The changes in asphyxia, in the warm-blooded animals, have, therefore, of late been generally thought to be as follows :—that the venous blood, though more or less noxious to all parts of the body which it fully penetrates, is nevertheless transmitted through the lungs in the first instance, in sufficient quantity to stimulate the left side of the heart, and is sent from thence in sufficient quantity to penetrate the brain ;—that by its action there it destroys the sensibility, but that it passes more and more slowly through the pulmonary vessels, and after a few minutes is no longer delivered to the left side of the heart in such quantity as to keep up regular and efficient contractions there ; and that thus, while the animal life is suddenly extinguished by the noxious influence of venous blood on the brain, the organic life is more gradually brought to a stand by its noxious influence in the lungs, and the consequent failure in the supply of blood to the left side of the heart.
This explanation is consistent with all the phenomena, and particularly with the very rapid restoration of the flow of blood by the admission of air to the lungs of half-asphyxiated animals, stated by Bichat himself as a difficulty in his view of the subject.
The more recent experiments by Dr. Kay had, however, led him to question the validity, even of that part of Bichat's doctrine, Which has been most generally admitted, viz. the ra pidly noxious effect of venous blood on the brain and nerves. He found, in various cases, that large quantities of blood from the veins of one rabbit could be injected (slowly and cau tiously, so as to avoid all injury of the cerebral matter) into the carotid arteries of another, with out causing more than muscular debility and lassitude ; so that he considers venous blood to be only a weaker stimulus to the brain than arterial, not a direct poison to it; and thinks the sudden insensibility of asphyxia is to be explained by the rapid diminution of the quan tity, not by the change of quality, of the blood sent to the brain from the heart.t And when we bear in mind the fact stated in the outset of this inquiry, that the motion and vivifying power of the nutritious fluid is dependent on its exposure to oxygen, not only in the higher animals, but even in the lowest tribes, and in vegetables, where neither heart nor nervous system exists ; it appears reasonable to suppose, that the chief impediment to the blood's motion, from the failure of the supply of oxygen, will be in the lungs themselves, where the venous blood is accumulated in the greatest quan tity, and where all the minute vessels carrying it must be most completely exposed to its action, But before we can be completely satisfied upon this subject, it will be necessary to carry the inquiry one step further, and to ascertain in what manner the change from venous to arterial blood so greatly promotes the flow of blood through the capillaries of the 'lungs and how the presence of venous blood in the begin nings of the pulmonary veins can so effectually retard it, that the action of the right ventricle of the heart, though continuing vigorous for a time thereafter, fails of its wonted effect, and the blood stagnates in those capillaries.
The common expression employed on this subject is, that arterial blood is a stimulus peculiarly adapted to excite the capillaries of the lungs and pulmonary veins ; and that venous blood stagnates in those capillaries for want of power to excite them. But it must be remembered that we have no distinct evi dence of the existence of coats, still less of irritable coats in the minute capillaries of the lungs;' that although the circulation there has been often examined with the microscope, no contraction of the vessels has ever been ob served ; that the only vital power of contrac tion which experiments authorize us to ascribe to any arteries, is a power of permanent or tonic contraction on their contents, which, when called into action, lasts for some time, and while it lasts must obviously impede the flow of fluids through these vessels ; that on these grounds Magendie and other eminent physio logists believe the only power, which arteries can exercise over their contents, to be simply a power of either relaxing, so as to give them a free passage, or contracting so as to lessen and re tard their flow ;t and that, conformably with these views, it was found by Wedemeyer, that when he injected stimulating liquids into the arteries of living animals, they were much longer of making their way into the veins, than mild liquids were.X These considerations evidently point to the conclusion, that, if the difference depend on any vital action of vessels, venous blood,which makes its way so slowly through the capillaries of the lungs, must be the stronger stimulus to them, and that arterial blood,which is transmitted so readily, must act as a sedative, to the only vital action of which these vessels are susceptible. But this conclusion is again strongly opposed by the fact, that in all other instances, in relation to muscular contraction, to the functions of the nervous system, and of secreting organs, arterial blood, and the oxygenated fluids in general, manifestly possess the stimulating power, and venous blood or carbonized fluids the sedative.