From all which distinguish man from other living beings, the most important is the brain. Some species of lower animals do not have brain, or have very small or poorly developed brain. For example, the earthworm has a brain as big as the head of a pin, the rabbit has a brain like a thimble. The human brain weighs an average of 1,350 grams, but the size of the brain is not the most important thing. The elephant has a bigger brain than a man, but his brain is not as developed as human.
The brain has three main parts: a large brain, cerebellum and medulla oblongata.
A large brain is considered the most important part because it manages all of our voluntary actions. It is the largest part of the human brain. It fills the entire surface of the top and back of the skull. It is divided into two equal halves, or hemispheres, and its surface is covered with grooves and convolutions, and consists of gray matter consisting of nerve cell bodies. Beneath this surface layer, called the cortex, is the white mass consisting of nerve fibers. Through this part been carried the messages to and from the crust. Specific parts of the cortex manage some functions of the body, so that each part of the crust is different. Science has discovered that in parts of the crust are centers for vision, feeling, hearing and movement of some muscles. Therefore, when the crust is damaged, only one part of the brain loses certain functions, such as, for example, speech.
The cerebellum is located in the back of the skull, below the cerebrum. It controls the maintenance of balance and muscle coordination. If the cerebellum is injured, man may become unable to walk or stand up straight right.
Medulla oblongata is large as the tip of the thumb and is located at the upper end of the spinal cord. It controls breathing, heartbeat, and many other activities for which it seems to be performed by themselves. It is a crossing of the nerve fibers that go from the brain to the spinal cord.
One half of the cerebrum controls one side of the body, and the other another. For example, the right half of the brain controls the left leg , etc.