experiments performed on the vessels of cold-blooded animals, or on 

 veins without valves. In bleeding at the bend of the arm, the blood can- 

 not come from that part of the vessel which is above the wounds: the 

 valves oppose insuperable obstacles to its retrograde flow; hence it is 

 very easy to distinguish the red blood which comes from the lower extre- 

 mity of the vein, from that which flows from the upper end, and which 

 is poured into the vessel by the veins which open into it, between the 

 puncture and the nearest valve. 



In its course to the parts among which the arteries are distributed, 

 the blood, vivified in 'its passage through the lungs, and fitted, as Four- 

 croy says, for a new life, loses its oxygen and caloric. Its capacity for 

 the latter, diminishes, in proportion as the oxygen, by combining with 

 hydrogen and carbon, restores it to the venous state. 



This theory of the process by which the blood parts with its oxygen? 

 in its progress along the blood-vessels, is rendered still more probable, 

 by recent discoveries on the nature of the diamond. This substance is 

 the only pure carbon, and that which is calle^ so by chemists, is an oxyde 

 of carbon which owes its dark colour to the oxygen with which it is 

 combined. Before these experiments, it was not easy to determine the 

 particular condition of the carbon which exists so plentifully in venous 

 blood. 



No precise calculation has yet been made, of the quantity of the oxy- 

 gen absorbed by the venous blood, nor of the quantity employed in the 

 combustion of hydrogen and carbon in the lungs, so as to form water and 

 carbonic acid*. 



Is the carbon, in venous blood, merely combined with oxygen, or is it 

 united with hydrogen, so as to form carburetted hydrogen ? It appears to 

 me more probable, that the oxygen which is absorbed by combining with 

 hydrogen, in every part of the body, produces the water which dilutes 

 the venous blood, renders it more fluid, and richer in serum than arterial 

 blood; while, by its union with carbon, it forms an oxide that gives to the 

 blood the dark colour, which is one of its most remarkable characters. 

 On reaching the lungs., which are real secretory organs, the water is ex- 

 haled, dissolved in the air, and forms the pulmonary transpiration; the 

 oxide of carbon, completely decomposed by an additional quantity of 

 oxygen, constitutes carbonic acid, which gives to the air that is expired, 

 power of forming a precipitate in lime water. 



The absorption of oxygen by the venous blood, explains how the phe- 

 nomena of respiration are continued into every part of the body, and 

 produce the warmth uniformly diffused over all our organs. In propor- 

 tion as the blood parts with its caloric, for which its affinity diminishes 

 as it becomes venous, the parts which give out their hydrogen and car- 

 bon, combine with it. If the lungs were the only organs in whieh caloric 

 might be disengaged, the temperature of those viscera ought considera- 

 bly to exceed that of other parts: experience, however, shows that the 

 temperature of the lungs is not sensibly more elevated. 



This theory of respiration, for which we are entirely indebted to mo- 

 dern chemistry, is contradicted by no one phenomenon. The greater the 



* Instead of saying that the venous blood absorbs oxygen, it will approach nearer the 

 state of our knowledge to believe that the venous blood gives off its carbon, which 

 combines, in the lung-s, with the oxygen of the inspired air. For a full view of the 

 latest opinions on this subject, see APPENDIX, Note W. 



