our tongues. This roughness is caused by little tubes with their pointed ends outwards. As soon as they are touched by food coming against them, they begin to sweat out a liquid, called gastric juice, which moistens the food, and changes its nature, separating the fluid from the solid part, and dissolving the latter (except fat). The food enters the stomach at the upper right-hand corner (c of the diagram), and the muscles contracting at that point, push it towards the left along the upper surface (a a) till it gets to P, which is closed by strong muscles. It then returns down the middle, b, and on reaching the end to the right hand, divides into two streams, one going along the top, aa, the other along the bottom, d. This constant circulation, which is kept up by the two sets of muscles in the stomach (one working lengthwise and the other circular-wise) both grinds all the hard parts of food against the rough surface, so reducing it to a powder, and also thoroughly moistens it all with gastric juice, until it is brought to a thin milky pulp, called chyme. As each portion of chyme in a sufficiently liquid state comes to P, the pipe (called Pylorus) opens a little and lets it out, the rest continuing to circulate, until all that can be dissolved is thus passed out. During this process, the stomach contracts at each flow of liquid out of the pylorus, so that the remainder is always being rubbed by the coat of the stomach. When all has been passed out that can be dissolved, the pylorus opens wide and lets out the sediment, and the stomach remains at rest till the next meal. This chyme now passes through a curved pipe, called the Duodenum D, where the bile from the gall-bladder (GB), of the liver (L), and the pancreatic juice from the sweet-bread (S w) are mixed with it. This mixture completes the digestion of the fat and sugar, which have not undergone any change in the stomach, and it is now called chyle, in which state it is ready to be poured into the veins to form new blood, and give nourishment to the body. A set of very small pipes, like hairs (called capillaries) touch the smaller intestines, into which the chyle runs next, and suck out through the walls of the intestines whatis nourishing, and convey it into one larger pipe (the thoracic duct, Th, fig. 9) which carries it up by the spine to the left side of the neck, where it flows into a large vein (vera cava superior), mingling with the return blood from the head, and is carried to the heart, which pours it into the lungs for purification, when it is fit for restoring the waste of the body. The refuse or fibrous part of the food, which does not contain any nourishment, or is incapable of being digested, now passes from the smaller into the larger intestine, through a little mouth or valve. This large pipe is called the colon, and first ascends the body for a short distance (and is called the ascending colon); then crosses over the body (and is called the transverse colon), next goes downwards (getting its name of descending colon), when its form changes into a straight pipe (the rectum), by which the refuse matter is conveyed away from the body; so this part of the intestines may be called the main sewer. The relative positions of the gullet A; stomach, B; liver, C (with its gall bladder, D); the duodenum, E; sweet-bread, F; smaller intestines, G; larger intestines, or colon, H; and rectum, I, will be best seen by reference to the accompanying figure. We have now got the food we cat distributed all over the body in a liquid state, and there it lies ready to be made into flesh and blood; but it is soft and liquid, like milk, only coloured red by the oxygen of the air. How, then, does it become solid flesh and bone? This is the last process. I dare say you know that a farmer's wife takes her milk and cream, warms it a little, then puts it into a small shallow tub, sprinkles a little salt, some colouring matter, and other things of that kind upon it, and then leaves it quite still to set, as she says, and in a few days it has got solid, and in time becomes a cheese, almost as hard as a piece of wood. Now, it is something like this with our food. This milk gets its colouring matter from the air, and a little salt and mineral matter is eaten with our food, and then it requires to be left quite still to set, until it becomes solid flesh and hard bone, like a piece of wood. This is why, when you have run about in the fresh air, and filled your blood with oxygen, you feel so sleepy. The milky food is ready for setting, and it wants to be left quiet and still to set. So you go to bed and are soon fast asleep. But what happens during sleep? Why, this new blood is set and made solid. The blood scarcely circulates at all during sleep; it is almost still, just moving a very little to keep up the current, and prevent the veins closing up or getting clogged with waste matter. The mind is at rest, the body does not move, the senses are all closed (you neither see, nor smell, nor hear, nor taste, nor feel, when you are asleep), the tongue is quiet; so, as there is no action, there is no work for the blood to do; but it is lying still to be set into solid matter. Sleep, then, is necessary for the repair of the body, as well as food and fresh air, for without it our food would remain liquid blood, and we should not restore the flesh and bone worn away by exercise. We take in our material by day, but we manufacture it into the various articles we want for repair of the body, and put the new material into its proper place, only during the sleep of night. CIRCULATION. We have seen how food is turned into blood; but it is not like blood at first, for it is white like milk, not red like blood. We have next to see how it gets its red colour. When it has been poured into the vein on the left side of the neck, it runs into the heart, which acts like a force-pump, and sends it up a pipe into the lungs. The lungs are very like two large sponges, one on each side of the heart, and a little above it. The heart forces the blood up the pipe (called the pulmonary artery), which then branches off into smaller pipes, and these divide into a great many very little ones, as fine as hairs, like the branches of a tree, and carry the blood into all parts of the lungs, just like those parts that form the substance of the sponge round the little holes. You can understand this if you dip a sponge into ink, and you will then see what part of the sponge is black, and that there are little holes where there is no black. blood is not quite black, but it is blue. The Look at the Fig. 11. veins on the back of your hand and you will see the colour. Now, draw in a breath, and the air rushes down S. VI. U |