712 TONE OF SKELETAL MUSCLES. [Boon in. 



But all these movements, even when most highly developed, 

 are very different from the movements, irregular and variable 

 in their occurrence though orderly and purposeful in their char- 

 acter, which we recognize as distinctly voluntary. Even admit- 

 ting 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. 



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

 rhythmic discharge of energy appears to be exclusively the out- 

 come 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 cer- 

 tain nerves ; but the actual existence of the beat is wholly inde- 

 pendent of these extraneous influences ; the rhythmic discharge 

 continues when they are entirely absent. The automatic rhyth- 

 mic discharge of respiratory impulses from the respiratory cen- 

 tre 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 continu- 

 ally 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 nutri- 

 tive processes of the centre itself, but also by the extrinsic ner- 

 vous 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 ; the metabolic events of this sub- 

 stance 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 un- 

 doubtedly 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 sustained 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 extrin- 

 sic 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 



