"The more wrinkles, the more wits," for that states the case concisely. "But what good do the creases do?" you ask again. "Give more surface for the gray stuff to be spread over," comes back the answer quick and positive. And

A, B, C, D, L show folds in the cerebrum; E, F show the gray and white of the cerebellum; K, H show the upper divisions of the spinal cord

that answer leads the doctor up to the point of his greatest enthusiasm, the gray and white substance of the brain.

Gray is all you have seen thus far, for it bends in and out with every fold and crease as if the whole substance of the brain were solid gray. "But look here," exclaims the doctor, as he presses open a deep cut which he has made with his knife through the gray cap, "see how

little gray there really is; only an outside layer about an eighth of an inch thick, and thinner than that in spots. But every thought you have, every pain you feel, every plan you make, every hope that thrills you, every purpose and ambition of your life is intimately connected with this thin gray layer that covers the white substance below it."

While you are thinking this over in amazement he will probably go on to say that the injury or disease of any part of that gray layer of the brain may rob you of one sense or another, or even destroy your brain power in the very direction where you thought you were strongest.

"If this particular brain had been injured here," the doctor will say, pointing to a certain spot on the gray surface, "its owner would not have been able to recognize anything that the eye looked at. And this is the worst sort of blindness, for when the sight center of the cerebrum is gone a man cannot so much as remember what seeing was like."

Accidents to the brain have taught some of these facts; diseases of the brain have taught others; while the study of the brains of animals has let in a flood of light on the whole subject. So that at the present time scientists know that a definite part of the gray layer is active for each separate sensation and for the power to move each separate part of the body.

This layer is called the cortex, and cortex means bark. It is clear then that the gray bark that covers both cerebrum and cerebellum is the most precious part of the human body. For this reason it needs a stout protection, and it gets it in the firmly knit, sturdy skull which surrounds it.

Instead of being a snug fit in its case, there is a little space filled with liquid, which separates the brain from the skull.

CHAPTER XXXVI

NERVE MACHINERY

From what looks like the confused tangle of fibers under the skin it would seem as if messages might sometimes get lost on their journey, - as if those intended for one particular spot might find themselves delivered at the wrong place, bringing despair to the brain. But

(Highly magnified)

this never happens. The confusion is only apparent; it is caused by the

way the bundles of fibers are variously bound together.

If we had eyes keen enough to see the fibers themselves, instruments delicate enough to do the work, and hands

steady enough to use the instruments without tearing the fibers, we might unwrap them, bundle after bundle, and trace them from start to finish. We should then find that every white nerve is a bundle of nerve fibers, each one of which is neatly and snugly wrapped by a fatty covering

that makes it look white, and that the difference between large nerves and small nerves is quite the same as the difference between large bundles of telephone wires and small bundles of wires, for in each the number of separate strands explains the size.

As we studied the nerves in this way we should discover for ourselves where the largest ones are and how they are related to the backbone. We should see that the bones of the back are so ingeniously locked together that a round opening is left on each side of each vertebra, and that as there are thirty-one vertebræ there must be sixty-two openings in all. We should then notice that the largest nerves of the entire nervous system are these sixty-two spinal nerves which find their way to the body through the backbone; and we should see that as soon as each leaves the bone the dividing begins. Large bundles, from the cord, become smaller through their dividing, then still smaller; they hold anywhere from two hundred to twelve hundred separate fibers; they continue to divide and subdivide, so that fibers which started in the same bundle are soon widely separated.

Often these fibers pass out of the wrappings of one bundle into the wrappings of another. They do this so constantly that these various bundles, as they grow smaller, are joined together like an intricate network. They twine and intertwine, but not a fiber loses its way. Each tiny one of the millions that form that lacework of