CHAPTER XIX. 



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 or vertebral column respectively. The general shape of these two 

 portions of the nerve system corresponds 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, occu- 

 pying 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 of the cranium; (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. (Fig. 227.) 



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

 spinal canal as far as the second or third lumbar vertebra. The central 

 nerve system is bilaterally symmetric, consisting of distinct halves united in 

 the median line. The cerebrum is subdivided by a deep fissure, running 

 antero-posteriorly, into two ovoid masses termed cerebral hemispheres; the 

 cerebellum is also partially subdivided into hemispheres; the isthmus like- 

 wise presents in the median line a partial division into halves; the medulla 

 oblongata and spinal cord are subdivided by an anterior or ventral and a 

 posterior or dorsal fissure into halves, a right and a left. 



The peripheral organs of the nerve system in anatomic and phy- 

 siologic relation with the central organs are the encephalic and the spinal 

 nerves. The encephalic nerves, twelve in number on each side of the 

 median line, are in relation with the base of the encephalon, and because of 

 the fact that they pass through foramina in the walls of the cranium they are 

 usually termed cranial nerves. 



The spinal nerves, thirty-one in number on each side, are in relation 

 with the spinal cord, and because of the fact that they pass through foramina 

 in the walls of the spinal column they are termed spinal nerves. As both 

 cranial and spinal nerves are ultimately distributed to the structures of the 

 body i.e., the general periphery they collectively constitute the periph- 

 eral 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, the dura 

 mater, the arachnoid, and the pia mater. 



The dura mater is a tough membrane composed of fibrous tissue. It 

 consists of two layers, the outer of which lines the cranial cavity and forms 



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