108 



blood drawn from his jugular vein, was the darkest 

 he had ever seen *. Lastly, we have endeavoured to 

 prove, that no gases either exist in the blood, or can 

 be transmitted through the cellular and vascular 

 structure interposed between the air and that fluid in 

 the lung? ; consequently, no oxygen gas can enter 

 into the blood to unite with its supposed carbon, 

 nor,, if such an union did take place, could the car- 

 bonic acid be afterwards expelled from that fluid. 



1)56. If, then, the carbon, which unites with the 

 oxygen gas of the inspired air during respiration, 

 come not directly from the blood flowing through 

 the pulmonary vessels, it must in some other way be 

 supplied by the animal system* It is generally ad- 

 mitted, that the cellular surface of the lungs is fur- 

 nished with exhalent vessels, like every other surface 

 of the body : and indeed, the excessive quantity of 

 fluid which is sometimes poured out by these vessels 

 into the bronchial cells, is not unfrequently the cause 

 of severe disease. This exhalation of fluid from the 

 lungs, contrary to what is the case in other exhalent 

 surfaces, seems to be chiefly supplied by the watery 

 parts of the venal blood ; for the pulmonary artery 

 in the lungs, not only terminates into veins, but, like 

 other arteries, and like the branchial artery in fishes 

 (65.), has an exhalent termination also. Dr Thrus- 

 ton injected a coloured liquor into the pulmonary 

 artery of a sheep, and the fluid partly passed into the 

 veins, and in part also escaped by the trachea f ; 



* On Animal Heat, p. 310. 



f De usu Respirationis, p. 48. an* 1671. 



