CHAPTER XIII. 

 THE BRAIN. 



The brain, or encephalon, develops jointly with 

 the spinal cord, and represents the anterior extremity 

 of the cerebrospinal axis. The average weight of 

 the brain is about forty-eight ounces, while the cord 

 weighs less than one ounce. Like the cord it is an 

 organ in which all the elementary tissues may be 

 found but in which the neurons predominate. 

 During embryonic growth the brain and cord are a 

 hollow tube and this cavity is never obliterated, but 

 remains in the brain as its ventricles. Develop- 

 mental history further discloses the fact that the 

 brain, like the cord, is made up of definite segments 

 or joints called neuromeres. These primary units 

 are soon replaced by three larger vesicles called 

 primary fore-brain, mid-brain, and hind-brain. It 

 is generally affirmed that the first of these repre- 

 sents 3 neuromeres, the second 2 neuromeres, and 

 the third 6 neuromeres. Later the primary fore- 

 brain divides to form the cerebrum and the 'tween- 

 brain, while the hind-brain also divides to form the 

 cerebellum and medulla. In the adult brain, there- 

 fore, five divisions may be recognized, each of them 

 presenting a central canal or cavity as designated in 

 the table below: 



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