英文科學(xué)讀本 第四冊(cè)·Lesson 03 Our Bodies

Lesson 03 Our Bodies

We can feel, in every part of our body, portions of the hard, solid framework on which the body is built. This strong framework is called the skeleton, and consists of upwards of two hundred distinct and separate bones of various shapes and sizes, intended to give strength and solidity, and to support the softer fleshy parts. If you call to mind the various skeletons of animals you have seen, you will at once understand that in every animal it is the skeleton which determines the shape of the body. The skeleton itself suggests at a glance the well-known shape of the individual animal.

Let us examine our own skeleton, and see how it is made. We will commence with the head, which comprises two parts—the skull and the face.

The skull is a hollow box intended to hold and protect the brain. It is built of eight separate bones, most of them broad flat plates. They are joined together firmly at their edges, because, as they simply form a box, there is no need for these bones to move. The face comprises all the rest of the head that is not included in the skull. It is formed of no less than fourteen separate bones. The only thing calling for special notice in the arrangement of these bones is the provision, which is made by them for the protection of the eyes, by lodging those organs in great hollows formed in the bones themselves. These hollows are called the orbits or sockets of the eyes. Notice how providentially they are surrounded by the broad frontal-bone of the forehead above, the nose-bone between them, and the cheekbones below. These effectually protect the delicate organs from injury. Only one of all these fourteen bones of the face is capable of movement. Which is it? The lower jawbone moves so as to open and close the mouth. It is attached on either side by a sort of hinge to the other bones of the skull.

Both jaws are armed with teeth for biting and chewing our food. We have during our lives two sets of teeth. The first, called the milk teeth, are twenty in number, and are shed while we are young. The others, known as the permanent teeth, last through the rest of our life. There are thirty-two permanent teeth in the complete set.

The trunk is that part of the body which would be left if the head and limbs were lopped off. The main pillar of the body is the backbone, which extends from the neck to the bottom of the trunk. It is not actually a single bone, but a string of separate bones called vertebra, and is called the vertebral column. The vertebrae are joined together by thick smooth pads of gristle, which form springy, yielding cushions between each bone and the one above it. The seven smallest and topmost vertebras form the neck; they support the head. Next to these come twelve vertebras, each of which supports a pair of ribs—one on either side. The ribs form a kind of hollow bony cage, which we call the chest. They are joined in front to the breastbone. It is worth noticing that the vertebras increase in size and solidity downwards. The bones at the base of the column are very thick, solid, and strong.

It is a curious fact, too, that the vertebral column of a child contains thirty-three distinct bones; but there are only twenty-six bones in the vertebral column of an adult. The explanation is this; as the child advances into adult life the four lowermost vertebras grow together and form one piece, corresponding to the tail of the lower animals—in fact, a sort of rudimentary tail; and at the same time the five next above these also become welded into one piece to form the sacrum. The sacrum and the great haunch or hip-bones form a sort of bony basin—the pelvis—at the base of the column.