NATURAL THEOLOGY. 167 



little skins or thin membranes lying close down to the 

 side of the vein, at suitable distances, and which allow 

 a free passage to the stream when it pursues its ascend- 

 ing course, but thrust out and bar up the passage 

 by the fluid getting behind them, whenever it would 

 attempt to flow the other way. 



B. Are there not the same valves in the vessels 

 which convey the blood from the heart ? 



A. There are not : except one at the mouth of 

 each artery which takes the blood from the heart. 

 There are none stationed along in these vessels as in 

 the veins ; and they would have been of no use, as 

 there is no danger of a reflux of the blood in its pas- 

 sage from the heart. 



J5. What endless examples of an all contriving 

 wisdom ! If one should attempt to account for the 

 formation of these loose skins or valves, by supposing 

 the force of the blood might have raised them up from 

 the sides of the vessels, they must, according to this 

 explanation, have been numerous in the arteries, 

 where the strength of the current is the most power- 

 ful ; but we find there are none. The operation of 

 natural causes seems to be exactly reversed in this, 

 as in many other parts of our wonderful structure. 



T. There is one part of the apparatus concerned 

 in the circulation of the blood, which you have omit- 

 ted, viz. the lungs. You may briefly describe these ; 

 and we will take our leave of the interior of the 

 animal frame. 



