Physiology and Hygiene for Secondary Schools

by Francis M. Walters

3. The Thorax.—This group contains twenty-four bones of similar form, called ribs, and a straight flat bone, called the sternum, or breastbone (Fig. 101). The ribs connect with the spinal column behind, and all but the two lowest ones connect with the sternum in front, and, by so doing, inclose the thoracic cavity. As already stated (page 85),[pg 226] the bones of the thorax form a mechanical device, or machine, for breathing. The ribs are so arranged that the volume of the thorax is increased by elevating them and diminished by depressing them, enabling the air to be forced into and out of the lungs.

Fig. 101—Bone groups of trunk.

4. The Shoulder and Pelvic Girdles.—These groups form two bony supports—one at the upper and the other at the lower portion of the trunk—which serve for the attachment of the arms and legs (Fig. 101). The shoulder girdle is formed by four bones—two clavicles, or collar bones, and two scapulæ, or shoulder blades. The clavicle on either side connects with the upper end of the sternum and serves as a brace for the shoulder, while the scapula forms a socket for the humerus (the large bone of the arm) and supplies many places for the attachment of muscles.

The pelvic girdle consists of two large bones of irregular shape, called the innominate bones. They connect behind with the sacrum and in front they connect, through a small pad of cartilage, with each other. On the inside of the girdle is a smooth, basin-shaped support for the contents of the abdomen, but on the outside the bones are rough[pg 227] and irregular and provide many places for the attachment of muscles and ligaments. Each innominate bone has a deep, round socket into which the end of the femur (the long bone of the leg) accurately fits.

5. The Arm and Hand Groups.—A long bone, the humerus, connects the arm with the shoulder and gives form to the upper arm. In the forearm are two bones, the radius and the ulna, which connect at one end with the humerus and at the other with the bones of the wrist (Fig. 102).

Fig. 102—Bone groups of arm and leg.

A group of eight small, round bones is found in the wrist, known as the carpal bones. These are arranged in two rows and are movable upon one another. Five straight bones, the metacarpals, connect with the wrist bones and form the framework for the palm of the hand. Attached to the metacarpals are the bones of the fingers and thumb. These form an interesting group of fourteen bones, called the phalanges of the fingers (Fig. 102).

The bones of the hand provide a mechanical device, or machine, for grasping, and the arm serves as a device for moving this grasping machine from place to place. The work of the arm, in this respect, is not unlike that of a revolving crane upon the end of which is a grab-hook. The hand without the arm to move it about would be of little use.