CHAP, i.] THE SPINAL CORD. 921 



brain, movements apparently spontaneous in nature are frequently 

 observed. But all these movements, even when most highly deve- 

 loped, are very different from the movements, irregular and variable 

 in their occurrence though orderly and purposeful in their character, 

 which we recognize as distinctly voluntary. Even admitting that 

 some of the movements of the brainless mammal may resemble 

 voluntary movements in so far as they are due to changes taking 

 place in the spinal cord itself independent of the immediate 

 influence of any stimulus, we are not thereby justified in speaking 

 of the spinal cord as developing a will in the sense that we 

 attribute a will to the brain. 



596. In the case of the beat of the heart, the automatic 

 rhythmic discharge of energy appears to be exclusively the outcome 

 of the molecular nutritive changes taking place in the cardiac 

 substance. The beat may be modified, as we have seen, by nervous 

 impulses reaching the cardiac substance along certain nerves; 

 but the actual existence of the beat is wholly independent of these 

 extraneous influences ; the rhythmic discharge continues when they 

 .are entirely absent. The automatic rhythmic discharge of respi- 

 ratory impulses from the respiratory centre is also dependent on 

 the intrinsic molecular changes of the centre, these being, as we 

 have seen, largely determined by the character of the blood 

 streaming through it ; but in this case extrinsic nervous impulses, 

 reaching the centre along the vagus and other nerves, play a much 

 more important part than do similar impulses in the case of the 

 heart. They act so continually on the centre and enter so largely 

 into its working, that we are compelled to regard the activity of 

 the centre as fed, if we may use the word, not only by the 

 intrinsic molecular nutritive processes of the centre itself, but also 

 by the extrinsic nervous influences which flow into the centre from 

 without. The automatism of the spinal cord as a whole resembles, 

 in this aspect, that of the respiratory centre rather than that of 

 the heart. It has for its basis doubtless the intrinsic molecular 

 changes of the grey matter, on whose remarkable constitution we 

 dwelt in a previous section ; the metabolic events of this substance 

 are so ordered as to give rise to discharges of energy; but the 

 discharge appears to be also intimately dependent on the inflow 

 into the grey matter of afferent impulses and influences. The 

 normal discharge of efferent impulses from the cord undoubtedly 

 takes place under the influence of these incoming impulses ; and 

 it may be doubted whether the grey matter of the cord would be 

 able, in the absence of all afferent impulses, to generate any sus- 

 tained series of discharges out of its merely nutritive intrinsic 

 changes. The automatic activity of the cord is fed not only by 

 intrinsic nutritive events, but also by extrinsic influences. 



In this feature we may, moreover, find perhaps the reason why 

 the automatic activity of the spinal cord is so limited, as compared 

 with that of the brain. In spite of certain striking but superficial 



