76 THE NERVOUS SYSTEM. 



perhaps half an inch, and the white matter forming the 

 rest. The outer or gray part is called the cortex and 

 is largely made up of nerve cells. It might be called 

 the active part of the brain. The white part consists 

 largely of nerve fibers which are given off from the nerve 

 cells and are carried down into the spinal cord. 



The surface of the brain is convoluted, the ridges 

 being separated by deep furrows or sulci, by which means 

 a great extent of gray matter is secured. The furrows 

 contain fluid from the subarachnoid spaces and vary 

 in number and depth according to intelligence. While 

 the convolutions are not uniform in all brains, the prin- 

 cipal ones are constant. 



Both the brain and the spinal cord are covered by 

 three membranes, the dura mater, the arachnoid, and 

 the pia mater. The dura mater is dense and fibrous 

 and lines the interior of the skull, being firmly adherent 

 to it at many points. In fact, it constitutes the internal 

 periosteum of the cranial bones. The arachnoid is a 

 delicate serous membrane, with two layers, lubricated 

 to prevent friction, which divides the space between 

 the dura mater and the pia mater, bridging over the 

 convolutions and enclosing the subdural and subarach- 

 noid spaces which are connected with lymphatics and 

 contain a serous secretion, the cerebro-spinal fluid. 

 This fluid forms an elastic water cushion, on which the 

 brain rests, and prevents concussion. The pia mater 

 is vascular, containing blood-vessels, lymphatics, and 

 nerves, and is closely attached to the surface of the 

 brain, dipping down into all the sulci. 



At the base or under surface of the brain are some 

 very important structures. The olfactory bulbs lie be- 

 neath the frontal lobe and projecting back is the olfac- 

 tory tract, through which the olfactory nerves come from 

 the brain. Back of the olfactory tract is the optic com- 

 missure where the optic nerves coming from the brain 

 cross each other. And back of the commissure again 

 is the optic tract, where the optic nerves emerge from 



