SYSTEMATIC SYNTHESIS OF TISSUES INTO ORGANS. 335 



977. MILLIONS OF SUCH CELLS cluster about the ex- 

 tremities of the divisions of the bronchii, presenting an 

 almost incomputable extent of surface to favor the ac- 

 tion of the air and blood upon each other. 



978. Inf. With such an extent of surface for the action of the 

 air, it can hardly be in the Lungs so short a time as not to have its ac- 

 tion complete, and therefore it should be at once thrown out, and a fresh 

 portion taken in. 



979. THE PULMONARY OR RESPIRATORY CAPILLARIES 

 are an exquisitely beautiful network of the minutest 

 vessels, the meshes between which are less than the 

 diameter of the vessels, existing in the walls of the air- 

 cells, and in part forming them. The appropriate con- 

 tents of the cells and of the capillaries pass through the 

 separating membrane, as freely as if it were not there ; 

 indeed, it appears to assist the action. 



980. THE ARTERIES (pulmonary or respiratory) ex- 

 tend into the lungs by the side of the bronchii, and di- 

 vide with them, till they reach the air-cell or pulmonary 

 capillaries, in which they terminate. 



981. THE NUTRIENT (bronchial or systemic) ARTE- 

 RY OF THE LUNG is another small artery that branches 

 from the aorta, and extending into the lung by the side 

 of the bronchus, divides and subdivides in all parts of 

 the lung, supplying its nutritive wants. 



982. Inf. There is a double circulation in each lung, one from the 

 right heart to the left, very voluminous, and the other small, from the 

 left to the right heart. 



983. THE PULMONARY OR RESPIRATORY VEINS com- 

 mence from the pulmonary capillaries, and lead back, two 

 in number, by the side of each division of the bronchii, 

 and pour their contents into the left or systemic heart. 



984. Remark. It is noticeable that while the systemic arteries 

 carry bright red, the pulmonary carry dark red, blood ; and while the 

 systemic veins carry dark, the pulmonary carry bright red, blood. The 



97T. What said of ? 978. How long should air remain in a lung ? 9T9. What 

 are ? 980. What said of ? 981. What is ? 983. What are ? 



