39. The Ribs and Sternum. The barrel-shaped framework of the chest is in part composed of long, slender, curved bones called ribs. There are twelve ribs on each side, which enclose and strengthen the chest; they somewhat resemble the hoops of a barrel. They are connected in pairs with the dorsal vertebræ behind. The first seven pairs, counting from the neck, are called the _true_ ribs, and are joined by their own special cartilages directly to the breastbone. The five lower pairs, called the _false_ ribs, are not directly joined to the breastbone, but are connected, with the exception of the last two, with each other and with the last true ribs by cartilages. These elastic cartilages enable the chest to bear great blows with impunity. A blow on the sternum is distributed over fourteen elastic arches. The lowest two pairs of false ribs, are not joined even by cartilages, but are quite free in front, and for this reason are called _floating_ ribs. The ribs are not horizontal, but slope downwards from the backbone, so that when raised or depressed by the strong intercostal muscles, the size of the chest is alternately increased or diminished. This movement of the ribs is of the utmost importance in breathing (Fig. 91). The sternum, or breastbone, is a long, flat, narrow bone forming the middle front wall of the chest. It is connected with the ribs and with the collar bones. In shape it somewhat resembles an ancient dagger. 40. The Hip Bones. Four immovable bones are joined together so as to form at the lower extremity of the trunk a basin-like cavity called the pelvis. These four bones are the sacrum and the coccyx, which have been described, and the two hip bones. Illustration: Fig. 17.—Thorax. (Anterior view.) The hip bones are large, irregularly shaped bones, very firm and strong, and are sometimes called the haunch bones or _ossa innominata_ (nameless bones). They are united to the sacrum behind and joined to each other in front. On the outer side of each hip bone is a deep cup, or socket, called the _acetabulum_, resembling an ancient vinegar cup, into which fits the rounded head of the thigh bone. The bones of the pelvis are supported like a bridge on the legs as pillars, and they in turn contain the internal organs in the lower part of the trunk. 41. The Hyoid Bone. Under the lower jaw is a little horseshoe shaped bone called the hyoid bone, because it is shaped like the Greek letter upsilon (Υ). The root of the tongue is fastened to its bend, and the larynx is hung from it as from a hook. When the neck is in its natural position this bone can be plainly felt on a level with the lower jaw and about one inch and a half behind it. It serves to keep open the top of the larynx and for the attachment of the muscles, which move the tongue. (See Fig. 46.) The hyoid bone, like the knee-pan, is not connected with any other bone. The Bones of the Upper Limbs.