There are several other complicated body systems, such as the skin and the reproductive system, but we will conclude this chapter with the nervous system.
Without nerves, your body could not send, relay, or receive any signals. Without nerves, you could not think or even live. A large part of your nerve activity is done without your conscious thought, and is called the autonomic nervous system.
Did you know that the best way to build a telephone switching station is to send in several dump trucks with sand, dirt, rock, and odds-and-ends junk? Then send in a bulldozer to scatter it around a little. After that leave it for several million years and return-and you will have a complete switching station, ready for operation? Well, that is how evolutionary theory would build one.
But within your body is a switching station and far more: a complete electronic computer system operated by something equivalent to an Intel chip 500,000. (As these words are being written, the largest home computers are Intel 486 in capacity.) Literally millions of connections are to be found inside just a pinhead of space in your brain. Main cables flow out from the brain and down through your spinal column, and then out to various parts of your body. And all that is supposed to have come about by chance?
Through a network of wires, messages come into the central switchboard, where the necessary connections are made to direct them out to the right places. Your nervous system is organized to bring messages into a center which relays them out to certain parts of the body. The brain and the spinal cord are the switchboard, and the nerves are the wires that carry incoming and outgoing messages. The deference is that thinking is a part of your switchboard system.
Your brain weighs about three pounds. It is similar to a bowl of jelly, yet it is the most fantastic creation in our world. The largest part is the cerebrum which fills the upper part of the cranium. Next is the cerebellum, located below the cerebrum. The third major part is the brain stem, with its pons and medulla.
The cerebrum is the main brain and is divided into two halves, one on either side, called hemispheres. The outer part is the cerebral cortex. This is soft grayish matter filled with nerve cells. Beneath it is the white matter, which has the nerve fibers, or “wiring,” leading out from the gray matter. The cortex or “gray matter” is heavily wrinkled. That is done to give it a much greater area. If it was flattened out, it would cover a surprisingly large area. Some centers in the cerebrum think, some are memory. Others are related to hearing, sight, movement, and speech.
Directly beneath the left and right cerebral hemispheres, and covered by them, are two other centers: the thalamus and the hypothalamus. The thalamus is a relay station; receiving impulses from every part of the body, it sends them to exactly the right part of the cortex. The thalamus also interprets sensations, and tells the brain whether they are pleasant or unpleasant. The main job of the hypothalamus is to regulate the action of various body organs in order to maintain normal conditions. For example, you shiver when you are cold because of the hypothalamus.
The cerebellum maintains body balance and coordinates groups of muscles. It is because of the cerebellum that you can walk across the room, or reach down and pick up a book. Skill in sports is related to good cerebellum connections.
At the top of the brain stem is the midbrain, which is an important reflex center. A reflex is an action that takes place automatically when something happens. If you look into a mirror and shake your head, your eyes will keep looking forward. It is the midbrain that tells them to do that.
The pons is the bridge between the cerebral cortex and the cerebellum, carrying messages from one to the other.
The medulla is just below the pons and is on the very bottom of the skull. It connects the brain with the spinal cord. It also controls certain factors on its own. One of these is the amount of carbon dioxide in the blood. The medulla, in some mysterious way, knows that percentage,-and then sends out signals instructing you to breath faster or more deeply. It also guides the rate of heartbeat. It even affects the muscles in the smallest arteries. The spinal nerves from the two halves (hemispheres) of the cerebrum cross over in the medulla before proceeding on down to the body.
The spinal cord is a long mass of nerve fibers reaching down through the central holes in all the vertebra in your spine. The spinal cord does two things: (1) conduct impulses from the brain to the body, and (2) operate as a reflex center apart from the brain. When you touch something hot, the spine sends the message to move your hand back quickly. That arrangement was wisely planned, for the nerve impulses warning of terrible danger did not have to travel as far before a message could be sent back to take proper action.
You have different types of nerve cells; we will not take the space here to describe them. Suffice to say that they are extremely complicated. Each nerve connects with thousands of other connections in nearby cells. The result is a massive electronic circuit board arrangement,-and all connected to part of a thinking mind.
The major nerves for your body exit the brain and travel down through the spine and then go outward at various points. There are 12 pairs of cranial nerves and 31 pairs of spinal nerves. The cranial nerves attach directly to the brain, and most of them carry impulses to and from the brain and various structures about the head (sensory organs, swallowing, speech, hearing, sight, tongue, jaw, etc.). However, other cranial nerves connect with organs in the thorax and abdomen.
The spinal nerves are attached to the spinal cord, and carry impulses from the skin and some internal structures to the central nervous system.