Читать книгу Physiology - M. Sir Foster - Страница 6

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7. When you want to make a snow man, you take one great roll of snow to make the body or trunk. This you rest on two thinner rolls which serve as legs. Near the top of the trunk you stick in another thin roll on either side—these you call the two arms: and lastly, on quite the top of the trunk you place a round ball for a head. Head, trunk, and limbs, i.e. legs and arms—these together make up a complete body.

In your snow man these are all alike, all balls of snow differing only in size and form; but in your own body, head, trunk, and limbs are quite unlike, as you might easily tell on taking them to pieces. Now you cannot very well take your own body to pieces, but you easily can that of a dead rabbit. Suppose you take one of the limbs, say a leg, to begin with.

First of all there is the skin with the hair on the outside. If you carefully cut this through with a knife or pair of scissors and strip it off, you will find it smooth and shiny inside. Underneath the skin you see what you call flesh, rather paler, not so red as the flesh of beef or mutton, but still quite like it. Covering the flesh there may be a little fat. In a sheep’s leg as you see it at the butcher’s there is a good deal of fat, in the rabbit’s there is very little.

This reddish flesh you must henceforward learn to speak of as muscle. If you pull it about a little, you will find that you can separate it easily into parcels or slips running lengthways down the leg, each slip being fastened tight at either end, but loose between. Each slip is what is called a muscle. You will notice that many of these muscles are joined, sometimes at one end only, sometimes at both, to white or bluish white glistening cords or bands; made evidently of different material from the muscle itself. They are not soft and fleshy like the muscle, but firm and stiff. These are tendons. Sometimes they are broad and short, sometimes thin and long.

As you are separating these muscles from each other you will see (running down the leg between them) little white soft threads, very often branching out and getting too small to be seen. These are nerves. Between the muscles too are other little cords, red, or reddish black, and if you prick them, a drop or several drops of blood will ooze out. These are veins, and are not really cords or threads, but hollow tubes, filled with blood. Lying alongside the veins are similar small tubes, containing very little blood, or none at all. These are arteries. The veins and arteries together are called blood-vessels, and it will be easy for you to make out that the larger ones you see are really hollow tubes. Lastly, if you separate the muscles still more, you will come upon the hard bone in the middle of the leg, and if you look closely you will find that many of the muscles are fastened to this bone.

Now try to put back everything in its place, and you will find that though you have neither cut nor torn nor broken either muscle or blood-vessel or bone, you cannot get things back into their place again. Everything looks “messy.” This is partly because, though you have torn neither muscle nor blood-vessel, you have torn something which binds skin and muscle and fat and blood-vessels and bone all together; and if you look again you will see that between them there is a delicate stringy substance which binds and packs them all together, just as cotton-wool is used to pack up delicate toys and instruments. This stringy packing material which you have torn and spoilt is called connective because it connects all the parts together.

Well, then, in the leg (and it is just the same in the arm) we have skin, fat, muscle, tendons, blood-vessels, nerves, and bone all packed together with connective and covered with skin. These together form the solid leg. We may speak of them as the tissues of the leg.

8. If now you turn to the trunk and cut through the skin of the belly, you will first of all see muscles again, with nerves and blood-vessels as before. But when you carefully cut through the muscles (for you cannot easily separate them from each other here), you come upon something which you did not find in