SEC. 4. THE AUTOMATIC ACTIONS OF THE SPINAL 



COED. 



§ 468. 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 automatic 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 conspicuously absent from a spinal cord when the brain has 

 been removed. 



A brainless frog placed in a condition of complete equilib- 

 rium 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 per- 

 fectly 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, internal or external, which has escaped observation. 

 In the mammal (dog) after division of the spinal cord in the 

 dorsal region regular and apparently spontaneous movements 

 may be observed in the parts governed by the lumbar cord. 

 \Yhen 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 sus- 

 pended 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 rhyth- 

 mic discharges from the central ' mechanisms of the cord. In 

 the newly born mammal too, after removal of the brain, move- 

 ments apparently spontaneous in nature are frequently observed. 



711 



