6 82 NERVOUS SYSTEM. 



not be so complete in some other classes of animals, which would account for the results 

 obtained by those who have denied decussation; but cases of disease of the cord in the 

 human subject all go to show that the crossed action is complete in man. 



Summary of the Action of the Spinal Cord as a Conductor. 



The antero-lateral columns of the cord, comprising that portion included between the 

 anterior median fissure and the origin of the posterior roots of the nerves, are insensible 

 to direct irritation, and serve as conductors of the motor stimulus from the brain to the 

 anterior roots of the nerves. If these columns be divided, voluntary motion is lost in all 

 parts below the section. If the rest of the cord be divided, leaving the antero-lateral col- 

 umns intact, the power of voluntary motion remains. Throughout the greater part of the 

 cord, this action is direct, and division of the antero-lateral columns upon one side pro- 

 duces paralysis of motion upon the corresponding side of the body. There is a decussa- 

 tion of the motor fibres at the medulla oblongata, and probably a partial decussation 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 con- 

 duction 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 im- 

 pressions 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 seg- 

 ments of the cord together by longitudinal commissural fibres; and this portion of the 

 cord has an important influence in coordinating the muscular movements. 



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. Division of one lateral half of 

 the cord is followed by complete paralysis of motion upon the corresponding side of the 

 body in all parts below the section, by anaesthesia in all parts below the section, upon the 

 opposite side of the body, and by hypersesthesia in the parts below the section, upon the 

 corresponding side of the body. 



The anatomical points bearing upon the physiological action of the cord are the fol- 

 lowing : 



^The fibres from the anterior roots penetrate the anterior gray cornua directly and are 

 in immediate connection with the prolongations of the motor cells. The motor cells also 

 have prolongations which pass to the brain in the white substance. The motor fibres 

 are thus directly connected with the cellular structures in the cord (the elements prob- 

 ably concerned in reflex movements) and the cells are in connection with conducting 

 fibres to the brain. 



The fibres from the posterior roots take several directions. Some of them pass to the 

 gray substance. A portion passes to the posterior columns, some extending upward and 

 others downward. The decussation, which is rendered certain by physiological experi- 

 ments, has not been satisfactorily followed by anatomists. It undoubtedly takes place 

 chiefly in the gray substance, probably in part by a crossing of the fibres themselves, 

 and in part by a crossing of prolongations from the cells with which certain fibres from 

 the posterior roots are connected. 



