Unfortunately, however, long and carefully as we have studied the brain, we really know little about the way in which it carries out these most important processes of memory, of judgment, and of thought, or even of the particular parts of it in which each of these is carried out.
[Illustration: THE NERVOUS SYSTEM Diagram to show brain, spinal cord, and larger nerves.] No part of the brain, for instance, seems to be specially devoted to, or concerned in, memory or reason or imagination, still less to any of the emotions, such as anger, joy, jealousy or fear; so all those systems which pretend to tell anything about our mental powers and our dispositions by feeling the shapes of our heads, or the bumps on them, are pure nonsense.
The most important and highest part of the brain is its surface, a thin layer of gray nerve-stuff, often spoken of as the _gray matter_ (the _cortex_, or "bark"), which is thrown into curious folds, or wrinkles, called _convolutions_.

This gray matter is found in the parts of the nervous system where the most important and delicate work is done.

The rest of the nervous system is made up of what is called white matter, from its lighter color; and this is chiefly mere bundles of telephone wires carrying messages from one piece of gray matter to another, or to the muscles.
We also know that a certain rather small strip of the upper brain-surface, or cortex, about the size of two fingers, running upward and backward from just above the ear, controls the movements of the different parts of the body.

One little patch of it for the hand, another for the wrist, another for the arm, another for the shoulder, another for the foot, and so on.

We can even pick out the little patch which controls so small a part of the body as the thumb or the eyelids.
So when we have a tumor of the brain or an injury to the skull in this region, we can tell, by noticing what groups of muscles are paralyzed, almost exactly where that injury or tumor is.