10 PROPERTIES OF 



contact with the air-bubbles, whilst in other parts it remained 

 of its natural colour. 



There is a difference between the arterial and venous blood 

 in colour ; the former, is of a florid red like the surface of the 

 crassamentum, the latter is dark or blackish like the bottom 

 of the crassamentum. This change in its colour is produced 

 on the blood as it passes through the lungs, as we see by 

 opening of living animals 1 ; and as a similar change is pro- 

 duced by air applied to blood out of the body, it is presumed 

 that the air in the lungs is the immediate cause of this change ; 

 but how it effects it, is not yet determined. 



As the blood is changed to a more florid red in passing 

 through the lungs, or from the venous to the arterial system, 

 so it loses that colour again in passing from the arteries to the 

 veins in the extreme parts, especially when the person is in 

 health ; but every now and then we observe the blood in the 

 veins more florid than is usual, and. it likewise frequently 

 happens in venesection, that the blood which comes first out 



1 That this change is really produced in the lungs, I am persuaded from experi- 

 ments, in which I have distinctly seen the blood of a more florid red in the left auricle 

 than it was in the right. But some authors of the greatest authority say that they 

 could not observe any such difference in a great number of experiments which they 

 made ; but this I should attribute to their having been later in opening the left auricle 

 after the collapsing of the lungs than I was ; for it seems probable, that whatever is 

 the alteration produced on the blood in its circulation through this organ, that 

 change cannot take place after it is collapsed. 



gation that the effect of many substances on the colour of the blood is 

 owing. 



The discordant observations as to the difference of colour between 

 arterial and venous blood may be partly owing to the facts observed by 

 Dr. Crawford^ and Dr. Davy, x that when dogs or sheep are exposed to 

 a temperature above 80, the venous blood becomes more florid, and 

 the arterial less so, than at a temperature several degrees lower. The 

 brightening of arterial blood seems to depend on the quantity of oxygen 

 consumed in respiration, which is greatest in cold weather. Harvey 

 probably made many of his experiments in Italy. Dr. Davy could 

 perceive no difference of colour between the blood of the jugular vein 

 and carotid artery of a sheep in the hot months, at Malta. Mr. 

 Thackrah/ on bleeding a man in a warm bath, observed that the 

 blood from the basilic vein was scarlet. 



w On Animal Heat, p. 307-8, 8vo, Lond. x Researches, ii, 140. 



1788. > On the Blood, ed. 1834, p. 123. 



