THE LUNGS. 611 



which he remarks are about the size of a bristle, and dis- 

 tinctly seen, are supposed to have somewhat the same at- 

 tachment to the lobule and its cells as a blow-pipe fixed to 

 the side of a small piece of sponge. 



The diameter of these cells is estimated from the l-50th 

 to the l-200th of an inch, and they have no regular shape 

 or size. By a calculation 18,000 of them are made to 

 belong to each lobule, and about six hundred millions to 

 the whole lungs, which will afford some idea of the im- 

 mensity of surface provided for the reception of air, and 

 the ample facilities for purifying the blood by such an 

 arrangement. 



The pulmonary artery, seemingly the next element in im- 

 portance, comes from the right ventricle of the heart, be- 

 neath the arch of the aorta, and divides into two branches. 

 The one on the right is larger, goes to the root of the right 

 lung, and thence divides and subdivides throughout the 

 substance of this viscus into capillary branches, which are 

 found to terminate upon, and completely surrounding and 

 lining the interior of the air cells. The left pulmonary 

 artery has the same distribution, but is smaller than the 

 right. Both these arteries convey dark, venous blood to 

 the air cells of the lungs, where this impure blood is 

 brought in contact with the air, and the change from dark 

 venous into red arterial blood occurs, a change constituting 

 the great leading object in the function of respiration. 



The pulmonary veins commence at the air cells, and are 

 formed by fine radicles from the ultimate terminations of 

 the pulmonary artery. These all successively converge into 

 four trunks, two for the right and two for the left lung, 

 which take up the red blood formed in the cells, and convey 

 it into the left auricle of the heart. 



The bronchial arteries come from the thoracic aorta, fol- 

 low the course of the blood-vessels above mentioned, ramify 

 in every direction, and are designed for the nourishment of 

 the lungs. 



The bronchial veins return the venous blood into the vena 

 azygos. 



