DERMIS

also called corium, the thicker, deeper layer of the skin underlying the epidermis and made up of connective tissue. It is present in varying degrees of development among various vertebrate groups, being relatively thin and simple in aquatic animals and progressively thicker and more complex in terrestrial species. The dermis from its earliest evolutionary appearance has been a depository of bone, as expressed in dermal armour (primitive fishes), scales (fishes and certain amphibians), and plates (crocodile, lizard, turtle, armadillo). The fin rays of fishes are dermal derivatives, as are many types of pigment cells. The dermis of mammals is of greater thickness relative to the epidermis than is that of other vertebrates, partly because it contains abundant collagenous connective tissue. When treated with tannic acid, the dermis becomes leather. In humans the dermis projects into the overlying epidermis in ridges called papillae. Nerves that extend through the dermis and end in the papillae are sensitive to heat, cold, pain, and pressure. Sweat glands and oil glands lie in the deeper stratum reticulare, as do the bases of hair follicles, the nail beds, and blood and lymph vessels.