[24] You can easily prove that a bone is made up of living tissue soaked and stiffened with lime, by putting it into a jar filled with weak acid. This will gradually dissolve and melt out the lime salts, and then you will find that the bone has lost three-fourths of its weight and that what remains of it is so soft and flexible that it can be bent, or even tied into a knot. [25] The hollow spaces in the bones of birds, however, are filled with air, which makes them lighter for flying. CHAPTER XX OUR TELEPHONE EXCHANGE AND ITS CABLES The Brain. We are exceedingly proud of our brain and inclined to regard it as the most important part of our body. So it is, in a sense; for it is the part which, through its connecting wires, called the _nerves_, ties together all the widely separated organs and regions in our body, and helps them to work in harmony with one another. We speak of it as the master and controller of the body; but this is only partially true. The brain is not so much the President of our Cell Republic as a great central telephone exchange, where messages from all over the body are received, sifted, and transmitted in more or less modified form, to other parts of the body. Three-fourths of the work of the brain consists in acting as "middle-man," or transmitter, of messages from one part of the body to another. In fact, the brain is far more the servant of the body than its ruler; and depends for its food supply, its protection, its health, and its very life, upon the rest of the body. The best way to keep the brain clear and vigorous is to keep the muscles of the stomach, the liver, the heart, and the entire body in good health. What the Brain Does. The brain is the very wonderful organ with which we do what we are pleased to call our thinking, and also a number of other more important things of which we are not conscious at all. It is a large organ, weighing nearly three pounds when full grown. In shape it is like an oval loaf of bread split lengthwise by a great groove down the centre, and with a curiously wrinkled or folded surface. The two halves of the brain, called _hemispheres_ (though more nearly the shape of a coffee-bean), are alike; and each one, by some curious twist, or freak, of nature, receives messages from, and controls, the opposite half of the body--the right half controlling the left side of the body, while the left half controls the right side of the body. Thus an injury or a hemorrhage on the left side of the brain will produce paralysis of the right side, which is the side on which a stroke of paralysis most commonly occurs.