The science of life : fully illustrated in tone and line and including many diagrams

BOOK 1

departments, dealing with ordinary matters of routine even up to a very high degree of complexity, while the cerebral hemispheres

Fig. 54. A slice of the cerebrum, showing the outer layer of grey matter and the core of white matter.

are the executive considering the state of affairs as a whole and making special decisions only when the routine responses are insufficient.

If we cut a slice off one of the hemispheres—the reader may do this for himself whenever sheep or calves’ brains come to table — we observe that there is a division into two kinds of tissue; running over the surface of the hemisphere there is a layer of grey matter ; and this is where the nerve-cells are seated and where the mental processes actually take place. And inside this there is a branching core of white matter, consisting of the fibres that put the cells of this grey matter into communication with the lower rautine parts of the brain and with other parts of the body, and also connect different areas of grey matter one with another. Impulses arriving from the senseorgans come first to the lower receiving centres whence fibres lead away in two directions ; some go to the reflex centres and others to the cerebral hemispheres. The impulse may be wholly reflected in a reflex or pass on in part and more or less modified to the hemispheres. Responsively

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THE SCIENCE OF LIFE

CHAPTER 93

a system of fibres takes impressions down from the hemispheres to the motor nervecells in the brain-stem or spinal cord. Thus the executive can consider special circumstances, command special movements, and even inhibit and overrule the normal activity of the automatic administrative centres—as for example the mind of Mucius Scaevola inhibited his reflexes when to impress Porsenna with the fortitude of a Roman he held his hand in the burning coals and kept it there in spite of his pain. We shall see in a later part that the extent to which this control by the personal executive can be exerted varies in different animals. A fish or a frog is to a large extent a_ reflex machine, but as we examine in turn the lower and higher mammals leading up to man, we find the lower centres relying more and more completely on the guiding control of the hemispheres. At the same time the hemispheres themselves are better and better developed ; in a frog they are relatively small, but in one of the higher mammals, even at the mouse level, they are large and their surface is thrown into wrinkles so that as much grey matter as possible can be fitted into the

cranial cavity, “78: 55: A bi of the grey There can be ™atier of Fig. 54 highly little doubt magnified.

that the dis- J¢ consists of nerve-cells with com-

plicated tangled processes. The

numbers indicate layers with different

characteristics. (Courtesy of Dr.

C. J. Herrick and W. B. Saunders Co.)

tinctively large cerebrum of man is associated with the