618 MANUAL OF PHYSIOLOGY. 



FIG. 241. 



Diagram illustrating the course taken by the fibres in the spinal cord. 

 (After Fick.) A, B, and c represent oblique views of three transverse sec- 

 tions of the cord, the tissue between which is supposed to be transparent. 

 The outline of the gray substance is marked with a line which incloses the 

 ganglion cells. At the lowest section (c) sensory nerve fibres (a) enter by 

 the posterior root, and, after connection with ganglion cells of the gray mat- 

 ter, communicate with the posterior white column, through which some 

 passes directly to the brain, as shown by the direction of the arrow-head 

 pointing to (6). This is the route which offers least resistance to an impulse 

 travelling to the brain through the cord. Hence it is that traversed by weak 

 peripheral (tactile) stimuli. By the same posterior root arrive impulses at 

 the cord which may traverse the finer, more irregular and resistant fibrils 

 of the gray matter shown by the fine lines. Through these channels pain- 

 ful sensations are carried. From many parts of the gray matter of the cord 

 ganglion cells may dispatch impulses by the motor root (d). Hence many 

 reflex actions are arranged. When an impulse comes directly from the 

 brain (voluntary centres) it adopts the direct route (c), which passes through 

 the white substance of the anterior columns before it excites the motor gan- 

 glion cells of the cord to coordinated activity. 



