454 RESPIRATION. 



Difference in Color "between Arterial cmd Venous Blood. 

 We have already considered this in treating of the proper- 

 ties of the blood, and will only take up in this connection the 

 cause of the remarkable change in the color of the blood 

 in the lungs. This change is instantaneous, and, long be- 

 fore the discovery of oxygen by Priestley, was recognized 

 by Lower, Goodwyn, and others, as due to the action of 

 the air. 



The celebrated experiment of Bichat showed the effect 

 on the color of the blood in the arteries, of preventing the 

 access of fresh air to the lungs. This observer adapted a 

 stop-cock to the trachea of a dog, by which he could regu- 

 late the entrance of air into the lungs, and exposing the caro- 

 tid artery, adapted a small one to this vessel. When he pre- 

 vented the air from getting to the lungs by closing the 

 stop-cock in the trachea, the blood became black in the 

 artery, but regained its florid hue when air was readmitted 

 to the lungs. 1 



The influence of air in changing the color of venous blood 

 may be noted in blood which has been drawn from the body ; 

 as is exemplified by the red color of that portion of a clot, 

 or the surface of defibrinated venous blood, which is exposed 

 to the air. If we cut into a clot of venous blood, the interior 

 is almost black, but becomes red on exposure to the air for a 

 very few seconds. 



We have been in the habit of illustrating the physiologi- 

 cal influence of the air on venous blood by the following 

 simple experiment : Removing the lungs of an animal (a 

 dog) just killed, the nozzle of a syringe is secured in the pul- 

 monary artery by a ligature, and a canula, connected with a 

 rubber tube which empties into a glass vessel, is secured in 

 the pulmonary vein. Adapting a bellows to the trachea, we 

 imitate the process of respiration ; and if defibrinated venous 

 blood be carefully injected through the lungs, it will be return- 



1 XAT. BICHAT, Rccherches Physiologiques sur la Vie d la Mori, 5me edition, 

 par F. MAGENDIE, Paris, 1829, p. 386. 



