II.

Anatomy

The outer layer of the mammalian body, called the integument, consists of the skin and its derivatives. The skin, equipped with varying amounts of hair, serves as a protective layer against mechanical injuries and invasion by germs, and prevents excessive loss or gain of body heat and moisture. In many mammals the colour of the skin or fur blends with the animal's natural surroundings. In others there is great contrast with the natural surroundings to favour visual signals that provide information about the identity of a species, and about the gender, age, or social status of an individual. The skin also functions as a sensory and excretory organ and contains specialized glands.

Mammary glands, which are present in fully developed form in all adult female mammals, and only in rudimentary form in most male and young female mammals, secrete milk to nourish the young. Sweat glands have been detected in almost all terrestrial mammals but appear to be lacking in some, notably the Cape mole rat and the two-toed sloth. Aquatic mammals, such as whales, dolphins, and sea cows, have no sweat glands. Sweat glands are usually located at the bases of hairs, except those in regions of skin bordering mucous membranes, such as the sweat glands surrounding the edges of the lips and covering the genitalia. Many mammals, however, have few functional sweat glands; in dogs and cats, for example, only the glands on the soles of the feet are functional. Meibomian glands, located on the edges of the eyelids, secrete an oily film that covers and protects the space between the eyeball and eyelid, and retains the film of tears that moistens the eyeball. A secretion from the wax glands of the ear prevents the intrusion of dust particles and small insects into the inner ear. Many mammals have scent glands located in the integument (outer layer) in various parts of the body; the fluids secreted by the glands repel enemies or attract the opposite sex.

The eyes, ears, and nose of mammals also have their external endings in the integument. All mammals have two eyes, but the eyes of several burrow-dwelling mammals, such as moles, have lost their function partly or completely or have become covered with skin. The ears of terrestrial and arboreal (tree-dwelling) mammals are visible as projecting cartilaginous processes, but the external ears of aquatic mammals, such as seals and walruses, are reduced to little more than small protective flaps. In whales' ears, the external openings are merely small holes.

The internal organs of mammals are essentially the same from the most primitive to the most complex. The internal features that distinguish mammals from lower vertebrates are the presence of two to four optic lobes in the brain, the presence of a muscular diaphragm separating the heart and lungs from the abdominal cavity, the presence of a single aortic arch located on the left side of the body, the possession of a four-chambered heart, including two auricles and two ventricles, and the absence of nuclei in red blood corpuscles after birth. All mammals except sea cows and certain sloths and anteaters have seven cervical (neck) vertebrae; this characteristic is possessed by the long-necked giraffe and the largest whales as well as by mice and other tiny mammals. Other unique skeletal features common to mammals are the articulation between the tibial and tarsal bones at the ankles, the chain of small separate bones in the ear, and the articulation of the mandible (lower jaw) and squamosal bones of the skull.