7. The structure of a long bone.—Take the long bones of the upper arms and upper legs, and examine them further. Break one across [Pg 224] the middle. Is it solid or hollow? What is the advantage of its being hollow? Is the tube empty or does it contain marrow? Are the ends of the bone hollow or solid? Place one of the bones on a bright hot fire. Is there any sign of burning? Does all the bone burn away? Carefully remove what is left, and compare the brittleness of the “burnt” bone with that of an unburnt bone. Place another bone for several days in a cupful of water to which has been added about two teaspoonfuls of strong hydrochloric acid. Then take it out, wash it; and try to bend it. Continue the treatment until the bone is soft enough to tie in a knot. Can you tie an ordinary bone in a knot? Why not? Put a “burnt” bone in similar dilute acid. Does it dissolve? What do you think gives a bone its hardness? 8. Bones moved by muscle.—Stretch out one of your arms and grasp the middle of the upper arm firmly with the other hand. Now bend the elbow, and notice that the great muscle (the biceps) of the front of the upper arm thickens and becomes shorter. Straighten the arm again, and notice that the muscle becomes thinner and longer again. Examine Fig. 160, which shows how the upper end of the biceps is attached at a to the shoulder-blade and its lower end to one of the bones of the fore-arm at P. If the biceps muscle shortens, the fore-arm must be pulled up, because the shoulder remains stationary. Carefully notice the various movements of which your arm is capable—e.g. extension of the arm; bending (flexure) on the elbow-joint; rotation of the fore-arm, so that either the palm or the back of the hand can be turned upwards; and grasping. Observe your power of touching the tip of the little finger with that of the thumb. If a human skeleton is accessible watch how one (which?) of the two long bones of the fore-arm rotates when the hand is turned over. How many of these movements can the rabbit make? The uses of the skeleton.—The bodies of vertebrate animals (p. 220) are supported, and their more delicate parts protected from injury, by an internal framework called the skeleton. In some of the simpler fishes this is composed of gristle or “cartilage”; but in more highly developed animals it consists almost entirely, in the adult [Pg 225] state, of bone. Further, almost all the bones are connected with strands or bundles of red flesh called muscle, which the animal is able to shorten or “contract” at will. When a muscle shortens, the bones to which its ends are fixed are of necessity pulled nearer to each other. If the bone at one end of the muscle remains stationary, that at the other end is moved into a new position. This is, generally speaking, the manner in which the numberless movements of the limbs, head, etc., are made. Fig. 160.—The action of the Biceps muscle.