Glands are organs that produce and secrete fluids. They are usually divided into two groups depending upon whether or not they have channels or ducts to carry their products away. Glands with ducts are called exocrine glands and include the glands found in the skin as well as the glands that produce digestive enzymes in the gut. Endocrine glands have no ducts and release their products (hormones) directly into the blood stream. The pituitary and adrenal glands are examples of endocrine glands.
Most vertebrates have exocrine glands in the skin that produce a variety of secretions. The slime on the skin of fish and frogs is mucus produced by skin glands and some fish and frogs also produce poison from modified glands. In fact the skin glands of some frogs produce the most poisonous chemicals known. Reptiles and birds have a dry skin with few glands. The preen gland, situated near the base of the bird’s tail, produces oil to help keep the feathers in good condition. Mammals have an array of different skin glands. These include the wax producing, sweat, sebaceous and mammary glands.
Wax producing glands are found in the ears.
Sebaceous glands secrete an oily secretion into the hair follicle. This secretion, known as sebum, keeps the hair supple and helps prevent the growth of bacteria (see diagram 5.6).
Sweat glands consist of a coiled tube and a duct leading onto the skin surface. Their appearance when examined under the microscope inspired one of the first scientists to observe them to call them “fairies’ intestines” (see diagram 5.1). Sweat contains salt and waste products like urea and the evaporation of sweat on the skin surface is one of the major mechanisms for cooling the body of many mammals. Horses can sweat up to 30 litres of fluid a day during active exercise, but cats and dogs have few sweat glands and must cool themselves by panting. Scent in the sweat of many animals is used to mark territory or attract the opposite sex.
Mammary glands are only present in mammals. They are thought to be modified sebaceous glands and are present in both sexes but are rarely active in males (see diagram 5.10). The number of glands varies from species to species. They open to the surface in well-developed nipples. Milk contains proteins, sugars, fats and salts, although the exact composition varies from one species to another.
Diagram 5.10 - A Mammary Gland