VOL. XXIII.] PHILOSOPHICAL TRANSACTIONS. 699 



chanical structure of the heart,* and a most ingenious rationale of its action : 

 yet there remain several doubts and difficulties about it, not sufficiently ac- 

 counted for ; towards the resolving of some of which, I shall offer what my 

 own thoughts have suggested to me, and leave it to the consideration of the 

 reader. 



Dr. Lower has so well accounted for the systole, or contraction of the heart, 

 from its mechanical structure, that he seems almost to have exhausted the sub- 

 ject ; and had he been as happy in discovering the true cause of the diastole, he 

 had left little room for the industry and sagacity of others about this viscus. But 

 having judiciously and solidly explained the systole, he contents himself with 

 ascribing the diastole to a motion of restitution, which is not satisfactory ; be- 

 cause, the systole being the proper, and, as he himself confesses, the only mo- 

 tion of the heart, a state of contraction seems to be its natural state ; and con- 

 sequently, without some external violence, it would have no diastole at all. 



This will appear more plain if we consider the circumstances of it, and its 

 motion, as a muscle, with respect to other muscles. That contraction is the 

 proper action, and state of all muscles, is evident from experience of fact, as 

 well as from reason. For, if any muscle be freed from the power of its anta- 

 gonist, it is immediately contracted, and is not by any action of the will or 

 spirits to be reduced to a state of extention. Whence it is plain, that the 

 muscles have no restitutive motion, but what they derive from the action of 

 their antagonists, by which they are balanced. Thus likewise the sphincters of 

 the gula, anus, and vesica, having no proper antagonists, are always in a state of 

 contraction, and suffer nothing to pass but what is forced through them by the 

 contrary action of some stronger muscles, which, though not properly to be 

 called antagonists, yet on all necessary occasions perform the office of such. 



That the heart is a muscle, furnished and provided for motion like other 

 muscles, is demonstrated beyond contradiction, by Dr. Lower and others. And 

 as it is a solitary muscle, without any proper antagonist, and not directly under 

 the power of the will, nor exercising voluntary motion, it approaches nearest to 

 the sphincter kind, which only has these conditions in common with it. But 

 in constant and regular alternations of contraction and dilatation, it differs ex- 

 ceedingly from all the muscles of the body. 



This reciprocal aestus of the heart has given much trouble to the learned 

 who, finding nothing peculiar in the structure, which should necessarily occasion 

 it, nor any antagonist, whose re-action should produce it, have been extremely 



* Of Dr. Lower's Treatise on the Heart, an account has been given at p. 330, vol. i.of thi§ 

 Abridgment 



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