432 



Our Surroundings 



increase its area, so that it would cover, if spread out, nearly four 

 square feet. All this area contains nerve cells in the outside layer. 

 Each of these cells has many fibers, one of which develops a 



covering, or sheath, and be- 

 comes a nerve thread which 

 connects it with other cells of 

 the brain. 



The Cerebellum. The cere- 

 bellum lies just below the cere- 

 brum. It makes up about six- 

 teen per cent of the brain and 

 is similar in structure to the 

 cerebrum. It acts as a balance 

 wheel and regulates the move- 

 ments of the body. It has 

 nothing to do with the origin 

 of thought or with the vital 

 processes of life. 



The Medulla. The medulla 

 is a wedge-shaped organ which 

 forms a connecting link between 

 the spinal cord and the brain. It has an outside covering of white 

 nerve tissue which lies over a central mass of gray tissue, and it 

 gives off several pairs of nerves. It presides over the action of 

 the respiratory system, controls the contraction of arteries, and 

 regulates the movements of the esophagus in swallowing. 



The Nerves. Certain cells in the brain and in the spinal 

 cord function as master nerve cells. Connected with them are 

 fine thread-like organs, called nerves or neurons, which pass to 

 all parts of the body, and afford a method of communication with 

 the brain. They make a system that may be compared with 

 a complicated telegraph system. When a nerve in any part of 

 the body is irritated, the nerve, transmits the impulse to the brain, 

 where it causes a sensation. The carrying of impulses is the chief 

 duty of the nerves. Nerves that carry impulses to the brain 

 are called sensory nerves and those that transmit orders from 

 the brain to any part of the body are called motor nerves. 



THE CEREBRUM OF A MAN'S BRAIN 



Showing the hemispheres and their 

 convolutions. 



