英文科學(xué)讀本 第四冊(cè)·Lesson 15 The Nerves

Lesson 15 The Nerves

You know that the skull is merely a hollow box intended to hold and protect the brain. You know, too, that this box rests on the topmost vertebra, to which it is jointed by a double-hinge joint. In the floor of the box, and just where it rests on the vertebra, there is a round hole, and in this vertebra, as well as in every one of the vertebra in the column, is a corresponding hole.

These vertebrae standing one above the other as we might stand a string of reels, form a long continuous tube from the bottom of the column upwards to the skull. We call this tube the vertebral canal, or the spinal canal.

The brain fills the whole cavity of the skull and a portion of it extends downwards from the skull and fills the spinal canal. This part is known as the spinal cord.

This brain matter is totally different from the muscles or flesh. It is a soft grayish-pink substance, more like marrow than flesh. From the brain and spinal cord extend outwards through the body long, white, silvery-looking threads—the nerves. Some of these nerves spread themselves all over the surface of the skin, and it is through them that we are able to feel the hardness or softness, the heat or cold, the roughness or smoothness of various bodies. Others spread themselves among the muscles of the body. They are the foremen or overseers who set the muscles to work.

When we wish to lift our hand or turn our head, it is the nerves that make the muscles do our bidding. If the nerves were destroyed or injured (as they are sometimes by paralysis), we should find ourselves quite unable to lift the limb, however much we might wish to do it.

But whence do the nerves get their orders? The brain is the chief center. The nerves may be aptly compared to a multitude of telegraph wires stretching into every part of the body. They carry messages to and fro.

Let me give you an idea of the kind of messages they carry. What would you do if someone, without your seeing him, placed a hot poker near your hand? You would instantly draw your hand away. To you this would appear the simplest of actions, but in reality the nerves have been very busy. Some of them first carried up a message to the brain, to say that the hand was being burned, then the brain sent back, by another set of nerves, an order to the muscles to pull the hand away. The muscles would not act without this order.

But besides being the overseer of the muscles and their work, the brain is the seat of the will, the intellect, the memory, the emotions and affections. Moreover, it is by the brain and its nerves that we see, hear, smell, taste, and touch.