110 OF VITAL MOTION. 



subjected to the same treatment. On examining the 

 heart in these cases, after it had ceased to act, it was 

 shrunken and collapsed, and the immediate operation 

 of the heat seemed to induce the diastole ; but owing 

 to the minuteness and indistinctness of the parts it is 

 not easy to speak positively upon this point. Be this 

 as it may, however, the influence of extra-organic 

 agents is clearly exhibited in these experiments. 



The same influence is also seen in the heart of a 

 cold-blooded animal of a frog, for example ; and here 

 we may obtain a distant glimpse of the causes which 

 induce the rhythm. It is well known that a heart of 

 this kind will pulsate for some time after removal 

 from the body, taking air into its cavities and expel- 

 ling it again, as it might be blood, the reactions 

 between the air and the heart being the chief source 

 of the force which is concerned in this action. Now, 

 at first sight, we have no more reason to suppose that 

 the air excites the systole than the diastole; but, if 

 we reflect awhile, we find it easier to entertain the 

 latter supposition than the former. There is no 

 elastic tissue to restore the diastole by its resilience 

 when the systole ceases, and on this account it is 

 difficult to suppose that the air can find entrance, 

 unless it possess some power by which it can resolve 

 the contracted state. Moreover it may be supposed, 

 that a certain generation of force must result from 

 the combination of the atmospheric oxygen with the 



