96 OP THE VESSELS 



blood in the heart itself is after this manner. The receiv- 

 ing cavities respectively communicate with the forcing cav- 

 ities, and, by their contraction, unload the received blood 

 into them. The forcing cavities, when it is their turn to 

 contract, compel the same blood into the mouths of the ar- 

 teries. 



The account here given will not convey to a reader ig- 

 rant of anatomy, any thing like an accurate notion of the 

 form, action, or use of the parts, (nor can any short and 

 popular account do this,) but it is abundantly suffi- 

 cient to testify contrivance ; and, although imperfect, 

 being true as far as it goes, may be relied upon for the 

 only purpose for which we offer it, the purpose of this con- 

 clusion. 



" The wisdom of the Creator," saith Hamburgher, '' is 

 in nothing seen more gloriously than in the heart." And 

 how well doth it execute its office ! An anatomist, who 

 understood the structure of the heart, might say before- 

 hand that it would play : but he would expect, I think, 

 from the complexity of its mechanism, and the delicacy 

 of many of its parts, that it should always be liable to de- 

 rangement, or that it would soon work itself out. Yet 

 shall this wonderful machine go, night and day, for eighty 

 years together, at the rate of a hundred thousand strokes 

 every twenty-four hours, having, at every stroke, a great 

 resistance to overcome ; and shall continue this action for 

 this length of time, without disorder, and without weari- 

 ness. 



But further ; from the account, which has been given of 

 the mechanism of the heart, it is evident that it must re- 

 quire the interposition of valves ; that the success indeed 

 of its action must depend upon these, for when any one of 

 its cavities contracts, the necessary tendency of the force 

 will be to drive the enclosed blood, not only into the mouth 

 of the artery where it ought to go, but also back again in- 

 to the mouth of the vein from which it flowed. In like 

 manner, when by the relaxation of the fibres the same cav- 

 ity is dilated, the blood would not only run into it from the 

 vein, w^hich was the course intended, but back from the ar- 

 tery, through which it ought to be moving forward. The 

 way of preventing a reflux of the fluid, in both these cases, 

 is to fix valves ; which, like flood-gates, may open a way to 

 the stream in one direction, and shut up the passage against 

 it in another. (PI. XVII. fig. 2, 3, 4.) The heart, constitut- 

 ed as it is, can no more work v/ithout valves than a pump 



