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. 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.