484 PHILOSOPHICAL TRANSACTIONS. [aNNO 1741. 



hiin, except some drawings. He does not undertake to give a description of all 

 the parts belonging to the heart, supposing tliein already sufficiently known 

 from the anatomical writers ; but only explains the surprising simplicity of its 

 muscular structure, as the ingenious Dr. Stuart has demonstrated it from vari- 

 ous preparations of boiled hearts ; viz. that the heart is nothing else but a single 

 muscle of nearly a semicircular form, whose fibres are all parallel. 



This structure the Doctor endeavours to imitate, by certain lines described 

 on a plane, which being cut out in a circular form, and then rolled up into the 

 shape of a truncated cone, gives a rude idea of the position of the fibres in the 

 heart, though perhaps not near so clear and intelligibly, as by proper prepara- 

 tion and exhibitions of the heart itself. 



The several courses of the fibres may be easily traced in a boiled heart ; and 

 if they are not found to answer to the directions of the lines on the paper-cone 

 with the strictest mathematical exactness, when rolled up, it must be ascribed 

 to the form of the heart, which is not exactly conic, though nearest reducible 

 to that figure ; and because the base is not a plane as in the paper-cone, but of 

 a convex round form; and the tendinous circle round it is of a smaller diameter 

 than the middle part of the heart. 



By this structure and circumvolution of the fibres, the muscle which con- 

 stitutes the heart, by a simple contraction of its length, by those external 

 fibres which encompass both ventricles, contract the diameter of the heart, 

 while by the internal fibres, that form the septum and inside of the left ventri- 

 cle, it shortens its length, or draws the apex up nearer to the base : this is done 

 without any contrariety in the action of these fibres, or destroying the force of 

 each other ; but, on the contrary, they being all parallel to each other, and a 

 continuation of the same fibres, assist each other in their action. 



The Doctor supposes this contraction is not caused so much by the influx of 

 the nervous spirits, as by the influx of the arterial blood, through the coronary 

 arteries into the substance of the heart ; and that the contraction of the auri- 

 cles comes from the same cause ; which will be alternate with that of the heart, 

 because the lateral branches, which arise out of the trunk of the coronary 

 artery, that encompasses ihe base of the heart and both auricles, are on one side 

 distributed into the substance of the heart, and on the other side into the coat 

 of the auricles; and will be alternately compressed, and alternately free, as the 

 auricles and ventricles are alternately full or empty of blood. 



