THE CENTRAL ORGANS OF THE NERVE SYSTEM 

 AND THEIR NERVES. 



The central organs of the nerve system are the encephalon and the spinal 

 cord, lodged within the cavity of the cranium and the cavity of the spinal 

 column respectively. The general shape of these two portions of the nerve 

 system correspond with that of the cavities in which they are contained. 

 The encephalon is broad and ovoid, the spinal cord is narrow and elongated. 



The encephalon is subdivided by deep fissures into four distinct, though 

 closely related portions: viz., (i) the cerebrum, the large ovoid mass occupy- 

 ing the entire upper part of the cranial cavity; (2) the cerebellum, the wedge- 

 shaped portion placed beneath the posterior part of the cerebrum and lodged 

 within the cerebellar fossae; (3) the isthmus of the encephalon, the more or 

 less pyramidal-shaped portion connecting the cerebrum and cerebellum with 

 each other and both with (4) the medulla oblongata. 



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 struc- 

 tures 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 protected 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 membrane, com- 

 posed 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. 



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