392 THE NEBVOUS SYSTEM 



part, distributed on the side next to the bones. In fact, 

 their principal function is to supply nourishment to the 

 bones, not to the brain. 



A second membrane, called the arachnoid, fits against the 

 smooth inner surface of the dura mater. This is a delicate, 

 transparent, serous membrane. Beneath it, enveloping 

 the brain itself, is a third, thin membrane, called the pia 

 mater. This pia mater is composed of a fine network of 

 small blood vessels held together by tissue, and these 

 blood vessels supply the nourishment and remove the 

 wastes of the brain itself. It is practicallj'- impossible to 

 remove this membrane, as it covers aU the folds of the 

 brain. 



These three membranes and the cranial bones furnish a 

 very efficient protection for the dehcate matter of the brain 

 proper. 



The brain proper. — Like the frog brain, it consists of 

 three portions, forebrain, midbrain, and hindbrain, but 

 these parts are so bent and doubled upon one another that 

 they form a figure similar to an interrogation point (?). 

 The two hemispheres of the forebrain occupy the bulk of 

 the cranial cavity and correspond in position to the top, 

 front, and part of the back of the curved portion of the 

 interrogation point. The olfactory lobes are tucked away 

 at the very front of the curve. In man, the forebrain con- 

 stitutes nine tenths of the bulk of the entire brain. 



The midbrain, with its optic lobes, is projected forward 

 in a position corresponding to the curve at the top of the 

 stem of the interrogation point. When viewed from the 

 under side, this projection bring-s the optic lobes just back 

 of the olfactory lobes. 



The hindbrain has a much more developed cerebellum 



