3dly. If there be adapted to the stop-cock a syringe full of air, and this fluid be pushed into the lungs, on opening the pipe, and then the pipe be suddenly shut again, the blood will become red, but much less evidently so, than when the entrance of the air is owing to voluntary inspiration. Here the portion of air injected must repel into the bottom of the cells whatever is already vitiated, while on the contrary, if the tube be simply opened, the vitiated air is at once rejected, and then replaced from without. The following experiment appears to confirm this idea.
4thly. If instead of pushing air upon that which is contained in the lungs, we pump out the vitiated air in the first place, and then inflate the organ, the colouring process will be more rapid, and the colour of the blood itself especially, more lively than in the preceding case, though less so than in the first of this latter suite of experiments.
5thly. The lungs being exposed on both sides by a lateral section of the ribs, the circulation will continue to go on for a certain time. Now, if by means of a syringe adapted to the stop-cock in the trachea, the pulmonary vesicles be alternately emptied and dilated, the changes from red to black, and from black to red, will be observed as in the above experiment, as long as the circulation lasts.
The following consequences may be inferred from the facts, which I have mentioned.
1st. The rapidity with which the blood becomes red again, on opening the pipe in the trachea, is a plain proof, that the principle from which this colour is gained, must pass into the blood across the membranous parietes of the air cells, and not by means of the absorbents. I shall establish this fact hereafter upon other proofs.
2dly. The celebrated experiment of Hook, in which the enfeebled movements of the hearts of animals in a state of asphyxia are accelerated by injecting air into the lungs, is very well explained. The red blood penetrates into the fibres of the heart, and puts an end to the debility induced, by the influx thither of the black blood.
3dly. I do not believe, that motion can ever be restored to the heart, when once it has been wholly annihilated by the presence of venous blood. In this I have never succeeded, though I have often attempted it. Many authors, however, pretend to have done so. If the heart be reanimated by arterial blood, it is necessary at any rate, that such blood, should pass into it, now in what way can it arrive there, if the circulation have entirely ceased.
