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APPLIED PHYSIOLOGY 



involve the medulla and cause instant death. But the medulla is 

 so situated that only the greatest violence can harm it. 



526. The cerebellum. Just above and overhanging the 

 medulla is a rounded mass called the cerebellum. It forms 

 less than one fifth of the brain. It consists of an interior 



white mass of nerve 

 threads, covered with 

 a layer of gray matter 

 about -j 1 ^- of an inch in 

 thickness. On the sur- 

 face are deep fissures 

 into which the gray 

 matter dips, so that its 

 amount is greatly in- 

 creased. In the gray 

 matter are nerve cells 

 which are connected 

 with the rest of the nervous system through the nerves 

 of its white matter. These nerve cells are the essential 

 part of the cerebellum. They have no connection with 

 any vital process of life, and do not take part in thought. 

 A man with a diseased cerebellum can perform a single 

 muscular act like raising his hand, but he cannot direct 

 changing and complicated movements, such as are required 

 in writing, walking, or balancing his body. Thus the cere- 

 ( bellum acts like a balance wheel, so that orders for com- 

 plicated movements may be sent with regularity and pre- 

 cision. 



527. The Optic tubercles. The optic tubercles are two small 

 collections of gray matter situated upon the main nerve tracts which 

 connect the cerebrum and medulla. They seem to be connected with 

 the reflex movements of the eye. Other collections of gray matter near 

 them seem also to be connected with the eye. 



Diagram of a human brain. 

 a cerebrum. b cerebellum. c medulla. 



