290 ADVANCED PHYSIOLOGY 



rest of the body, they are connected with the individual 

 organs by a network of nerves and nerve endings, which make 

 up the peripheral nervous system. In addition to the central 

 and peripheral systems, there is a third, partially independent 

 collection of nerve fibres and cells, which is known as the 

 sympathetic nervous system. 



THE CENTRAL NERVOUS SYSTEM 



The Brain. In all the higher animals, the brain is present 

 and is the seat of the mental life. One can scarcely say that it 

 is more important than the heart or the kidneys, for without 

 any one of them life could not continue; but it is the sole 

 directive agent of the higher functions. 



The brain is inside the skull, the bones surrounding it and 

 making up the brain box being called the cranium. Where 

 the backbone joins the skull the spinal cord passes into the 

 cranium through a large opening, the foramen magnum. 



Membranes about the Brain. Applied closely to the inner 

 surface of the cranium is a tough lining called the dura mater. 

 This serves at the same time both as a brain covering and a 

 sheath, which functions as the periosteum does in other bones. 

 Closely applied to the brain itself is a thin, rather delicate 

 covering, the pia mater. This follows the brain surface com- 

 pletely, dipping into every groove and covering every wrinkle. 

 The pia mater is very full of blood vessels, and thus forms 

 both a protecting and a nourishing agent. 



Between the dura and the pia mater is a thin tubular mem- 

 brane, the arachnoid, the space within it being filled with the 

 arachnoid fluid. This serves the very apparent purpose of a 

 cushion. The brain is exposed to countless jars as one walks 

 or merely moves the head, and it is easy to see how much dif- 

 ference this fluid cushion must make in saving this delicate 

 nerve center from wear and tear. 



Main Divisions of the Brain . All animals with backbones 

 show the same brain parts as man. Assuming the brain to be 



