NATURAL THEOLOGY. 171 



mouth of the artery where it ought to go, but also 

 back again into the mouth of the vein from which 

 it flowed. In like manner, when by the relaxa- 

 tion of the fibres the same cavity is dilated, the 

 blood would not only run into it from the vein, 

 which was the course intended, but back from the 

 artery, through which it ought to be moving for- 

 ward. 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. The heart, constituted as it is, can no 

 more work without valves than a pump can. 

 When the piston descends in a pump, if it were 

 not for the stoppage by the valve beneath, the 

 motion would only thrust down the water which 

 it had before drawn up. A similar consequence 

 would frustrate the action of the heart. Valves, 

 therefore, properly disposed, i, e., properly with 

 respect to the course of the blood which it is ne- 

 cessary to promote, are essential to the contri* 

 vance. And valves so disposed are accordingly 

 provided. A valve is placed in the communica- 

 tion between each auricle and its ventricle, lest, 

 when the ventricle contracts, part of the blood 

 should get back again into the auricle, instead of 

 the whole entering, as it ought to do,- the mouth 

 of the artery- A valve is also fixed at the mouth 

 of each of the great arteries which take the blood 

 from the heart ; leaving the passage free, so long 

 as the blood holds its proper course forward; 



