The underlying layer of the skin, known as the dermis, is much thicker but much more uniform in structure than the epidermis (see diagram 5.1). It is composed of loose connective tissue with a felted mass of collagen and elastic fibres. It is this part of the skin of cattle and pigs etc. that becomes commercial leather when treated,. The dermis is well supplied with blood vessels, so cuts and burns that penetrate down into the dermis will bleed or cause serious fluid loss. There are also numerous nerve endings and touch receptors in the dermis because, of course, the skin is sensitive to touch, pain and temperature.
When looking at a section of the skin under the microscope you can see hair follicles, sweat and sebaceous glands dipping down into the dermis. However, these structures do not originate in the dermis but are derived from the epidermis.
In the lower levels of the dermis is a layer of fat or adipose tissue (see diagram 5.1). This acts as an energy store and is an excellent insulator especially in mammals like whales with little hair.