WHAT IS THE SKIN? (PART 3)
Sweat glands are a specialized group of cells lying in the dermis which produces sweat. These glands are found over the whole skin surface, with considerable regional variation in density of distribution. They are most numerous on the palms, soles, forehead and armpits. The duct of the sweat gland opens on to the skin surface independently of hair and sebaceous gland openings. On the forehead or armpits there are frequently 200-300 sweat glands per square centimetre, and under extreme climatic provocation an individual may produce two litres of sweat an hour. In this way sweat glands are able to flood the skin surface with water, which has a cooling effect, and hence they are very important as part of a heat exchange mechanism. The closely associated blood vessels dilate or constrict to either dissipate or conserve body heat. This is therefore a very effective thermoregulatory system, one which maintains a constant internal environment, enabling man to escape the rigid climatic limitations imposed upon cold-blooded animals.
Hair follicles are finger-shaped folds of epidermis dipping into the dermis, which are responsible for hair formation. Hair then is a derivative of epidermis, arising from deep within the dermis, and composed of the protein keratin. The most superficial part of the follicle forms a duct, in which the hair stands free. Since the sebaceous duct also opens into the follicular duct, the hair shaft emerges through the same pore which secretes the sebum, ensuring its direct lubrication. In the deepest part of the follicle the follicular wall and the hair are fused. This section constitutes its root, the lowest part of which is known as the bulb.
Hair is an extremely complex structure which broadly speaking consists of a central cortex surrounded by several protective layers. Hair contains neither nerves nor blood vessels and is therefore a ‘dead’ structure. There are many different types of hair, which in one form or another cover the entire surface of the skin, with the exception of the palms and soles. In most areas the hair is short and fine, like that found on a child or on the cheeks of a woman, and is known as vellus hair. The longer, broader, and usually coloured hair, such as that on the scalp, is known as terminal hair. There is no clear distinction between these types. In fact, the same follicle may produce either type under different conditions. For example, vellus may change to terminal hair on the chin of an adolescent, or terminal may change to vellus on the scalp of a balding man. The protein-synthesizing capacity of this tissue is enormous. When one considers that scalp hair grows at the rate of 0-35 millimetre daily, and that the average number of scalp hairs is about 100000, this means that about 30 metres of hair is produced every day.
*3\44\4*
Random Posts
Tags: Skin Care