Towards 1846, I wanted to make experiments on the cause of poisoning by carbon monoxide. I knew that this gas had been described as toxic, but I knew absolutely nothing about the mechanism of this poisoning; I therefore could not possess any pre-conceived opinion. What was to be done, then? I had to give birth to an idea by making a fact appear, that is to say, make an experiment to see. In fact, I poisoned a dog by forcing him to breathe carbon monoxide, and immediately after his death, I opened his body. I looked at the state of the organs and the fluids. What instantly attracted my attention was the the blood was crimson in all the vessels; in the veins as in the arteries, and in the right and left chambers of the heart. I repeated this experiment on rabbits, birds, and frogs; everywhere I found the same general crimson coloration of the blood.....
In 1856.....I took up again the study of carbon monoxide poisoning which I had begun in 1846. I found myself then in an ambiguous situation, because, at that time, I already knew that carbon monoxide poisoning renders the blood crimson throughout the entire circulatory system. It was necessary to make hypotheses and to establish a pre-conceived idea based on this first observation before proceeding further...and the following reflections presented themselves to my mind. The crimson color of the blood, I told myself, is peculiar to the arterial blood and is related to the presence of oxygen in great proportions, while the dark coloration is caused by the disappearance of oxygen and to the presence of a greater proportion of carbonic acid, so the idea came to me that the carbon monoxide, in preserving the crimson color in the venous blood, may have perhaps hindered the oxygen from changing into carbonic acid in the capillaries. However, it seemed difficult to understand how all this could be the cause of death. But still continuing my internal and pre-conceived reasoning, I added; if all of this were true, blood taken from the veins of animals poisoned by carbon monoxide should contain oxygen like arterial blood; that was what was necessary to see.
........I carried out an experiment to verify my hypothesis relating to the persistence of oxygen in the venous blood... I passed a stream of hydrogen through the crimson venous blood taken from an animal poisoned by carbon monoxide, but I could not, as usual, displace the oxygen. I attempted the same on the arterial blood with no further success. My pre-conceived idea was therefore false. But this impossibility of obtaining oxygen from the blood of a dog poisoned by carbon monoxide provided a second observation for me which suggested new ideas according to which I formed a new hypothesis. What could have happened to the oxygen in the blood? ........ I exhausted myself in conjecture concerning the manner in which the carbon monoxide could have made the oxygen disappear and, since gases displace one another, I naturally thought that the carbon monoxide could have displaced the oxygen and driven it from the blood. In order to confirm this, I resolved to change the experiment and to place the blood in artificial conditions which would permit me to recover the displaced oxygen.
I then studied the action of carbon monoxide on the blood by artificial poisoning. To do this, I took a certain quantity of arterial blood from a healthy animal, and I placed this blood under mercury in a test tube containing carbon monoxide, and I consequently agitated the entire set up in order to poison the blood while protecting it from contact with the outside air. Then after a certain time, I looked to see if the air in the test tube, in contact with the poisoned blood, had been modified, and I determined that the air in contact with the blood was notably enriched with oxygen, at the same time that the proportion of carbon monoxide was diminished. It appeared to me ..... that there had been a simple exchange volume for volume between the carbon monoxide and the oxygen in the blood. But the carbon monoxide in displacing the oxygen which it had driven from the blood, remained fixed in the blood corpuscles and could no longer be displaced by oxygen or by any other gas, such that death occurred by the death of the blood corpuscles, or to put it another way, by the cessation of the exercise of their physiological which is essential to life.
Source: Claude Bernard. "Introduction a l'etude de la medecine experimentale" (Paris: J.B. Bailliere et Fils, 1865, pp. 85-92, 101-104., 107-112, 265-301. Translated by A.S. Weber.)
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