296 NEKVOUS SYSTEM. 



tary motion remains. Throughout the greater part of the 

 cord, this action is direct, and division of the antero-lateral 

 columns on one side produces paralysis of motion on the cor- 

 responding side of the body. There is a decussation of the 

 motor fibres at the medulla oblongata, and a partial decussa- 

 tion in the cord itself in the upper cervical region. In the 

 dorsal region and below, the motor conducting fibres are 

 situated chiefly in the anterior columns ; but in the cervical 

 region, these fibres pass to the sides and are contained chiefly 

 in the lateral columns. The conduction of motor stimulus 

 is probably not effected exclusively by the white substance, 

 but is transmitted in part by the gray matter. 



The gray substance of the cord serves as the medium of 

 transmission of sensory impressions to the brain. This is 

 effected chiefly by the gray matter surrounding the central 

 canal, but it may take place to some extent in other portions. 

 If the entire gray matter be divided, with but slight injury 

 to the white substance, sensation is lost in all parts situated 

 below the section. The white substance does not conduct 

 sensory impressions to the brain, either in the antero-lateral 

 or the posterior columns. The most probable function of 

 the white substance of the posterior columns is to unite 

 the different segments of the cord together by longitu- 

 dinal commissural fibres ; and this portion of the cord has 

 an important influence in coordinating the muscular move- 

 ments. 



The sensitive nerve-fibres from the posterior roots of the 

 spinal nerves pass in the cord for a short distance upward 

 and downward. They then penetrate the gray matter, and 

 decussate throughout the entire length of the cord. Divis- 

 ion of one lateral hah of the cord is followed by complete 

 paralysis of motion on the corresponding side of the body in 

 all parts below the section ; anaesthesia in all parts below the 

 section, on the opposite side of the body ; and hypersesthesia 

 in the parts below the section, upon the corresponding side 

 of the body. 



