CHAP. XVIIL] THE NERVOUS SYSTEM. 203 



other organs, forming the mediums of communication between 

 these distant parts and the great nerve-centre. 



Connected with the nerves in various situations are smaller 

 nerve-centres, called ganglia. A series of these ganglia, joined 

 together by nervous bands, form two large chains on either side 

 of the vertebral column, 

 extending from the base 

 of the cranium to the 

 coccyx. These ganglia, 

 with their nerves, are 

 generally called the sym- 

 pathetic system, and it is 

 usual to speak of the 

 nervous system as consist- 

 ing of the cerebro-spinal 

 system and of the sympa- 

 thetic system. 



For convenience of de- 

 scription, we will divide 

 the cerebro-spinal system 

 into 



(1) Brain and cranial 



nerves. 



(2) Spinal cord and spi- 



nal nerves. 



Brain and cranial nerves. 

 -The brain, the most 

 complex and largest mass 

 of nervous tissue in the 

 body, is contained in the 

 complete bony cavity FlG . 118 . _ DlAGRAM ILLUSTRATING ~THE GEN- 

 formed by the bones of ER AL ARRANGEMENT OF THE CEREBRO-SPINAL 



SYSTEM. 



the cranium. It is cov- 

 ered by three membranes (also named meninges), the dura 

 mater, pia mater, and arachnoid. 



The dura mater, a dense membrane of fibrous connective 

 tissue, lines the bones of the skull, forming their internal perios- 

 teum, and covers the brain. It sends numerous prolongations 

 inwards for the support and protection of the different parts of 



