Chap. 8 AN AGENT OF EVOLUTION THE BODY COVERING 129 



push down into the dermis (Fig. 8.2). Their great variety includes the 

 stinging cells of hydra, wax glands of honeybees, the mucous glands whose 

 secretion earthworms leave, behind them in shiny trails, and the mucous 

 glands that make the slipperiness of fishes. More familiar are the oil glands 

 of hair and the sweat glands whose products have become the symbol of 

 human toil, the lacrimal or tear glands, and the mammary glands which pro- 

 duce food for all young mammals. The activity of these glands is deeply 

 associated with human experiences. The epidermis has earned a high place 

 in human history; Sir Winston Churchill gave it two-thirds of Blood, Sweat 

 and Tears, 



Dennis. The dermis is the inner and thicker layer of the skin, the one 

 where the prick of a needle first hurts (Fig. 8.2). The bulk of it is composed 

 of the crisscrossing fibers of connective tissues familiar in leather. Dermis 

 is a nutrient layer containing lymph and blood capillaries and fat cells, the 

 latter often extremely abundant. There are many nerve endings in it; the 

 autonomic (involuntary) nerves control the contraction and dilatation of 

 the capillaries and consequent paling or flushing of the skin. The dermis is 

 the scene of blushing. Heat regulations also occur there; blood may be spread 

 out and cooled in the dilated surface capillaries or driven into the warm 

 deeper parts of the body when they are contracted. The colors of frogs and 

 other lower vertebrates are mainly due to pigment-bearing cells (chromato- 

 phores) in the dermis. Epidermal structures, glands, and feather and hair 

 follicles project into the dermis where dermal structures such as blood ves- 

 sels, nerves, and smooth muscle are associated with them (Fig. 8.2). 



Skin Derivatives 



Such notable developments from the skin layers as horns, claws, nails, and 

 hoofs should be added to the scales, feathers, and hair already mentioned. 

 Teeth have a history of close association with the skin and in certain sharks 

 there are rows of them just outside as well as inside the mouth cavity. The 

 plates of whalebone that hang from the upper jaw of toothless whales are 

 composed of cornified epidermal cells. 



Epidermal Glands. The epidermis contains glands. Lobsters, grasshoppers, 

 and every other arthropod are completely clothed in the secretion of their 

 epidermal glands. Natural pearls are epidermal secretions as are shells of 

 the giant clam {Tridacna gigas) weighing 300 pounds or more, often used 

 as basins for holy water. 



Scales. The scale of an insect, a butterfly, or moth is a minute plate of 

 cuticle secreted by one or more epidermal cells. It is solely a secretion and 

 does not contain any cells. The "hairs" and spines of other invertebrates are 

 similar. In contrast to these, the scales of bony fishes and other vertebrates 

 are composed of cells that originate from groups of skin cells. 



