126 LETTERS TO BROTHER JOHN. 



into an infinite number of infinitely minute 

 branches, which traverse every part of the lungs. 

 The black blood, therefore, carried to the lungs 

 by the pulmonary artery, is divided into number- 

 less minute streamlets, which are conducted, in 

 every direction, through the lungs, by the innu- 

 merable hairlike branches of the pulmonary artery. 



The lungs are made up of a countless number of 

 small cells, through and among which the little 

 streamlets of black blood are of course conveyed : 

 and every time we draw in our breath, these cells 

 become filled with air ; and the air which they then 

 contain comes in contact with the little vessels con- 

 taining black blood ; and acting through the deli- 

 cate coats of these, it operates those changes in 

 the blood which it was sent to the lungs for the 

 purpose of undergoing. 



What the whole of these changes are, is not 

 thoroughly understood: but this much is certain 

 that, whereas the blood enters the lungs of a 

 black colour, and in a condition unfit to effect the 

 nutrition of the body, it no sooner becomes exposed 

 to the influence of the air in the cells of the lungs, 

 than it loses its black colour, acquires the brilliant 

 hue of vermilion, and becomes at once endowed 

 with all the properties necessary to the nutrition 



