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SKIMMER

1764

SKUNK

Twenty-four remain distinct through life, while the lower nine become consolidated into two groups: the sacrum and the coccyx. Twenty-four ribs bound the chest, and most of them are connected by cartilage with the breast-bone or sternum. The shoulder-blades, the collar-bones and two hipbones are connected with the arms and limbs respectively There are 30 bones in each arm and the same number in each leg, counting the knee-pan. The bones are held together by ligaments. See the illustration for further names. See FOOT, HAND and SKULL.

Skim'mer, a class of web-footed, long-winged sea-birds, belonging to the gull family. They have a long, thin bill with the upper half shorter than the lower. There are only three known species, each being confined to one country. The American skimmers, called shearwaters, skim along the surface of the water in flocks, using the lower part of the bill as a plow to bring up the small fish on which they feed and holding them with the upper half of the scissors which their bills form. They are also called scissors-bill or razorbill shearwaters. The other varieties are found in Asia and Africa. See GULL.

Skin, the investing membrane of the body of an animal. In vertebrated animals it consists of two layers: an outer epidermis and an inner dermis or true skin. The former is largely for protection, the latter plays an important part in physiology. The epidermis has neither blood-vessels nor nerves, but the true skin is richly provided with both. The dermis is made of a dense felt-work of fibers, and in the human body is from one fiftieth to one tenth of an inch in thickness in different parts of the body. Its outer surface is raised into little hillocks called papillae. These are of two kinds — the vascular papillae containing a network of minute blood-vessels, and the nervous papillae in which the nerves terminate. There often is an oval touch-corpuscle in which the nerve has its ending. The epidermis lies on the dermis and dips into all of its depressions. It is composed of cells, the outer layers of which are reduced to flattened, scale-like elements, which are continually falling off or being removed by bathing and friction. They are as continually renewed from a deeper layer of plump cells which divide rapidly. Thus the outer skin has two layers — a horny layer and a mucous layer. The latter is the seat of the pigment or coloring granules which impart color to the skin. In negroes it includes a rather dense layer of dark granules. All over the body the skin has minute orifices which are the openings of small tubes connected with the sweat-

tlands. These consist of coiled tubes in the eepest part of the true skin or just below it. A  tubular   duct   extends   from   each

gland to open on the surface, and as it passes through the layer of the skin it runs in spirals like a corkscrew. The number of sweat-glands varies greatly in different parts of the body; they are fewest in the back and neck, being about 400 to the square inch. On the skin of the palm and sole there are two or three thousand to the square inch. Hairs, nails, hoofs, scales and like structures are outgrowths from the epidermis of the skin. (See HAIR and FEATHERS.) The skin performs important offices. The watery vapor passed off by it in the form of perspiration amounts to about 2j pounds a day. It also liberates carbon dioxide. In some animals, as frogs, it is an important aid in respiration. It is also concerned in regulating the temperature of the body. When the body is covered with a varnish or layer of leaf-metal, death results, not from stopping the perspiration as was formerly supposed, but by radiation of heat and fall of bodily temperature — the skin being unable under the conditions to regulate the temperature. The touch-corpuscles in the skin give it an important sensory action in connection with the nervous system.

Skull, the hard framework of the cranium including the lower jaw. The skull in fishes and in mammals, although so different in general appearance, is composed of corresponding bones. They are more separated in the fish, and more modified and united in the higher animals. In sharks and other cartilaginous fishes the skull may be entirely cartilaginous throughout life. The question of the nature of the skull perplexed naturalists for a long time. Early in the igth century Oken, the German philosopher, and Goethe, the poet-philosopher, declared the skull to be composed of modified vertebrae. This idea was extensively developed by Richard Owen, in his great work on Comparative Anatomy of the Vertebrates. Thus arose what was called the vertebral theory of the skull. Huxley, however, in 1859 showed that the bony cranium is preceded by a skull of united cartilage, having neither sutures nor joints. The bones are formed later, being mostly laid down in the membranes surrounding the original cartilaginous skull, which disappears. Consequently the bones are not primary structures, and the history of the skull shows that they cannot represent modified vertebras. By comparing the skulls of different animals corresponding bones can be identified, but the study is technical. Consult Huxley's Anatomy of Vertebrated Animals and Parker and Bet-tany's Morphology of the Skull.

Skunk, a carnivorous animal widely known on account of the offensive odor of a fluid which it ejects when frightened or disturbed. There are several varieties found only in the New World both in North