LETTERS TO BROTHER JOHN. 125 



cavity of the heart, like two Fleet ditches disem- 

 boguing- into the Thames. 



We have now completed what is called " the 

 greater circle of circulation"; that is, we have 

 traced the vermilion nutritious blood from the 

 heart to every point of the body. We have seen 

 it there part with its nutritious particles, in order 

 to repair the waste of the body : and thus, deterio- 

 rated in quality, altered in colour, and rendered 

 oppressive and unwholesome in its properties, we 

 have traced it back to the same organ from which 

 it set out ; viz. the heart. But, although we have 

 brought it back to the same organ from which it 

 started, we have not yet brought it back to the 

 same side of that organ. It set out from the left 

 side of the heart ; and we have only traced it back 

 to the right. Let us therefore proceed. 



When the black deteriorated blood has been 

 brought back from every part of our structure, 

 and collected into the two large veins, which I have 

 denominated " Fleet ditches," and has been poured 

 by them into the right cavity of the heart, the 

 walls of that cavity contract upon it, and propel it 

 into a large vessel, termed the pulmonary artery, 

 by which it is conveyed to the lungs. In the lungs, 

 the pulmonary artery is divided, and subdivided, 



