receptors The receptors present in the skin known as cutaneous receptors helpful to know the sensations like ü Tactile receptors – sensitive to touch ü Thermo receptors – sensitive to cold or hot ü Pacinian receptors – sensitive to pressure ü Nociceptors – sensitive to pain Ø Hypo Dermis It is present below the dermis consisting loose connective tissue and stores large amount of fats which helps to insulate body to loss or gain and also useful for shock absorption in storage of energy. Ø Functions · It gives protection from the entry of micro organisms. · It helps to eliminate the waste materials in the form of sweat. · It secretes oil which keeps the hair and skin moist. · It is useful for the formation of vitamin-D.
| 
