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 CHAPTER VIII

THE NERVOUS SYSTEM

Review Questions introducing this Subject.—What is a cell? What are the five supporting tissues? What are the two master tissues? Why are they so called? What kind of cells have many branches? Does the food ever come in contact with the salivary glands? When you look at a basket of apples, the sight "makes your mouth water." Is there a connection between the eye and the mouth? What two tissues enable the skin to blanch and to blush? Do the different organs share the blood in the same proportions at all times? How can this proportion be changed? How is the brain protected from injury? How is the spinal cord protected? Is the hole for the spinal cord through the main body of the vertebra, or behind the main body?

Harmonious Activity.—Strike suddenly at the eye of another, and the lids fall to protect it, and the hands rise to ward off the blow. If a grain of dust gets into the eye, the tear glands form tears to wash it out. If you touch the hand unexpectedly to a hot iron, the muscles of the arm jerk the hand away. If the foot of a sleeping person is tickled, the muscles of the leg pull it away. Many muscles coöperate in the act of running. If the human being were merely an assemblage of working organs, the organs might act independently, and there would be such confusion that the body would be powerless, and life could not be maintained. The nervous system enables the organs to work together for the common good. Why does an ameba not need a nervous system?

The Need of Nerve Centers as well as Nerves.—If there were no central office in a telephone system of one thousand subscribers, then every subscriber, in order to com