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.
All the nerve fibres in each half or hemisphere of the upper brain run downward and inward like the sticks of a fan, to meet in a strap-like band, or stalk, which connects it with the base of the brain and the spinal cord.

A very small amount of damage at this central part, or base, of the brain will produce a very large amount of paralysis.

We may have large pieces of the bones of the skull driven into the outer surface of the brain, or considerable masses of our upper brain removed, or destroyed by tumors or disease, without very serious injury.

But any disease or injury which falls upon the base of the brain, where these stalks run and big nerve-knots (_ganglia_) lie, will cause very serious damage, and often death.
The whole upper brain is a department of superintendence, which has grown up from the lower brain to receive messages, compare them with each other, and with the records of previous messages which it has stored up, thus giving us the powers which we call memory, judgment, and thought.