THE NERVOUS SYSTEM 293 



approximately parallel or concentric, as is shown in Figure 147. 



Cavities in the Interior of the Brain. The interest which 

 anatomists have felt in the brain has led them to make minute 

 studies of its interior, but we shall notice only a few points. 

 If the cerebral hemispheres be forced apart at the 

 top, they are found to be joined together toward their 

 centers by a large cross band of fibres, called the corpus 

 callosum. There are also empty spaces in the brain. We 

 know that this is true of the larger bones but we seldom 

 think of cavities in the brain. If we assume the parts of the 

 brain to be arranged in a straight line, one part behind the 

 other, and the whole to be cut in a vertical plane near the 

 middle, the cavities or ventricles will appear as in Figure 148, 

 as a continuous series from the front through the brain and 

 down the cord. It must not be supposed that these passages 

 are open cavities, for the sides of the ventricles are in close 

 contact with one another. They are, however, cavities, just as 

 there is a cavity in a rubber water bottle when it is empty 

 even though its sides are collapsed. Just what purpose these 

 cavities serve is not known. The fluid in them is like that 

 between the dura and pia mater coverings, of a thin watery 

 consistency, and may act in connection with the blood from 

 which it is derived, as a means of nourishing the brain. 



Gray and White Matter. If a part of the brain be cut open, 

 its tissues will appear to be of two sorts: on the outside or 

 cortex, is gray matter, and inside this, white matter; Fig. 148. 

 This difference in color would be in itself of no consequence if 

 the microscope did not show these layers to be made up of es- 

 sentially different materials. The greater part of the gra> 

 matter contains numerous nerve cells, while the white matter 

 underlying the gray, appears to be made up almost entirely of 

 nerve fibres, which are, essentially, outgrowths from tha 

 nerve cells. This distribution of the white and gray matter 

 in the different parts of the brain will be noted as each divi- 

 sion is considered. 



