VOL. XXIII.] PHILOSOPHICAL TRANSACTIONS. 705 



the body conspire towards the constriction of the heart, which is the state of 

 quiescence, to which it naturally tends. Yet we find it alternately in a state of 

 violence, that is, of dilatation ; and this upon necessity, because upon this alter- 

 nation depends all animal life. 



Some sufficient external cause must therefore be found out, to produce this 

 great phenomenon ; which cause must be either in the air or atmosphere, because 

 we have no constant and immediate commerce with any other mediums. Some 

 great physicians observing this, and that deprived, by whatever means, of com- 

 munication with the external air, we become instantly extinct, they have ima- 

 gined, that in the act of inspiration certain purer parts of the air mixed with 

 the blood in the lungs, and were conveyed with it to the heart, where they 

 nourished a sort of vital flame, which was the cause of this reciprocal aestus of 

 the heart. Others, not quite so gross, rejecting an actual flame, have fancied 

 that these fine parts of air, mixing with the blood in the ventricles of the heart, 

 produced an effervescence which dilated it. But these fancies have been long 

 since exploded and condemned on ample conviction, and it is a point yet unde- 

 termined, whether any air does mix * with the blood at all in the lungs. 



But supposing that some air may insinuate itself into the pulmonary vein, it 

 can no other way dilate the heart, than by an effervescence in the left ventricle ; 

 which would not dilate the right. But this opinion is contradicted by autopsy, 

 and too laboriously confuted by others, to be brought upon the stage again here. 

 There remains therefore only the gross body of the atmosphere to be consi- 

 dered, which is undoubtedly the true antagonist to all those muscles, which serve 

 for ordinary inspiration and the constriction of the heart. This will appear 

 more evidently, if we consider not only the power, but the necessity of its 

 action upon animal bodies, as well as the want of other sufficient agents. 



The heart is a solitary muscle, of very great strength, and the intercostal 

 muscles and diaphragm, which likewise have no antagonists, are a vast addi- 

 tional force, which must be balanced by the contrary action of some equivalent 

 power or other. For, though the action of the intercostal muscles be volun- 

 tary, that does not exempt thegi from the condition of all other muscles serving 

 for voluntary motion, which would be in a state of perpetual contraction, not- 

 withstanding any influence of the will, were it not for the libration of antago- 

 nist muscles. This libration between other muscles is answered by the weight 

 of the incumbent atmosphere, which presses upon the thorax and other parts 

 of the body. And, as in all other voluntary motions, the influence of the will 



• The experiments of modern physiologists have placed it out of all doubt that a quantity of air is 

 absorbed by the blood iu respiration. 



VOL. IV. 4 X 



