Central Nervous System 97 



myelinatcd) it is functionally imperfect as a 

 conductor of nervous energy. (Fig. 12.) 



The central nervous system of vertebrates 

 consists of the spinal cord and brain ; the former 

 is enclosed in the vertebral column (backbone) 

 and the latter in the skull. The spinal cord is 

 formed of a cylinder of nervous substance 

 extending throughout the length of the spine; 

 it has a deep fissure in front and another behind, 

 the two halves of the cord being connected by 

 a strand of nervous substance which contains a 

 canal extending throughout the length of the 

 cord, and continued upwards into spaces in the 

 interior of the brain. Each half of the spinal 

 cord is divided longitudinally into three 

 columns, by the line of attachment of two 

 parallel series of filaments which constitute the 

 roots of the spinal nerves. If the anterior root 

 fibres are irritated the muscles to which these 

 fibres are distributed contract, but no pain is 

 felt; this is therefore known as a motor root. 

 If the posterior roots are irritated pain is felt, 

 referable to the area of the skin to which the 

 nerve fibres of these roots are distributed; the 

 posterior spinal roots are therefore known as 



G 



