[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.
[82] The observation of Bichat is very just; and I have myself often observed in cases of apoplexy, that the motions of the heart continue many hours after the arteries contain only black blood.
[83] At the period Bichat wrote, it was impossible to know whether the arterial or venous blood contained most hydrogen and carbon. At the present day even, when the means of analysis are much more perfect and animal chemistry farther advanced, we are hardly better informed.
[84] It seems that when Bichat wrote this work, he had not fixed in his own mind what part the arteries take in the circulation; at least, in this paragraph, he seems to give them a contractile power, which, in his other works, he accords only, to their ultimate ramifications.
[85] It is certain, whatever Bichat may say concerning it, that numerous angles in the course of a blood-vessel, by increasing the friction, must retard the course of the fluid which runs through its cavity. We may in fact believe that it is one of the causes, which, in the flattening of the lungs, embarrass the circulation; this cause exerts its action principally upon the last ramifications of the pulmonary artery which, by their interlacing, form the bronchial cells; another more powerful cause, which acts equally on the divisions of the vein and the artery is, as we have said, the pressure made on the lungs by the flattening of the thorax, and the elevation of the diaphragm.