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The Skeleton.The bone in the living animal is furnished with blood vessels and nerves, and is just as much a living tissue as the skin or any other organ. The long bones are hollow on the inside to contain the marrow which has a certain function to perform in the nourishment of the bone. The greater part of the nutrition of the bone comes, however, from a membrane which covers its outside, called the periosteum. When this membrane is torn off, the bone suffers and often dies ; this is what happens in many cases of felons. In early life the bones consist chiefly of gristle or cartilage ; the salts of lime and magnesium are deposited in this gristly mat ter during childhood and youth. Some of the bones remain in this cartilaginous condition for years after birth ; the thigh-bone, indeed, becomes completely converted into bone only about the twentieth year of life. Hence it happens that the. bones of children are less liable to fracture than those of adults. The Spine.-The spine consists of twenty-six pieces called vertebrae. Each of these is a ring of bone surrounding a central canal which contains the spinal cord. The lower vertebrae comprise also a thickened part called the body, which rests upon the corresponding part of the vertebrae below, and serves to transmit the weight of the body to the lower limbs. The vertebrae are provided with numerous bony projections which serve for the attachment of muscles and ligaments. The first vertebra, the one just below the skull, is called the atlas. It is so arranged as to permit a movement of the skull, which rests upon it, forward and backward. The movement of the head from side to side is accomplished by the rotation of this vertebra around a pivot which projects from the second vertebra. The Head.- The head comprises twenty-two bones, eight of which are included in the cranium, fourteen in the face. The cranial-bones are thin curved plates, united to each other by serrated edges known as sutures. Each of the bones forming the vault of the skull is composed of two plates, between which there is a layer of spongy, bony tissue. The outer one of these plates is somewhat elastic, while the inner one is very brittle and inelastic. It sometimes happens that a blow upon the head fractures the internal brittle plate, while the external escapes by virtue of its elasticity. The occipital bone constitutes the back part of the head and the base of the skull. The sides of the cranium are the parietal bones. The forehead is made of the frontal bone. The lower part of the side of the skull is made up of the temporal bone. The structures composing the internal ear are contained in this bone. The bridge of the nose is formed by the nasal bones. The upper jaw is made up of the two superior maxillary bones. This bone is hollowed out so as to constitute a cavity which communicates with the mouth. The prominence of the cheek is formed by the malar bone. The turbinated bones are contained in the nostrils. The partition between the nostrils is made up in part of a bone called the vomer. The lower jaw is a semicircle of bone, its ends terminating in an upright piece, the whole being shaped much like an inverted horseshoe. The upright pieces lie in contact with the lower surface of the temporal bone, making the joint of the jaw. The tongue is attached at its base to a small bone shaped like a letter U, and hence called the hyoid bone. The Chest. - The chest is a space bounded by the spinal column behind, the breast bone in front, and the ribs on the side. The breast bone is composed of three pieces, and terminates in a projection of gristle or cartilage, which is situated just above the pit of the stomach. The ribs are twenty-four in number, twelve on each side. They are all connected with the spinal column at the back. The upper seven on each side are joined by means of cartilages to the breast bone. The next three ribs are joined by their cartilages to the cartilages of the ribs above ; the lowest two ribs are connected with the backbone only. The seven upper ribs are called the true, the remaining five the false ribs. The direction of the ribs is obliquely downward and forward from the backbone. By means of the cartilaginous attachments to the breast bone, the ribs are capable of considerable motion, whereby the size of the chest can be increased and diminished, as in the acts of breathing. The Upper Extremity. - This consists of the shoulder, the arm, the forearm and the hand ; the bones included are the shoulderblade, the collar-bone, the arm-bone (humerus), the two bones of the forearm (ulna and radius), the eight bones of the wrist, five of the hand, and fourteen in the fingers. The only bones of the upper extremity requiring special notice are those whereby the rotary movements of the hands are effected. The ulna is firmly fixed at the elbow joint, where it is attached to the bone of the arm. At the wrist, on the other hand, it constitutes but a small part of the joint. The radius is scattered to the arm-bone, as to enjoy free rotary movement, while it is fixed firmly to the bones of the wrist, which, therefore, accompany its movements. The rotation of the hand is effected by the movement of the radius around the ulna. When the hand is held with the palm upward, the two bones of the .forearm are almost parallel; when the hand is turned over, so that the back is upward, the radius lies obliquely across the ulna, the latter bone not having, changed its position. The Lower Extremity.-This includes the hip, the thigh, the leg and the foot. The projection called the hip is the upper margin of the bony ring called the pelvis. This bony basin contains some of the important organs of the body. On either side is a deep socket, which receives the head of the thigh-bone. The thigh-bone (femur) is the largest, the longest and the strongest bone in the entire skeleton. The part which fits into the socket in the pelvis is connected with the shaft of the bone at an obtuse angle. The lower extremity of the thigh-bone is very broad and thick, so as to afford a large surface for the transmission of the weight of the body to the leg. The front of the knee-joint is covered with a disk-shaped bone - the knee-pan ox patella. This bone is contained in the sinew or tendon of the large muscle which constitutes the front of the mass of the thigh. The back part of the foot is composed of seven bones, which together are called the tarsus. The largest of these is the bone which forms the projection of the heel. To this bone is attached the tendon of the muscles forming the calf of the leg. 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