THE NERVOUS SYSTEM 343 



substance is both white and grey, and is found in the white matter 

 of the brain and spinal cord. 



558. The Brain consists of soft white and grey nervous matter, 

 and fills the irregular cavity of the skull or cranium. It is divided 

 into four parts : 



1. The cerebrum, or large brain [Plate XXXVII., A), com- 



posing about three-fourths of the whole organ. 



2. The cerebellum, or small brain (Plate XXX VII., B), 



situated immediately behind the cerebrum. 



3. The pons varolii (Plate XXXVIII. , B 4), or bridge, con- 



necting the right and left portions of the cerebellum. 



4. The medulla oblongata {Plate XXXVII., C), or connect- 



ing-link between the spinal cord and the brain proper. 



559. The Brain is covered by three coats or membranes, called 

 the meninges. These are: (1) dura mater, a tough, fibrous 

 membrane, lying in immediate contact with the bony skull ; (2) the 

 arachnoid membrane (spider-web), or middle coat, a very fine, 

 delicate membrane ; and (3) the pia mater, which covers the surface 

 of the brain — a very fine, extensive membrane, made up of a net- 

 work of small bloodvessels. The last-named membrane follows and 

 covers all the convolutions of the brain, and supplies it with blood. 



560. The Cerebrum [Plate XXXV III., A 1 1 and B 1 1), or large 

 brain, is divided by a fissure into two hemispheres, each of which is 

 mapped out into numerous convolutions of grey matter, covering 

 white fibrous nerve material. The cerebrum is said to be the main 

 seat of sensation, reason, and will. 



561. The Cerebellum (Plate XXXVIII., A 2, B 2 2), or small 

 brain, has a middle lobe (vermis) and two lateral hemispheres, made 

 up of convolutions that are smaller than those of the cerebrum, and 

 differently arranged. The grey nerve substance is abundant, and 

 external to the white nerve tissue. The cerebellum is thought to 

 control and regulate the voluntary muscular actions of the body. 



