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BRAIN

258,

BRAIN

tions. It is easy to go from this condition to that in the human brain, in which the cerebral lobes are so large that they cover the cerebellum. When the human brain is looked at directly from above, we can see only the cerebral lobes; all the other parts are covered, and the surface is much more convoluted than in the case of the dog's brain. Therefore, the line of change in brain,

I--BRAIN OF PERCH

2--BRAIN OF FROG

3BRAIN OF ALLIGATOR

4BRAIN OF PIGEON

5BNAIN OF RABBIT

6BRAIN OF DOG

from simpler to higher forms, has been the great expansion of the cerebral lobes, and there have also been internal changes.

The outer layer of the central lobes is called the cortex. It consists of gray matter which under the microscope is

made tcp of an^ immense number of nerve-cells, arranged in layers. Nerve fibres lead away from the nerve-cells, and serve to carry the impulses that arise within the latter. Just as electricity generated in a battery cell may be carried away by wires, so the nervous energy that arises in the nerve-cell is carried away by the nerve fibre. The many fibres leaving the cortex serve to connect it with other parts of the brain, and also with the tissues of the body. Bundles of fibres form fibre-tracts, which pass into the spinal cord, and from which smaller bundles of fibres run in the nerve trunks and are distributed to muscles and all other tissues of the body. These are motor, secretory nerves, etc. Similar groups of fibres run in the opposite direction from the surface of the body inward. These are sensory fibres.

It is extremely interesting to know that certain groups of nerve-cells in the cortex preside over particular movements and particular activities of the body. The entire outer surface of the brain has been explored by use of the electrical current and divided into territories called areas. The stimulation of a particular area, for example, will cause movements of muscles of the leg (see sketch), that of an adjacent area, movements of the arm, etc. The diagram shows the parts of the cortex connected with the leg, arm, face, tongue, larynx, with hearing, sight, speech, word-blindness and sensation. Any disorder or deficiency in the cortex of these areas makes disturbances or disorders of the muscular movement, or the sensations connected with them.

The cerebellum, likewise, contains clusters of nerve-cells, which connect with fibres, and regulate a large number of muscular movements. The medulla ob-

longata c o n -tains clusters of cells called centers, which control and regulate breathing, ac-tion of the heart, swallowing, movements of the stomach, the blood supply to different parts, etc. Any injury to the medulla is, therefore, very serious.

Some of the most interesting recent experiments show that the nerve-cells undergo changes, that can be detected by the microscope, similar to the changes in the cells of glands and other tissues, when they are active. This brings the activity of the central nervous system into line with that of other physiological processes.

Diagram of  the human brain, showing area of   the cortex