Читать книгу Natural Wonders - Edwin Tenney Brewster - Страница 11

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By “we” I mean all living things, trees and grass and dogs and cats and boys and girls. For as you, my reader, have I hope already discovered, we who have the breath of life in us are a good deal alike, whether we are oaks or men. We don’t look much alike, to be sure. But when we consider the things that are not alive—the stones and stoves and bats and balls and such—and see how very different we are from these, then we get some idea of what being alive is, and understand how being alive makes us blood-brothers with everything else that is alive also.

Now things that are alive usually do more or less growing. We have already learned something of this growing of little creatures in the egg—how the eye buds out as a ball and afterwards folds into a cup, how the limbs sprout out from the body as shapeless lumps which only gradually turn into hands or feet or wings. We have learned something of the way the bones grow, and the skin, and the hair, and the nails. Now we have to learn something more about the way a little child or a little tree grows up to be a big one.

The tree, we already know, grows larger round only between bark and wood. It grows taller only at the tips of its branches. The solid wood, once formed, does not change. If then, you drive two nails into the trunk of a little tree, say a foot apart and one above the other, even if that tree should grow to be a hundred feet high, those two nails would remain just where they were, a foot apart and just the same height from the ground as before. The little tree looks so much like the big one, that one cannot help thinking that it has simply grown thruout, so that the same branch which was once at the height of one’s head has now been lifted to the height of the house eaves. But this is not the fact. The lower branches of the little tree have died and dropped off; what are now the lower branches of the large tree, were once the top twigs of the little one, which have always been at the same level where they now are. The top branches of the large tree, as the tree grows still larger, will in their turn become, first the middle branches, then the lower ones, then will drop off entirely.

Now this growing at one end which looks like growing thruout is pretty common in our own bodies. We have seen how the hair grows at the inner end only, and the nails likewise, and the skin. Ignorant people will tell you that cutting off the ends of your hair, or singeing the ends; and that smearing various messes on the outside of your skins will change the quality of either. Don’t you believe them. After wood and skin and hair and teeth are once grown, all we can do is to protect them. Really to affect their growth for good or ill, we have to do something to the growing end.

The bones also grow in spots. The child’s leg bones and arm bones and finger bones do not simply swell up to become the man’s. The head of each bone, the rounding end, that is, where it touches the next, grows on the outside. But the shaft does its growing chiefly at two spots, one at each end where the shaft joins the head.

The bony part of the tooth, on the other hand, starts as a paper-thin sheet, but full sized. The living cells which build the hard bone, lie on the inside of this shell. They keep building on more bone on the inner surface, pushing themselves toward the middle of the tooth, until the tooth wall is so thick that only a narrow space is left in the center. But their long roots which they leave behind, still reach thru to the outer surface of the bone, ready to ache when there is occasion. Meantime, the outside of the tooth pocket, as we have learned, has been plastering on enamel on the outside of the shell, and pushing itself farther and farther away.

A plant’s roots, like its branches, grow at the tips; and the nerves in our own bodies grow in somewhat the same way, beginning at the inner end, and somehow finding their way thru and around the other tissues of the body, till they find the place to which they were sent. But the muscles and the fat grow thruout their mass, like dough being raised for bread. Most of the hollow tubes of the body—the blood vessels, for example, and the red lane down which our breakfast goes—grow in this way. But the hollow bones, as they grow, are taken down on the inside to enlarge the hollow, and built up again on the outside with old material and new to enlarge the shaft.

Even the blood grows, the watery portion coming from the food we eat and the water we drink; but the red and white corpuscles which float in the watery part, are made in special factories in the body (some of them in the marrow of the bones) and turned loose in the blood stream.

Growing, you see, tho easy to do, is by no means so simple as it appears.