THE BRAIN 



233 



and form the intricate convolutions found upon the 

 brain's exterior. The intelligence of animals has been 

 found to depend more or less upon the number of 

 the convolutions, that is, upon the num- 

 ber of nerve cells contained in the gray 

 matter of the cortex. 



Gray matter. The gray matter is 

 made up mainly of two kinds of cells, 

 the true nerve cells, and the smaller con- 

 nective tissue cells, the neuroglia, which 

 by their close meshwork support the 

 nerve cells. The nerve cells are con- 

 nected with one another and with other 

 cells in various parts of the brain and 

 the spinal cord, either directly by 

 means of their tree-like branches and 

 axis cylinders, or indirectly by means of 

 intervening neurons. 



White matter. The white matter of 

 the brain consists, as we have seen, 

 mainly of axis cylinders and their 

 sheaths, together with a certain amount 

 of connective tissue to support them. 

 The axis cylinders tend to follow 

 parallel courses and thus form definite 

 bundles of nerve fibres or tracts of com- 

 munication. 



Protection of brain. The brain is 

 protected from external injury by the 

 bony case, the skull, in which it lies. It 

 is protected from contact with the skiill 

 by a small amount of fluid and by 

 several membranes that both cushion it and help to 

 supply it with blood-vessels for its nutrition. 



FIG. 128. Diagram 

 of cortex (gray 

 matter) of brain, 

 showing nerve 

 cells and their 

 connections. 



