The Human frame consists of a skeleton (fig. 73) formed of hard and unyielding bone, having joints to admit of motion in certain directions; this skeleton performs certain great and important offices, it forms strong boundaries or protecting cases for most of the vital parts, so as to shield them from external violence, such as the brain and spinal cord, the lungs, heart, &c., it also furnishes a series of levers to be acted on by the muscles, for the purposes of motion, and a firm support for the soft parts of the system. The skeleton is divided into the head, trunk, and extremities; the head is again divided into cranium and face; the trunk is divided into spine, thorax, and pelvis; the extremities into the upper and lower, the upper consisting of arm, fore-arm, and hand; the lower of thigh, leg, and foot. The bones are divided into "flat bones" (fig. 74) and "long bones" (fig. 75), the flat bones (as a general rule) form the boundaries of the cavities, and the long bones the levers. [Illustration: FIG. 74.--FLAT BONE (Scapula).] [Illustration: FIG. 75.--LONG BONE (Femur).] [Illustration: FIG. 76.--SECTION OF BONE, MAGNIFIED.] The bones consist of earthy matter, chiefly phosphate of lime, held together by a sort of cartilage, and arranged in fine scales or plates forming interstices or cells (fig. 76), these are large in the centre of the bone so as to make it nearly (and in some instances quite) hollow, and very small at the surface, so as to make it there nearly solid; this arrangement is found to be the very best to secure strength and lightness, together with a very slight degree of flexibility. The general form of a long bone is that of a shaft, having an enlargement at either extremity, the shaft is generally curved slightly so as to give elasticity, and is more solid than the ends, which are expanded to give size and firmness to the joints, and porous to keep them light; the shaft is small and close in its texture, that there may be plenty of room for the muscles which lie on and cover it. The skeleton then, consists of bony cases which enclose the viscera, a series of levers covered with, and moved by the muscles, and the whole forming a solid framework to support the soft parts. All the various motions of the body, from the winking of the eyelid and the wonderfully adjusted motions of the eye itself--which are so accurate, that a certain perceptible motion is required to look alternately at one side and the other of a pin's head--to the most powerful stroke of the arm, are performed by the contraction of the muscles. But there are several actions which would at first appear to be produced by elongation instead of contraction, such as the protrusion of the tongue, the winking of the eyelids, and the closure of the lips; but the first of these is produced by the contraction of a muscle which is attached at one extremity to the under part of the root of the tongue, and at the