30 ELEMENTARY PHYSIOLOGY. 



bring it pleasure or pain ; and they largely influence 

 its decisions, its orders and its temper. 



The brain is an exceedingly soft and 



Structure. . & J 



delicate organ. If it were not enclosed 

 in a triple sac and nicely fitted into its bony cham- 

 ber it would fall apart from its own weight. It is 

 composed of two kinds of substance, one of which is 

 gray in color and the other white. The outer por- 

 tion of the brain is composed of the gray matter. 

 The white matter occupies the inside portion. 



The brain is surrounded by three coats 

 Protection or me mbranes. The one lying next to 



to the Brain. J & 



it is a delicate covering containing 

 vessels which supply the brain with blood. This 

 membrane takes its name from its purpose of careful 

 protection ; so it is called the pia mater which 

 means a tender mother. It lies very close to the 

 surface, stretching over the little hills and dipping 

 down into the little valleys, with which the outside 

 of the brain is covered. Next to the pia mater lies 

 a membrane so delicate that it was named after a 

 spider's web arachnoid. This membrane performs 

 its work of protection by collecting from the blood a 

 watery fluid to moisten the surface of the brain and 

 prevent any possible friction. The outer coat is quite 

 tough and substantial: so it is called the dura mater, 

 or hard mother. It lies close to the inside surface 

 of the skull bones. Now wo can see how the brain 

 is protected, for instance, against a blow on the head. 

 The effectof such a blow would be diminished, first, by 



