SEC. 3. AS A CONDUCTOR OF AFFERENT OR EFFERENT 



IMPULSES. 



When we feel something touching our foot, or when we move 

 our foot, afferent or efferent impulses must evidently pass along the 

 whole length of the spinal cord on their way to and from the brain. 

 We may say at once that it is impossible that sensory impulses 

 should be conveyed straight along a fibre from the periphery to 

 the sensorium, and volitional impulses straight along another 

 fibre from the 'organ of the will' to the muscular fibre; the 

 number of fibres in the cord is wholly insufficient for such a 

 purpose. Moreover not only anatomical but physiological con- 

 siderations shew that conduction along the cord is not simple, but 

 carried out by a more or less intricate system of relays. 



The phenomena of reflex action have shewn us that the cord 

 contains a number of more or less complicated mechanisms capable 

 of producing, as reflex results, coordinated movements altogether 

 similar to those which are called forth by the will. Now it must 

 be an economy to the body, that the will should make use of these 

 mechanisms already present, rather than that it should have 

 recourse to a special apparatus of its own of a similar kind. It is 

 therefore a priori probable that when the foot is pricked the 

 sensory impulses so generated, on reaching the cord pass into the 

 grey matter there to undergo a certain amount of transformation 

 and thence to be transmitted, either by direct and simple, or by 

 indirect and complicated, paths to the brain. Similarly, we may 

 suppose that when the leg is moved by an effort of the will, 

 volitional impulses starting from the brain, pass by a more or less 

 direct path to certain portions of grey matter in the lumbar cord, 



