HUMAN BODY: SKIN-HAIR-NAILS;

 Skin, Hair and Nails:

    The body has its own living mantle, the skin, an impermeable barrier blocking the road to bacteria. If exposed to the ultraviolet rays of the sun, a brown pigment "melanin" colours it and thus protects it from harmful rays. Millions of sensors detect the most diverse sensations: contact of a fur, pressure of a weight, pain of a sting,...ect. Hair and nails are then extensions of the skin. Millilons of hair cover almost the entire body. The thickest ones, the hair, grow on the head that they protect from the sun and the cold. While the others, more refined, do not play this protective role.

 >>Skin, hair and nails owe their strength to a protein, keratin.

Nails:

    These plates last and protect the tips of the fingers and toes. The living cells of their root divide endlessly, pushing the nail forward. As the cells move towards the fingertips, they fill with keratin and die. The nails grow about 5 mm per month, and faster in summer than in winter. The nails also allow to take small objects more easily.

Prints:

    When you touch an object, especially a hard glass or metal object, you leave fingerprints. These are the oily traces of the very fine streaks of the skin on the fingertips. These streaks, and the sweat that comes out of them, help the finger grasp things. With its volutes and curls, each fingerprint is unique!

The protective layers:

    The skin is less than 2 mm thick and consists of two layers: 

•The outer layer is the epidermis. It consists of dead, flat, tangled, resistant and impermeable cells, constantly destroyed and then replaced by living cells. 

•Below it is the dermis containing the sensors, nerves, blood vessels, sweat-secreting glands and hair roots.

The growth of a hair:

    The hairs are keratin tubes growing in small cavities of the skin, the follicles. This big hair just came out of one of 100,000 head follicles. It’s straight because the follicle has a round opening. If that opening were oval, it would be curly. On the right, the two fine hairs are older and covered with flat cells that overlap like roof tiles. This prevents hair from getting tangled.

Sweating:

    Perspiration allows the body to maintain a normal temperature of 37°C, even if it is very hot. Intense physical activity, such as running, raises the body temperature because the effort produced by the muscles releases heat. But too high a temperature is bad for the body. Also, at the first signs of overheating, about 3 million tiny glands secrete salty sweat all over the skin. As it evaporates, the sweat releases heat and cools the body.