Bones of the arm and hand.
Here are the bones of the arm and the hand. The head of the arm bone that goes into the socket at the shoulder is, as you see, a smooth round ball. It fits into a sort of cup. The joint here is what we call a ball-and-socket joint. The ball turns in the socket very easily in making any whirling motion with your arm, as you do when you jump the rope.
Shoulder joint and elbow joint.
The joint at the elbow is of a different kind: it is what we call a hinge joint. You can not make any whirling motion at your elbow as you can at the shoulder; the motion is all one way, like a hinge. The chief motion at the wrist also is a hinge motion, as you can see by working your hand back and forth. There are two bones, you notice, in thearm below the elbow: these roll on each other in such a way that you can turn the palm of your hand in different directions.
There are a great many little bones in the body of the hand and in the fingers. There is a very great variety in their motions, so that the hand can do almost any thing that you want it to do. I shall have something more to tell you about this when you come to the chapter on the hand.
Bones of the leg and foot.
You have here the bones of the leg and foot. You see only the lower end of the stout thigh bone, at the knee joint: it makes a hinge joint with the large bone of the leg. The motion of this joint is only one way, backward and forward, as you see in walking. The small, thick bone, called the knee-pan, is left out in the figure. One of the uses of this bone is to be a shield to the joint. If you fall down in running, you are apt to come upon the knee, and this shield keeps the joint from being hurt.
You see that long, very slender bone at the side of the large one: one would suppose that this would be very easily broken, but it is not, because it is so well covered up with muscles. Its lower end is quite thick and strong, and makes the outer part of the ankle. The ankle joint is a hinge joint like that of the knee.
Why there are so many bones in the foot.
There are as many bones in the foot as there are in the hand. Why is this? You remember that I told you that the hand had so many bones because it had to perform so many different motions. But it is not so with the foot; it does not have much variety of motion. There is some other reason, then, for its having so many bones. It is this. If the bones of the foot were all in one, the foot would be a very stiff and clumsy thing; it would not be springy as it is now. You would make awkward work in walking and running with such feet.
Skeleton of the bat.
The bones of different animals are made differently, according to the work which they do. Those that do heavy work have heavy, stout skeletons; but those that have only light work to do have their bones slender. A bird has a light skeleton, for it could not fly so well with a heavy one. Here is the skeleton of a bat. The bones are exceedingly light and slender, for it is light and nimble work that he does in flying.
The bones in an old person are more brittle than those in a child. If the child’s bones were brittle they would be very often broken, because he so often tumbles down. If old personswere as careless as children are, there would be broken limbs to be taken care of in almost every house. They would not get off with a short crying spell and a bruise, as children commonly do when they have a fall.
The bones of a child’s head.
There is one contrivance in the child’s head that prevents the bones from breaking in its frequent falls. In the grown person the bones of the head are fastened tightly together, and are almost like one bone. But it is not so with the child. In an infant’s head they are very loose, and you can feel quite a space between the bones at the top of his forehead. Now, when the child falls and hits his head, the loose bones give and do not break.
Though the teeth are like the bones, they are different from them in one thing. The bones grow with the rest of the body, but the teeth never grow any larger than they are at first. When the tooth first pushes up through the gum, it is as large as it ever will be. Look at the reason of this. The outside of the tooth—the enamel, as it is called—is made very hard. It needs to be so, that the tooth may do its work well. Such a hard substance, when once made, is finished. It never can grow. No blood can get into it to make it grow, as it can into the bones.
Why we have two sets of teeth.
And now you see the reason that every person has two sets of teeth. If the teeth that one has when a child should remain in his head, they would be too small for him when he became an adult; and as the jaws grew they would become quite far apart, and so would look very strange. To get rid of these difficulties, the first set begin to be shed about the seventh year, and a new set of larger teeth take their places. As the new teeth are notonly larger, but are more in number, they fill up all the room designed for them in the enlarged jaws.
Skeletons of crabs and lobsters.
All the bones of our bodies are inside, and are covered with muscles, cords, and ligaments; and over all is the skin. But the bones of some animals are outside. This is the case with crabs and lobsters. Their bones make a sort of coat of mail to defend the soft parts from being injured. The hard coats of many insects also may be considered as their skeletons.
How they are shed every year.
Such animals as crabs and lobsters have new skeletons every year. The old skeletons are too small for their growing bodies, and so they must be cast off. The animal crawls into a retired place to go through the operation. It is painful, and sometimes proves even fatal. He makes a great effort, and the shell comes apart. He then, by hard struggling, pulls himself out. He now keeps still a few days in his retirement, and another case or skeleton, as hard as the old one, is formed. When he comes out with his new armor on, he is as brave and as ready to fight as ever.
Questions.—What is said about the shoulder joint? The elbow joint? The wrist? How is it that you can turn the palm of the hand one way and another? Why are there so many little bones in the hand? What is said about the knee joint? What is one of the uses of the knee-pan? What is said about the slender bone in the leg? What about the ankle joint? Why are there so many bones in the foot? What is said of the difference in brittleness between the bones of the old and of the young? What is said about the bones in a child’s head? How are the teeth unlike the bones? Why do we have two sets of teeth? What is said about the bones of some animals? What is related of crabs and lobsters?
Questions.—What is said about the shoulder joint? The elbow joint? The wrist? How is it that you can turn the palm of the hand one way and another? Why are there so many little bones in the hand? What is said about the knee joint? What is one of the uses of the knee-pan? What is said about the slender bone in the leg? What about the ankle joint? Why are there so many bones in the foot? What is said of the difference in brittleness between the bones of the old and of the young? What is said about the bones in a child’s head? How are the teeth unlike the bones? Why do we have two sets of teeth? What is said about the bones of some animals? What is related of crabs and lobsters?