HOW THE BODY IS GOVERNED
Making the Parts of the Body Work.—Each of the hundreds of organs in the body has a certain work to do and this must be done at the right time. In order that all may work together and each one do its part when needed, there is a chief manager called the brain and a helping manager named the spinal cord. Millions of tiny threads for sending messages connect the two managers with every part of the body. These threads form the nerves.
The Brain.—The brain is a soft bunch of matter filling the inside of the skull. The bones of the skull are a quarter of an inch thick and prevent any common knocks from hurting the brain. It is surrounded by three coverings which also help shield it from injury.
The surface of the brain is very uneven. There are a great many folds separated by grooves. Some of these are more than an inch deep.
Fig. 86 —The under side of the brain and the spinal cord with the chief nerves of one side of the body viewed from in front.
Parts of the Brain.—The brain is divided into three chief parts. The upper and larger part is called the big brain or cerebrum. The lower part behind is the little brain or cerebellum. The part under the little brain and round like the thumb is the stem of the brain. It connects the larger parts of the brain with the spinal cord.
The big brain is partly separated into halves by a deep cut called a fissure. Each half is a hemisphere.
The outer layer of the brain is gray. It is made of millions of tiny lumps of matter which are the bodies of nerve cells. These are connected by threads much finer than hairs with other parts of the brain and spinal cord. Over these threads called nerve fibers one cell can talk to another somewhat as we talk over a telephone wire.