=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. [Illustration: FIG. 87.--Side of the skull cut away to show the brain. _B_, backbone.] =The Spinal Cord.=--This is a bundle of nerve matter about as thick as your finger. It extends from the stem of the brain down the canal in the backbone. The outer layer of the spinal cord is white because it is made of the tiny threads, _nerve fibers_. The inner part is made of the bodies of nerve cells and therefore looks gray. The fibers are branching threads from the cells in the cord and brain. =The Message Carriers or Nerve Fibers.=--In order that the managers may send messages, these fine threads, the nerve fibers, extend from them to all parts of the body. In many places from five to five hundred or more of these fibers are united in one white cord called a _nerve_. Twelve pairs of nerves are joined to the under side of the brain and thirty-one pairs are connected with the spinal cord (Fig. 86). The nerves of the brain branch to all parts of the head and neck, and one pair goes down to the lungs, heart, and stomach. The nerves connected with the spinal cord branch to every part of the muscles, bones, and skin of the arms, trunk, and legs. =How the Nerves do their Work.=--On a telephone wire we can send a message in either direction. A message can travel on a nerve in only one direction. For this reason there must be two kinds of nerves. One kind is called _sending nerves_ because the brain and cord send orders over them to make the organs act. The other kind carries messages to the brain from the eyes, ears, skin, or other organs of sense, telling it how they feel. On this account these are named _receiving nerves_. When we wish to catch a ball, the brain sends an order along the nerve threads down the spinal cord and out through the nerves of the arm to the fingers to get ready to seize a ball. The fingers are spread to grasp the ball, but they do not close until a message goes from the skin of the finger tips to the spinal cord, telling it that the ball is in the hand.