219.WHY DO WE HAVE TWO NERVOUS SYSTEMS?
The human nervous system is like a network of wires that lead from the brain, establishing contact with every portion of the body The brain is the center of this system. From the brain, orders go forth over the nerve wires and make us move, laugh, eat, and otherwise behave like human beings.
The nerves also serve the brain as messengers. Through a special part of the nervous system, the brain is informed of everything that happens to the body. The main line of communication connecting brain and body is the spinal cord. It is enclosed within the backbone. The spinal cord is really a grouping together of the nerve trunks, similar to the grouping together of wires to form a cable.
218.WHY DO WE HAVE TO EAT?
If we didn’t eat, we couldn’t keep our bodies alive, we couldn’t grow, we wouldn’t have energy, and various parts of our bodies couldn’t function as they must. Modern scientists have been able to find out which substances in food our bodies need, and how much of these substances we need to be healthy. In other words, it isn’t enough to just eat; we must take in enough of each type of food substance for a specific purpose.
217.WHY DO OUR TEETH DECAY?
Among the lower animals, there is a whole succession of teeth throughout their lives. As teeth are fully developed and used up, they fall out and new ones take their place.
Man, however, has only a single replacement of his teeth. By the time a baby is about two years old, it has a total of 20 teeth, called the milk teeth.
