THE NERVOUS SYSTEM. Ill 



8. The Coverings of the Brain. Three membranes 

 envelop the brain, an outer, middle and inner. The outer is 

 closely attached to the bone, and forms the periosteum. It is 

 a tough, strong membrane, composed of fibrous and connec- 

 tive tissue. Besides lining the skull, it forms a strong upright 

 partition between the two hemispheres of the cerebrum, and 

 also the floor upon which the back part of this portion of the 



FIG. 35. Vertical section of the Brain. 



brain rests. This floor for the cerebrum is the roof for the 

 cerebellum. 



The inner membrane is a thin, firm tissue, in close contact 

 with the brain, and dipping down with it into the furrows. 

 It is abundantly supplied with meshes of blood-vessels, and 

 many of these extend into the brain substance to nourish it. 



The middle is a very thin membrane, so thin that it has 

 been likened to a spider's web. It envelops the brain loosely, 

 and is more or less separated from the other membranes by 

 loose tissue and by fluifJ The quantity of fluid varies in 



