Dermis
The dermis is the layer beneath the epidermis, considerably thicker than it, and it contains the blood vessels, nerves, sweat glands, hair follicles and the collagen and elastin that give skin its strength and elasticity.
If the epidermis is the surface, the dermis is the substance. It is the thick layer where the working parts of skin are: blood vessels, nerve endings, sweat glands, the follicles that hair grows from, and the protein framework holding all of it together.
That framework is mostly collagen, which provides tensile strength, alongside elastin, which lets skin return to shape after being stretched. Changes in both account for much of how skin alters over a lifetime.
Because the blood supply is here, conditions involving the dermis look different from surface ones. Redness, warmth and swelling all require vessels, so they signal that something is happening at this depth. Urticaria is a clear example: fluid leaking from small dermal vessels lifts the skin into a weal, and the whole thing resolves when the vessels recover.
Depth also determines consequence. Damage confined to the epidermis heals without leaving a mark, because the layer renews itself completely. Damage extending into the dermis disturbs a structure that does not rebuild identically, which is the reason scarring is described in terms of this layer rather than the one above it.
Clinicians use dermal as a locating word, and hearing it tells you that whatever is being described sits below the surface rather than on it.
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