The union of male sperm and the female ovum accomplishes reproduction. In coitus, the male organ ejaculates millions of sperm into the vagina, with some making their way to the uterus. Ovulation is the release of an egg into the uterus; the uterus is prepared for the implantation of a fertilized ovum by the action of estrogens. In some primates, if a male cell fails to unite with a female cell, other hormones cause the uterine wall to slough off during menstruation. After childbirth, pro-lactin, a hormone secreted by the pituitary, activates the production of milk.
Skin, the natural outer covering of the body, is an important part of the integumentary system. The skin is an organ of double-layered tissue stretched over the surface of the body and protecting it from drying or losing fluid, from harmful external substances, and from extremes of temperature. The inner layer, called the dermis, contains sweat glands, blood vessels, nerve endings (sense receptors), and the bases of hair and nails. The outer layer, the epidermis, is only a few cells thick and contains pigments, pores, and ducts, and its sur face is made of dead cells that it sheds from the body. The sweat glands excrete waste and cool the body through evaporation of fluid droplets; the blood vessels of the dermis supplement temperature regulation by contracting to preserve body heat and expanding to dissipate it. Separate kinds of receptors convey pressure, temperature, and pain. Fat cells in the dermis insulate the body, and oil glands lubricate the epidermis.
See also: Antennae; Beaks and bills; Bone and cartilage; Brain; Cell types; Claws, nails, and hooves; Digestive tract; Ears; Endoskeletons; Exoskel-etons; Eyes; Feathers; Fins and flippers; Fur and hair; Heart; Horns and antlers; Hydrostatic skeletons; Noses; Kidneys and other excretory structures; Lungs, gills, and tracheas; Muscles in invertebrates; Muscles in vertebrates; Scales; Sense organs; Shells; Skin; Tails; Teeth, fangs, and tusks; Tentacles; Wings.
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