CENTRAL ORGANS OF NERVE SYSTEM. 181 



The spinal cord is narrow and cylindric in shape. It occupies 

 the spinal canal as far down as the second or third lumbar vertebra. 



The nerves in relation with the central organs of the nerve system 

 are the encephalic or cranial and the spinal nerves. 



The encephalic nerves are twelve in number on each side of the 

 median line. Because of the fact that they pass through foramina in 

 the walls of the cranium they are usually termed cranial nerves. 



The spinal nerves are thirty-one in number on each side of the 

 cord. 



The cranial and spinal nerves are ultimately distributed to all the 

 structures of the body e. g., the general periphery, and for this 

 reason they are collectively known as the peripheral organs of the 

 nerve system. 



The central organs of the nerve system are supported and pro- 

 tected by three membranes named, in their order from without inward, 

 as the dura mater, the arachnoid and the pia mater. 



The dura mater, the outermost of the three, is a tough mem- 

 brane, composed of white fibrous tissue arranged in bundles, which 

 interlace in every direction. In the cranial cavity it lines the inner 

 surface of the bones, and is attached to the edge of the foramen 

 magnum ; it sends processes inward, forming the falx cerebri, falx 

 cerebelli, and tentorium cerebelli, supporting and protecting parts of 

 the brain. In the spinal canal it loosely invests the cord, and is 

 separated from the walls of the canal by areolar tissue. 



The arachnoid, the middle membrane, is a delicate serous structure 

 which envelops the brain and cord, forming the visceral layer, and is 

 then reflected to the inner surface of the dura mater, forming the 

 parietal layer. Between the two layers there is a small quantity of 

 fluid which prevents friction by lubricating the two surfaces. 



The pia mater, the most internal of the three, composed of 

 areolar tissue and blood-vessels, covers the entire surface of the 

 brain and cord, to which it is closely adherent, dipping down be- 

 tween the convolutions and fissures. It is exceedingly vascular, 

 sending small blood-vessels some distance into the brain and cord. 



The cerebro-spinal fluid occupies the subarachnoid space and the 

 general ventricular cavities of the brain, which communicate by an 

 opening (the foramen of Magendie) in the pia mater, at the lower 

 portion of the fourth ventricle. This fluid is clear, transparent, alka- 



