170 NATURAL THEOLOGY 



fibres are continuous with the fibrous substance of the heart itself. 

 Now when the ventricle is distended with blood, the valves are 

 drawn by their tendons in such a manner as almost to close the 

 orifice; and certainly so to dispose them, that the instant the 

 blood takes a direction backward into the vein by the contraction 

 of the ventricle, they fall together, and like a flood-gate stop the 

 current in that direction. Were there no cor dm tendinece, or coluntr 

 nee, carnece, these valves' would be floated back into the auricle, and 

 lose their office. But the most adinirable part of the contrivance 

 is, that the columncR carnece receiving the same impulse to contract 

 as the walls of the heart itself, act at the same instant with it; and 

 by contracting in proportion as the walls approach each other, 

 they hold the margins of the valves like the leeches of a sail when 

 bagged by the wind. The blood being prevented passing back- 

 ward is urged into the great artery still in the direction of the ar- 

 row. And now it will be observed that the artery must be dilated 

 when the heart contracts. And the artery itself being both elastic 

 and muscular, reacting upon this impulse, it will contract while 

 the ventricle is dilating. The blood would fall back from the 

 great artery into the ventricle, were it not again prevented by the 

 mechanical intervention of valves, a represents the semilunar 

 valve of the aOrta at the root of that great artery ; and it is a sur- 

 prising thing to see how offices so nearly alike are performed by a 

 mechanism entirely diflTerent. This valve consists of three little 

 bags, which are driven up by the force of the blood in the natural 

 course of the circulation ; but when, by the action of the aOrta, 

 the blood makes a motion backwards, it fills these three little 

 bags, and they fall together, and prevent the blood flowing back 

 into the heart.] 



But further : from the account which has been 

 given of the mechanism of the heart, it is evident 

 that it must require the interposition of valves; 

 that the success indeed of its action must depend 

 upon these ; for when any one of its cavities con- 

 tracts, the necessary tendency of the force will 

 be to drive the enclosed blood, not only into the 



