CHAPTER XVII. 



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 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, occupying 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. 

 201.) 



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 likewise 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. 



From the inferior surface of the encephalon and from either side 

 of the median line twelve nerve-trunks emerge, which as they pass 

 through foramina in the walls of the cranium are termed cranial 

 nerves. From each side of the spinal cord thirty-one nerve-trunks 

 emerge, which as they pass through foramina in the walls of the 

 spinal column are termed spinal nerves. As both cramaL^nd ^spinal 

 nerves are ultimately distributed to the structures of the body, i. e., 

 the general periphery, they collectively constitute the perjj&sz&l 

 organs of the nerve system . 



The organs of tHFcentral nerve system are supported and pro- 



440 



