696 A MANUAL OF PHYSIOLOGY 



Functions of Spinal Cord (including Medulla Oblongata). 



The functions of the spinal cord may be classified thus : 



1. The conduction of impulses set up elsewhere either in 



the brain or at the periphery. 



2. The modification of impulses set up elsewhere (reflex 



action). 



3. The origination of impulses (?). 



i. Conduction of Nervous Impulses by the Cord. The old 

 controversy as to whether the white fibres of the spinal 

 cord are directly excitable may be considered as definitely 

 settled in the affirmative. The inquiry was complicated by 

 the presence of the spinal roots, which since the experi- 

 ments of Charles Bell have been known to be capable of 

 excitation by artificial stimuli. But at length the difficulty 

 was overcome in this way. The posterior (dorsal) portion 

 of several segments of the cord with the attached posterior 

 roots and the grey matter was excised. Long strands of 

 the white matter of the anterior (ventral) portion of the 

 cord were isolated, and laid on electrodes, and contractions 

 of muscles were seen to follow stimulation, even when the 

 anterior roots nearest the stimulating electrodes had been 

 cut, and every precaution taken to avoid escape of current 

 on to the distant anterior roots of the nerves supplying 

 the muscles. Indeed, apart from direct experimental 

 evidence, the fact that the white fibres of the brain are 

 universally admitted to be excitable by artificial means 

 would be of itself almost sufficient to decide the question, 

 for we know of no essential difference between the cerebral 

 and the spinal fibres. But the conditions must rarely occur 

 under which direct stimulation of white fibres in their course 

 is possible in the intact body ; and the only impulses with 

 which we need concern ourselves here are those that reach 

 the conducting paths from grey matter in the cord itself or 

 in the brain, or from the peripheral organs. 



What sort of impulses, then, do the various tracts of the 

 spinal cord conduct ? For the posterior roots this question 

 was first fully answered by Magendie ; for the anterior roots 





