the joint, just as your taking hold of the lid of a box and raising it works the hinges; or pushing the weight at the bottom of a clock pendulum makes it swing on the pivot where it is hung. This will be seen by the accompanying diagrams. The contraction of the muscle, P, shortens and tightens it, which makes the joint, F, bend, drawing the hand up towards the face. At the same time, if we throw the body forward a little, the tightening of P (fig. 7), pulls the string (or tendon), and raises the heel, so that a step forward is made in walking. Then the contraction of the muscle of the thigh pulls its tendon, and straightens the femoral bone, drawing it into a straight line with the leg below the knee, thus pulling the body into an erect position. But as these joints would soon wear out by rubbing against one another, they are kept well supplied with a kind of oil that makes them work smoothly. Muscles are what we generally call flesh, being red from the blood-vessels in them. We could not do without muscles, as they cause all movement by contracting and expanding, so as to turn the bones on their joints, work the lungs in respiration, and keep up the pumping of the heart, the thinking of the brain, and the movement of the food in mastication, swallowing, and digestion; so that we can neither work nor play, stand, kneel, nor sit without them; and, as they are equally required for circulation, respiration, and absorp tion, we cannot even live, when asleep, without their aid. ON DIGESTION. You know that your bodies wear out by use, just as your shoes do by walking on them. But every now and then your shoes have to go to the cobbler, and a new sole is put on in place of the one that was worn off. Now, when we run, or walk, or play cricket, or think, or talk, or breathe, or eat, we wear off some part of our bodies; but we do not every now and then send the body to a workman to have a new piece put on here and a patch there, where the old parts have been worn away; but we keep mending every bit of the body as fast as it wears off, and putting in fresh material as good as the old. The cobbler takes a piece of leather, and nails it or stitches it on to the shoe; but we cannot take a piece of meat and stitch it on to our arms to thicken them, if they have got thin, but we do get it on to our arms in another way. You may have seen people put wool or cotton into a machine at one end, and then have seen it come out in threads of worsted or cotton at the other; or you may have watched people in a paper-mill put a quantity of old rags into a large vat, and then seen it come out in large sheets of paper at the end of the machine. Your body is a machine which manufactures what you eat into flesh and blood and bone, just like the papermachine turning rags into paper; only it manufactures everything it wants for its own use, it does not turn it out for sale. It uses up all it wants to mend itself with, and only turns out what it does not want. Let us see how it does this. When your body has worn away some parts by your working or playing for a few hours, you feel tired and hungry. You come in and get your dinner, and by the time you have finished, you do not feel tired, but you are ready to run off and play again. When you felt hungry, your machine wanted something put into it, and you were tired, because it was worn and almost standing still for want of something to work upon. you put in more stuff, and it went to work again, and mended what was worn, and then you could run about as fresh as ever. What did you put into the machine? A piece of bread and butter, which you bit off a slice with your two front teeth (the cutting teeth). This was very quickly rolled by your tongue into the middle of your mouth, and you felt your mouth water, and the dry bread began to get moist. Your tongue and your cheeks kept working it backwards and forwards between your back teeth, till it was ground quite small, so those teeth are called grinders. Now, let us see what has happened to it? I daresay some of you have chewed some wheat in that way, and then have taken it out of your mouth and found it a sticky paste, and made bird-lime of it. You will find, too, that after you have begun to chew it, it tastes sweet. This sweet taste is because it is changed by the moisture of the mouth (which is called saliva), into sugar, and it is sticky, because some part of it is also changed into gluten. So we have these two changes of bread in the mouth into sugar and gluten, and this is the first part of digestion. This paste is now fit to make fat in your body, and keep you warm. The butter mixed with it is also fat. If you eat any potatoes, just the same thing happens to them as to the bread. But now you put a piece of meat into your mouth, and chew it in the same way till it is quite small and mixed with saliva, but it is not changed, like the bread, into sugar. You swallow them both by the action of the tongue, which rolls the food up into a little ball, and pushes it to the back of the throat, where it drops into the gullet. This gullet is a pipe fitted with rings, and as the food touches each ring, the ring closes and squeezes it on to the next, and so it is pushed gently down into the stomach. You may see these rings working in a horse's throat, if you watch it drink. A little gristly curtain at the back of the mouth (called the palate) prevents any food from going up the nose, the entrance to which is closed by it in the act of swallowing. The same act also pinches together the sides of the windpipe, and covers them with another piece of gristle, called the epiglõttis. The stomach, into which the food falls, is a bag of muscle, something like a bladder, with a small pipe at each end. Its inside is lined with a velvety sort of covering, like the rougher side of tripe and the rough part of |