]'2'2 PHYSIOLOGY OF THE CIKCULATION. 



oxygen of the absorbed air combines with certain 

 portions of the blood during the systemic circulation, 

 and that the residual nitrogen is, with the carbonic 

 acid, discharged through the walls of the exhalm" 



fJ O O 



portion of the pulmonary vessels. 



It is known that the opposite change from arterial 

 to venous blood depends upon the presence of car- 

 bonic acid in the latter fluid; and it is generally 

 believed that this change is effected in the systemic 

 capillaries and minute veins, by the absorption of 

 various products resulting from the decomposition of 

 the tissues. Among other matters carbonic acid is, 

 of course, copiously generated, and by its absorption 

 the change of colour is produced. In like manner, 

 when the main artery of a limb is tied, the blood 

 which enters the trunk below the ligature, and which 

 has necessarily passed through minute anastmosing 

 branches, is immediately found to possess the colour 

 of venous blood. Now this blood has evidently beeh 

 j daced under nearly the same physical conditions as 

 the blood contained in the veins ; for to arrive at the 

 lower portion of the main artery, it must have tra- 

 versed two systems of arborescent tubes ; in the first 

 of which the streams diverge, and in the second 

 converge. 



Those streams, therefore, which flow in the same 

 course as the currents contained within the veins, 

 exercise the same property of absorption; whence 

 the change of colour resulting from the entrance into 

 the blood of a quantity of carbonic acid. 



It thus appears that while the absorbing blood- 

 vessels of the lung take in atmospheric air. those 



