618 MANUAL OF PHYSIOLOGY. 



immediate result of an impulse coming from the skin, and this 

 coordination is the result of mutual relationships existing between 



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 sections 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 matter, 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 traveling 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 painful 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 cen- 

 tres) it adopts the direct route (e>, which passes through the white substance of the 

 anterior columns before it excites the motor ganglion cells of the cord to coordinated 

 activity. 



