106 THE BLOOD. 



Color of the Blood. The color of the blood is due to the 

 corpuscles. In the arterial system it is uniformly red. In 

 the veins it is dark blue and sometimes almost black. This 

 difference in color between the blood in the arterial and in 

 the venous system, was a matter of controversy at the time 

 of Harvey. By the discoverer of the circulation, the differ- 

 ence, which is now universally known and admitted, as re- 

 gards most of the veins, was supposed to be merely accidental, 

 and dependent on external causes. Fifty years later, Lower 1 

 demonstrated the change of color in the blood as it passes 

 through the lungs, and associated it with the true cause, vis. 9 

 the absorption of oxygen. The color in the veins, however, 

 is not constant. Many years ago, John Hunter observed, in 

 a case of syncope, that the blood drawn by venesection was 

 bright red ; a and more recently Bernard has demonstrated 

 that in some veins the blood is nearly, if not quite, as red as 

 in the arterial system. The color of the venous blood de- 

 pends upon the condition of the organ or part from which it 

 is returned. The red color was first noticed by Bernard in 

 the renal veins, where it contrasts very strongly with the 

 black blood in the vena cava. He afterwards observed that 

 the redness only existed during the functional activity of the 

 kidneys ; and when, from any cause, the secretion of urine 

 was arrested, the blood became dark. He was led from this 

 observation to examine the venous blood from other glands ; 

 and directing his attention to those which he was able to 

 examine during their functional activity, particularly the 

 salivary glands, found the blood red in the veins during 

 secretion, but becoming dark as soon as secretion was arrested. 

 These observations may be easily verified by opening the 

 abdomen of a living animal so as to expose the emulgent 

 veins, introducing a canula into the ureter so as to be able 

 to note the flow or arrest of the urine. As long as the urine 



1 LOWER, Tractatus de Corde item de Motu & Colore Sanc/uinis, Amstelodami, 

 1669, p. 180. 



2 The Works of John Hunter, Philadelphia, 1840, vol. iii., p. 93. 



