578 THE SPINAL CORD. 



THE AUTOMATIC ACTIONS OF THE SPINAL CORD. 



508. We speak of an action of an organ or of a living body as being 

 spontaneous or automatic when it appears to be not immediately due to any 

 changes in the circumstances in which the organ or body is placed, but to 

 be the result of changes arising in the organ or body itself and determined 

 by causes other than the influences of the circumstances of the moment. 

 Some automatic actions are of a continued character ; others, like the beat 

 of the heart, are repeated in regular rhythm ; but the most striking auto- 

 matic actions of the living body, those which we attribute to the working of 

 the will and which we call voluntary or volitional, are characterized by their 

 apparent irregularity and variableness. Such variable automatic actions 

 form the most striking features of an intact nervous system, but are conspic- 

 uously absent from a spinal cord when the brain has been removed. 



A brainless frog placed in a condition of complete equilibrium in which 

 no stimulus is brought to bear on it, protected, for instance, from sudden 

 passing changes in temperature, from a too rapid evaporation by the skin 

 and the like, remains perfectly motionless until it dies. Such apparently 

 spontaneous movements as are occasionally witnessed are so few and seldom, 

 that we can hardly do otherwise than attribute them to some stimulus, inter- 

 nal or external, which has escaped observation. In the mammal (dog) after 

 division of the spinal cord in the dorsal region regular and apparently spon- 

 taneous movements may be observed in the parts governed by the lumbar 

 cord. When the animal has thoroughly recovered from the operation the 

 hind limbs rarely remain quiet for any long period ; they move restlessly in 

 various ways ; and when the animal is suspended by the upper part of the 

 body, the pendent hind limbs are continually being drawn up and let down 

 again with a monotonous rhythmic regularity, suggestive of automatic 

 rhythmic discharges from the central mechanisms of the cord. In the 

 newly born mammal too, after removal of the brain, movements apparently 

 spontaneous in nature are frequently observed. 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 character, 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. 



509. In the case of the beat of the heart, the automatic rhythmic dis- 

 charge 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 sub- 

 stance along certain nerves ; but the actual existence of the beat is wholly 

 independent of these extraneous influences ; the rhythmic discharge con- 

 tinues when they are entirely absent. The automatic rhythmic discharge of 

 respiratory 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 in- 



