ABSTRACT
In humans, the surface of the skin contains numerous hair and sweat pores, wrinkles, and different types of hair. In particular, types of hair such as downy hair are primarily round and flat, and form the stratum corneum, which is responsible for the tactile sense. The color of the skin varies depending on the amount of pigment and blood present in it. The color also depends on the fraction of light reflected from off of the skin surface or on the transparency of the skin surface. In addition to race, gender, age, and body region differences, the presence of nutritional conditions, endocrine disorders, visceral diseases, and skin diseases are important factors that have an effect on skin color. The skin consists of the epidermis (outermost layer), dermis (inner layer), and subcutaneous tissues. Many cells constitute the skin tissue. Skin thickness varies depending on the body region in question: the thickness of the epidermis ranges from approximately 0.03 to 1.00 mm and that of the dermis is approximately 10 times more than that of the epidermis. Most of the subcutaneous tissues are formed by lipids; therefore, the thickness of the subcutaneous tissues is different at different body regions, and also varies among individuals. In the skin, many blood vessels are distributed like meshes in the dermis and subcutaneous tissues; however, there are no blood vessels in the epidermis. These blood vessels circulate blood and supply nutrients. Furthermore, the well-distributed network of sensory and autonomic nerves in the skin are responsible for the different sensations felt, the movement of smooth muscle and blood vessels, and the secretions from the sebaceous and sweat glands.
