168 LUNGS. 



THE LUNGS. 



A. The LUNGS are two large spongy substances 

 which fill the upper part of the chest, and are admir- 

 ably fitted for the purpose they answer, viz. to 

 ventilate the blood, or afford it the air by which it is 

 purified and rendered fit for the support of life. 



B. I thought you were going to say, for the pur- 

 pose of breathing. 



A. It is the same thing. The principal use of 

 breathing is the introduction of air for the benefit of 

 the blood ; it being found, that the blood derives 

 some of its most essential properties from the air 

 which we inhale into the lungs. All the blood of the 

 body is, for this purpose, thrown up into the lungs by 

 the heart in the course of every circulation : and here 

 we may notice the curious mechanism of the lungs. 

 They are full of air holes running in every direction ; 

 and the blood is exposed to the air in innumerable 

 vessels which are spread over the sides of these air 

 holes, for this purpose. Again, what we find in no 

 other part of the body, these passages for the air are 

 made of a substance like horn : this preserves them 

 open and free for the circulation of the air. How 

 comes it, we may ask, they are not made of the same 

 substance with the veins ? We can only answer by 

 referring to design. The blood can force its way 

 through a skin tube ; but air would be liable to bo, 

 obstructed. Lastly, there is a double passage for 



