THE CENTRAL NERVOUS SYSTEM 701 



pass down to the nucleus of Deiters and thence along the antero- 

 lateral descending tract to the anterior horn of the cord, and indirectly 

 to the periphery. 



All the paths enumerated, as well as others to which it 

 would be tedious to formally refer, and which the ingenuity 

 of the reader may be profitably employed in constructing 

 for himself, from the data already given, are to be looked 

 upon as possible channels for the passage of impulses between 

 the brain and the periphery. But it must be distinctly 

 pointed out that what is certain is in this case much more 

 limited than what is possible. Among the efferent paths it 

 is certain that the pyramidal tracts are the conductors of 

 voluntary motor impulses, and that in most individuals the 

 great majority of such impulses decussate in the medulla 

 oblongata, only a small minority in the cord. For a lesion 

 involving the pyramidal tract above the decussation of the 

 pyramids causes paralysis of the opposite side of the body, 

 a lesion below the decussation paralysis of the same side. 

 It is not certain, although it is possible that when one 

 pyramidal tract has been destroyed, in some animals at 

 least, the motor cortex from which it leads may to a certain 

 extent place itself again in communication with the paralyze*.' 

 muscles through its commissural connections with the 

 opposite hemisphere. 



Decussation of the Sensory Paths. On the other hand, it is 

 certain that pathological or traumatic lesions, involving the 

 destruction of one lateral half of the cord in man and ex- 

 perimental hemisections in some mammals, are followed by 

 symptoms which suggest that the sensory impulses decus- 

 sate chiefly in the spinal cord viz., diminution or loss of 

 sensibility on the opposite side below the level of the lesion, 

 with little or no impairment, and often increase of sensi- 

 bility (hypersesthesia) on the same side. 



This was first pointed out by Brown-Sequard, although long after 

 he saw cause to retract this interpretation of his experiments. It is 

 a curious circumstance that while clinical observation has on the 

 whole tended more and more to confirm the view that in man an 

 important portion of the sensory path decussates in the cord, experi- 

 mental physiologists have for the most part obtained contradictory 

 results. Thus Mott, working with monkeys, found that sensation, 

 far from being abolished, is, as a rule, impaired in a smaller degree 



