Your shoulder has a joint that lets your arm swing round and round, as well as move up and down. Your hip has another that lets your leg move in much the same way. [Illustration: _The hip-joint._] This kind of joint is the round end or ball of a long bone, which moves in a hole, called a socket. Your joints do not creak or get out of order, as those of doors and gates sometimes do. A soft, smooth fluid, much like the white of an egg, keeps them moist and makes them work easily. BONES. What parts of our bodies are jointed together so nicely? Our bones. How many bones have we? If you should count all your bones, you would find that each of you has about two hundred. Some are large; and some, very small. There are long-hones in your legs and arms, and many short ones in your fingers and toes. The backbone is called the spine. [Illustration: _Backbone of a fish._] If you look at the backbone of a fish, you can see that it is made up-of many little bones. Your own spine is formed in much the same way, of twenty-four small bones. An elastic cushion of gristle (grĭs´l) fits nicely in between each little bone and the next. When you bend, these cushions are pressed together on one side and stretched on the other. They settle back into their first shape, as soon as you stand straight again. If you ever rode in a wheelbarrow, or a cart without springs, you know what a jolting it gave you. These little spring cushions keep you from being shaken even more severely every time you move. Twenty-four ribs, twelve on each side, curve around from the spine to the front, or breast, bone. (_See page 38._) They are so covered with flesh that perhaps you can not feel and count them; but they are there. Then you have two flat shoulder-blades, and two collar-bones that almost meet in front, just where your collar fastens. Of what are the bones made? Take two little bones, such as those from the legs or wings of a chicken, put one of them into the fire, when it is not very hot, and leave it there two or three hours. Soak the other bone in some weak muriatic (mū rĭ ăt´ĭk) acid. This acid can be bought of any druggist. You will have to be careful in taking the bone out of the fire, for it is all ready to break. If you strike it a quick blow, it will crumble to dust. This dust we call lime, and it is very much like the lime from which the mason makes mortar. [Illustration: _Bone tied to a knot._] The acid has taken the lime from the other bone, so only the part which is not lime is left. You will be surprised to see how easily it will bend. You can twist it and tie it into a knot; but it will not easily break. You have seen gristle in meat. This soft part of the bone is gristle. Children's bones have more gristle than those of older people; so children's bones bend easily. I know a lady who has one leg shorter than the other. This makes her lame, and she has to wear a boot with iron supports three or four inches high, in order to walk at all.