THE CENTRAL NERVOUS SYSTEM 715 



tone is due, and therefore reflex, or different in nature and 

 automatically discharged. Now, degeneration of a muscle is 

 not usually caused, or at least not for a long time, by interrup- 

 tion of its afferent nerve-fibres, as in locomotor ataxia, or 

 after section of the posterior nerve-roots (Mott and Sherring- 

 ton). We can hardly suppose that in any case the trophic 

 influence of the cells of the spinal or sympathetic ganglia 

 to which all other reflex powers have been denied, is of 

 reflex nature. And there is, indeed, more evidence in favour 

 of trophic tone being an automatic action of the cord than 

 for any of the other tonic functions hitherto considered. 



Respiratory Automatism. But the evidence upon which the 

 spinal cord has been credited with true automatic action is 

 chiefly connected with the central respiratory mechanism. 

 It is known (p. 211) that a section above a certain level 

 in the medulla oblongata does not abolish the respiratory 

 movements. The respiratory centre, then, must be con- 

 tinually sending out impulses which are not originated by 

 impulses reaching it from the brain. But this is far from 

 being a proof of definite automatic action by the spinal 

 cord, for although afferent impulses do not, under the con- 

 ditions of that experiment, reach the respiratory centre from 

 the brain, they may and do reach it from the periphery; 

 and the only true test of automatic activity would be to 

 sever the whole of the afferent paths leading to the centre, 

 and then to observe whether or no the respiratory move- 

 ments continued. This is an experiment which it is difficult, 

 if not almost impossible, to carry out. But to say this is 

 merely to confess that, in the present state of experimental 

 physiology, it is difficult or impossible to apply a crucial 

 test to the doctrine of respiratory automatism. 



The ' Centres ' of the Cord and Bulb. We have frequently used 

 the word ' centre ' in describing the functions of the spinal cord, but 

 the term, although a convenient one, is apt to convey the idea that 

 our knowledge is far more minute and precise than it really is. When 

 we say that a centre for a given physiological action exists in a definite 

 portion of the spinal cord, all that is meant is that the action can be 

 called out experimentally, or can normally go on, so long as this 

 portion of the cord and the nerves coming to it and leaving it are 

 intact, and that destruction of the 'centre' abolishes the action. 

 For example, a part of the medulla oblongata on each side of the 



