404 THE SPINAL CORD. 



acute pain become manifest whenever the irritation is applied to 

 the posterior columns ; but no muscular contractions follow, other 

 than those of a voluntary character. Longet has found 1 that if the 

 spinal cord be exposed in the lumbar region and completely divided 

 at that part by transverse section, the application of any irritant to 

 the anterior surface of the separated portion produces at once con- 

 vulsions below ; while if applied to the posterior columns behind 

 the point of division, it has no sensible effect whatever. The an- 

 terior and posterior columns of the cord are accordingly, so far, 

 analogous in their properties to the anterior and posterior roots of 

 the spinal nerves, and are plainly composed, to a greater or less ex- 

 tent, of a continuation of their filaments. 



These filaments, derived from the anterior and posterior roots of 

 the spinal nerves, pass upward through the spinal cord toward the 

 brain. An irritation applied to any part of the integument is then 

 conveyed, along the sensitive filaments of the nerve and its pos- 

 terior root, to the spinal cord ; then upward, along the longitudinal 

 fibres of the cord to the brain, where it produces a sensation corres- 

 ponding in character with the original irritation. A motor im- 

 pulse, on the other hand, originating in the brain, is transmitted 

 downward, along the longitudinal fibres of the cord, passes outward 

 by the anterior root of the spinal nerve, and, following the motor 

 filaments of the nerve through its trunk and branches, produces at 

 last a muscular contraction at the point of its final distribution. 



CROSSED ACTION OF THE SPINAL CORD. As the anterior columns 

 of the cord pass upward to join the medulla oblongata, a decussa- 

 tion takes place between them, as we have already mentioned in 

 Chapter I. The fibres of the right anterior column pass over to 

 the left side of the medulla oblongata, and so upward to the left side 

 of the brain ; while the fibres of the left anterior column pass over 

 to the right side of the medulla oblongata, and so upward to the 

 right side of the brain. This decussation may be readily shown 

 (as in Fig. 130) by gently separating the anterior columns from each 

 other, at the lower extremity of the medulla oblongata, where the 

 decussating bundles may be seen crossing obliquely from side to 

 side, at the bottom of the anterior median fissure. Below this 

 point, the anterior columns remain distinct from each other on each 

 side, and do not communicate by any further decussation. 



1 Traite de Physiologie, vol. ii. part 2, p 8. 



