602 CONDUCTION OF IMPULSES. [BOOK in. 



to the brain, whereas efferent impulses of volition cross wholly in 

 the region of the medulla oblongata, and afterwards keep to the 

 same side of the cord along its whole length. Other observers, and 

 these perhaps are deserving of the greater confidence, find that a 

 section of a lateral half of the cord affects both volitional and 

 sensory impulses of both sides, though to different degrees, the 

 loss both of sensation and motion being greater on the side operated 

 on than on the other. Hence they maintain that the decussation 

 of both kinds of impulses is gradual, extending some distance 

 along the cord. Thus they hold that the volitional impulses 

 for the hind limb, while crossing over largely in the upper part 

 of the cord, continue to cross over right down to the lumbar 

 region, and similarly that the sensory impulses from the hind limb, 

 while crossing largely in the lumbar region of the cord, continue to 

 cross in the dorsal or even in the cervical region. 



Admitting this latter view as the one most in accordance with 

 facts, we have still to ask what are the exact paths taken by the 

 volitional and sensory impulses in their respective courses, that is 

 to say, What are the particular parts of the spinal cord which serve 

 to conduct volitional impulses on the one hand and sensory 

 impulses on the other ? Upon the discovery of the distinctive 

 functions of the anterior and posterior roots, it seemed natural to 

 conclude that the anterior columns with which the anterior roots 

 are more directly connected, should serve as the path for volitional 

 impulses, and similarly that the posterior columns should afford a 

 path for sensory impulses. But this view was soon found to be 

 untenable ; and it became modified into the conception that sensory 

 impulses pass along the posterior columns and the grey matter, 

 while volitional impulses descend in the antero-lateral columns. 

 Further, this somewhat general statement has been reduced to 

 greater definiteness by some authors in the following way. They 

 hold that the impulses which when they reach the brain give rise 

 to feelings of general sensibility only or of pain, and the impulses 

 which form part of the chain of a reflex act, as when the fore leg or 

 hind leg moves in response to a stimulus applied to the hind leg or 

 fore leg respectively, are transmitted by the grey matter, and by all 

 parts of the grey matter and that in any direction. Distinct tactile 

 sensations on the other hand, they contend, travel exclusively by 

 the posterior columns, and distinct volitional impulses by the antero- 

 lateral columns. That these several kinds of impulse should travel 

 by separate paths, does not in itself seem improbable, and is to a 

 certain extent suggested by pathological experience. For carefully 

 observed cases have been recorded of disease of the cord, and ap- 

 parently of the cord alone, in which the patient could appreciate 

 even a slight touch but felt no pain when a needle was thrust 

 into the skin, or when the skin was otherwise treated in a way 

 which, under normal circumstances, would give rise to pain. Con- 

 versely the sense of touch has been found to be absent, while pain 



