THE NERVOUS SYSTEM. 



d. The posterior columns of the cord appear to have a great 

 share in reflex movements, and this is the principal cause of 



The above diagram (after Brown-Sequard) represents the decussation of the con- 

 ductors for voluntary movements, and those for sensation : a, r, anterior roots and 

 their continuations in the spinal cord, and decussation at the lower part of the 

 medulla oblongata, TO, o; p, r, the posterior roots and their continuation and decus- 

 sation in the spinal cord ; g g, the ganglions of the roots. The arrows indicate the 

 direction of the nervous action ; r, the right side ; I, the left side. 1, 2, 3, indicate 

 places of alteration in a lateral half of the spino-cerebral axis, to show the influence 

 on the two kinds of conductors, resulting from section of the cord at any one of these 

 three places. 



the peculiar kind of paralysis so often observed in disease of 

 these columns. 



e. Impulses of the will, leading to voluntary contractions of 

 muscles, appear to be transmitted principally along the anterior 

 columns, and the contiguous gray matter of the cord. 



/. Decussation of motor impulses occurs, not in the spinal 

 cord, as is the case with sensitive impressions, but, as hitherto 

 admitted, at the anterior part of the medulla oblongata. This 

 decussation, however, does not take place, as generally sup- 



