474 



separated head, which will cease at once on destruction of the medulla 

 oblongata and brain. 



The evidence of this is in the continued action of the heart after 

 it is cut out of the body. It may remain at rest for a few minutes 

 after the excision ; but then, as if recovering from shock or fatigue, 

 it again begins to act, and thus will continue for many hours acting 

 as regularly as it did when its connexions were unbroken. 



The time during which the action of the cut-out heart will be 

 maintained is different in the several classes of the Vertebrata ; but 

 some such continuance may be observed in all, and in all the expe- 

 riment is enough to prove that the rhythmic action does not depend 

 either upon distant nervous organs, or upon the blood which na- 

 turally flows through the cavities of the heart ; for as soon as the 

 heart is cut out, its cavities are emptied and no blood flows through 

 them. 



Thus, then, for a first conclusion, we may be sure that the cause 

 of the rhythmic action of the heart is something in the heart itself; 

 and this, notwithstanding the variations of the rhythm, which may 

 be produced by morbid or artificial states of organs far distant from 

 the heart. 



But the cause, whatever it be, is not equally in all parts of the 

 heart; for when its parts are in certain manners separated, some 

 continue to act rhythmically, and others cease to do so. If, for 

 example, the cut-out heart be divided into two pieces, one com- 

 prising the auricles and the base of the ventiicle, the other com- 

 prising the rest of the ventricle, the former will continue to act 

 rhythmically, the latter will cease to do so, and no rhythmic action 

 can be, by any means, excited in it. The piece of ventricle does 

 not lose its power of motion, for if it be in any way stimulated, it 

 contracts even vigorously ; but it never contracts without such an 

 external stimulus, and, when stimulated, it never contracts more 

 than once for each stimulus. Without losing motility, it has lost all 

 rhythmic power, and all appearance of spontaneous action. We can, 

 indeed, make it imitate a rhythm by stimulating it at regular inter- 

 vals, such as every two or three seconds ; but in any experiment of 

 this kind, it will presently appear how much sooner the motor force 

 is exhausted by the artificially excited actions, than by the apparently 

 equal actions of the natural rhythm. 



