Besides, if the lungs flattened in the dead body the instant the thorax was opened, it would be owing to the pressure of the external air, a pressure which would expel through the trachea what was contained in these organs. Now if, to prevent the escape of air, you close hermetically the canal by fixing a tube to it the stopper of which is tight, and the thorax is afterwards opened, the lungs still flatten; the air had already gone out of them. Make, on the contrary, the same experiment on a living animal, you will always prevent the flattening of these organs, by preventing the expulsion of the air. In this point of view Goodwyn has gone on a wrong principle in measuring in a dead body, the quantity of air remaining in the lungs after each expiration. Besides, if you open bodies ever so little, you will hardly find two in which there is the same arrangement in the lungs. The infinitely various manner in which life terminates, by accumulating more or less blood in these organs, by retaining more or less air in them, &c. gives them so variable a size, that no general data can be established respecting them. On the other hand, can we hope to be more successful on the living body? No; for who does not know that digestion, exercise, rest, the passions, tranquillity of mind, sleep, watchfulness, temperament, sex, &c. make an infinite variety in the forces of the lungs, the rapidity with which the blood circulates through them, and the quantity of air that penetrates them? All the calculations on the quantity of this fluid which enters or goes out according to the inspiration or the expiration, appear to me to be physiological errours, inasmuch as they assimilate the nature of vital forces with that of physical forces. They are as useless to science as those which had formerly for their object the muscular force, the velocity of the blood, &c. Besides, observe if their authors agree better among themselves then they used to do on this much agitated point. [81] It is inconceivable how Bichat could think of confirming his opinion by the example of hydrothorax. Who does not know that when an effusion takes place in the cavity of one of the pleuras, that that portion of the lungs only which is above the level of the water serves the purposes of respiration; that when the effusion has arrived to the summit of the cavity, the lungs of that side, which can no longer dilate, are of no use in respiration; and those of the other, being compelled alone to make the necessary modifications in the blood, must be traversed by the greatest part of this fluid? It is known, finally, that in this case the patient cannot lie down an instant on the sound side, because this position prevents the dilatation of the lungs of that side which alone serve for respiration, and the danger of suffocation is therefore imminent.