. A healthy body. A textbook on anatomy, physiology, hygiene, alcohol, and narcotics. For use in intermediate grades in public and private schools . jellj-like mass, smTounded by membranes, andwell protected from injury by the bones of the is full of blood-vessels; some of these are quite large,but the majority are minute capillaries. The surfaceof the brain is not smooth, but is thrown into ridges,between which are depressions. The White and the Gray Matter. The brain consists oftwo kinds of matter, — the white and the gray former is composed largely of nerve-fibres, while
. A healthy body. A textbook on anatomy, physiology, hygiene, alcohol, and narcotics. For use in intermediate grades in public and private schools . jellj-like mass, smTounded by membranes, andwell protected from injury by the bones of the is full of blood-vessels; some of these are quite large,but the majority are minute capillaries. The surfaceof the brain is not smooth, but is thrown into ridges,between which are depressions. The White and the Gray Matter. The brain consists oftwo kinds of matter, — the white and the gray former is composed largely of nerve-fibres, whilethe latter consists principally of nerve-cells. The graymatter is on the outside of the brain. It is the graymatter that commands, while the white matter gray matter originates, and the white matter con-veys the messages. The Cerebrum. The part of the brain above the earsis called the cerebrum, or the great brain; the part THE NERVOUS SYSTEM. 161 at the back of the head, beneath the cerebrum, is calledthe cerebellum, or the lesser brain. It is supposed thatthe cerebrum is the organ of the mind. Here we think,and know, and Fig. 53. Various forms of nerve-cells. The cerebrum is divided into two parts by a naturalfissure in the middle line, passing from the front back-wards. From this it would seem that we have twobrains, — a right brain and a left brain. But at thebottom of this fissure the halves are united by a bandof nervous tissue (Fig. 55, 3) ; so doubtless their actionis in some way connected. The ridges, or convolutions,of the cerebrum vary in different animals. As a rule,the more intelligent the animal, the more numerous arethese convolutions; the deeper the depressions betweenthem. 11 162 A HEALTHY BODY. The Weight of the Brain. The brain of the elephant isthe heaviest known, which weighs from eight to tenpounds. The brain of the whale comes next, whichweighs from six to eight pounds. The average weight
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