RESPIRATION. 



343 



Within the alveolar walls exists a dense capillary network. 

 They are placed more toward the inner side of the vesicle, being 

 covered only by the thin lining of the air-sacs. So densely are they 

 arranged that the spaces between the capillaries are even narrower 

 than the diameter of the capillaries, which here are about one three- 

 thousandth of an inch in diameter. In man between the folds of 

 two adjacent air-cells there is found but a single layer of capillaries, 

 while on the boundary line between two air-cells the course of the 

 capillaries becomes so twisted that they project into the cavities of 



Fig. 115. Section of the Parenchyma of the Human Lung, Injected 

 Through the Pulmonary Artery. (SCHULZE.) (From Mills's "Animal 

 Physiology," copyright, 1889, by D. Appleton and Company.) 



a, a, c, c, Walls of the air-cells. 6, Small arterial branch. 



the alveoli. By these arrangements, and particularly since the inter- 

 vening septa are so very thin and permeable, the exposure of the 

 blood to the air becomes complete, as two sides of a capillary are 

 thus exposed at the same time. 



Blood-supply. The lungs receive a copious supply of blood from 

 two sources : (1) the pulmonary and (2) the bronchial arteries. The 

 )ronchial arteries furnish nutriment for the lung-tissues. Six thou- 

 id liters of blood pass through the lungs in twenty-four hours, 



