604 CONDUCTION OF IMPULSES. [BOOK in. 



or cord tracts of degeneration travel downwards and upwards re- 

 spectively in the lateral columns (though accompanied by degenera- 

 tion in other parts); moreover the method of development spoken 

 of at p. 600 teaches that a large part of each lateral column is asso- 

 ciated with the pyramids (and hence sometimes called pyramidal 

 tracts) and so with the crura cerebri and the brain. It may 

 be added that the decussation of the pyramids in the medulla 

 oblongata is chiefly a decussation of the lateral columns, though 

 obviously, from what has been said before, decussation of impulses 

 does not take place exclusively here. And such pathological evidence 

 as is forthcoming also to a certain extent supports this same view. 

 For while disease of the posterior cornua and posterior columns 

 seems to affect the sensory impulses passing along the nerves which 

 pass into the cord at the diseased part, and disease of the anterior 

 cornua and (though this is less clear) of the anterior columns is 

 similarly confined in its action to motor impulses passing out by 

 the nerves belonging to the diseased part, disease of the lateral 

 columns seems to affect chiefly the transmission of impulses along 

 the length of the cord and especially the transmission of volitional 

 impulses, for the evidence as to the interference in the conduction of 

 sensations by disease other than of the posterior columns and 

 cornua is by no means large or conclusive. 



Accepting this view provisionally we may form some such 

 conception as follows of the conduction of volitional and sensory 

 impulses. Volitional impulses cross, to a considerable extent in 

 the medulla oblongata, but continue to cross probably to a less 

 and less extent all the way down. In either case they appear to 

 travel along the cord in the lateral columns. Eventually they 

 become connected (possibly through part at least of the anterior 

 columns) with the grey matter of the anterior cornua, where 

 they join the local nervous mechanisms of which we have spoken 

 above. From the grey matter the impulses proceed by the anterior 

 roots to the appropriate muscles. Similarly sensory impulses make 

 their way, with the intervention possibly of the posterior columns, 

 first into the grey matter of the posterior cornua, and thence 

 into the lateral columns, and so up to the brain, decussation being 

 effected at first largely, and afterwards to a less extent, though 

 continued upwards for some distance. 



It must be remembered however that such a conception 

 can only be regarded as provisional. And indeed continued 

 experimental investigations teach us that this view also is in 

 turn beset with many difficulties. We have already called and 

 shall have occasion again to call attention to the importance 

 of distinguishing between the immediate and the more permanent 

 effects of any operation on the central nervous system. Now 

 cases have been recorded where section of the lateral columns on 

 both sides in the dorsal region, has had for its immediate effect 

 loss of sensibility and voluntary power in the hind legs, but where 



