THE CENTRAL NERVOUS SYSTEM. 2 17 



with one another by means of the nerve-fibres of the 

 white substance. 



The white substance of both cerebrum and cere- 

 bellum consists of coarser and finer, but all very 

 small, nerve-fibres, running in various directions, and 

 supported by a delicate connective-tissue frame- 

 work, similar to the neuroglia of the cord. The - 

 gray matter consists here, as in the cord, of gang- 

 lion-cells, and fine gray fibres supported by connec- 

 tive tissue. In parts of the gray as of the white 

 substance, certain cellular and fibrous elements oc- 

 cur, of which it is at present impossible to say 

 whether they are connective or nerve-tissue. In- 

 deed, it is the difficulty of determining the nature of 

 certain structures in the brain, together with their 

 extreme delicacy, and the difficulty of isolating 

 them, which renders the histology of the brain so 

 difficult a theme, and explains the unsatisfactory 

 state of our knowledge concerning it. 



We cannot do more in these lessons than to study 

 briefly the structure of two of the best known and, 

 at present, perhaps most interesting parts of the 

 brain namely, the cortical portions of the cere- 

 brum and cerebellum. 



a. Cortex of the Cerebrum. The structure of this 

 part of the brain differs somewhat in different re- 

 gions, but that of one of the frontal lobes is suffi- 

 ciently typical for our purpose. Here, in a section 

 perpendicular to the surface, and extending through 



