As venous seems to be converted into arterial blood by the discharge of are naturally led to conjecture that the opposite change, from arterial to venous, must be produced by a reverse operation. Yet as no addition of carbon can be obtained from the blood in this part of the circulation, we are reduced to the necessity of supposing that the nutritive and secretory matter, which is separated by the capillary arteries, contains a smaller proportion of carbon than the general mass of blood, and consequently increases the proportion of carbon in what is left. We have indeed no positive evidence that this is the case, nor perhaps will it be ever possible to prove it by any direct experiments ; but the conclusion appears, notwithstanding, to be sufficiently warranted, that there must be some outlet from the sangmferous system to balance that of the lungs ; and we know of none except the secretory and nutritive arteries. Upon the whole, then, we may vcmure to draw the probable conclusion, that venous differs from arterial blood in containing a larger quantity of carbon.
An interesting question occurs to us in this part of our investigation,—upon what part of the blood is it that the air more immediately acts? According to one hypothesis, the immediate subject of inquiry will be, from what part does the oxygen abstract the carbon ? and, according to the other, by what part is the oxygen attracted ? Although we are obliged to form our conclusions rather from con jecture than from any direct facts, yet it appears natural to suppose, that the red particles are the immediate agents in this operation. These globules are the only constitu ents of the blood which possess any specific characters, and which can therefore impart any specific properties to it, while, at the same time, they are known to be easily de composed, and to he more readily affected by various che mical re-agents than either the serum or the fibrin. The nature of the action is indeed obscure ; for there does not appear to be any foundation for the idea that prevailed at one time, that these red particles attract the oxygen in consequence of the iron which they contain. All the most recent experiments tend to discountenance this idea, which is farther objectionable, as involving the doubtful hypo thesis of the absorption of oxygen by the blood.
One circumstance still remains to be noticed, in which arterial has been supposed to differ from venous blood, that the former contains a larger proportion of crassamen turn. Although we have perhaps no very decisive evi dence of the fact, yet it is not in itself improbable ; for when the arterial blood is sent into the minute arteries of the muscles, we imagine that one operation which it per forms is, to afford a supply of fresh fibrin, to repair the waste which is continually going on in all these organs.
It may indeed be said, that the membranous parts will re quire the same supply of matter from the serum ; but there are reasons for supposing that membrane is a sub stance of a more fixed nature than fibrin, and that there is a less frequent change of its constituent particles.
We have now reviawed in succession the various changes which are supposed to be produced in the blood by respi ration, and it will be perceived that, notwithstanding the discovery of many important facts, the subject still remains involved in much obscurity. This depends, in a great measure, upon the difficulty of performing experiments on a substance like the blood, composed of a number of in gredients, connected together by a complicated system of affinities, which is liable to be disturbed by the operation of almost every external body. The difference between arterial and venous blood is therefore rather inferred from the comparison of a number of observations that have been made upon it in different states of the body, or from its operation on substances that are subjected to its action, than from the direct result of experiment. The present state of knowledge may, we think, be comprised in the following propositions: 1. The blood, when it leaves the right side of the heart, is of a purple colour; during its passage through the lungs it is converted to a bright scar let, and it again acquires the purple hue when it enters the capillaries of the veins. The change from purple to scarlet would appear to he effected by the oxygenous part of the atmosphere, which is received into the vesicles of the lungs. 2. The blood, in passing through the lungs, emits a quantity of carbon, which is expired in combina tion with oxygen, under the form of carbonic acid gas. 3. The change of colour from purple to scarlet may be produced in the crassamentum of venous blood out of the body by exposure to atmospheric air, or still more to oxy gen, while scarlet blood is rendered purple by exposure to azote, hydrogen, or carbonic acid. 4. A quantity of aqueous vapour is expired from the lungs ; but there is no proof that it is formed there by the union cf its com ponent parts. On the contrary, it is more probable that it proceeds from the evaporation either-of a pot tion of the water that is united with the blood, or with some of the mucous secretions with which the pulmonary organs arc furnished. 5. There is no direct proof of the absorption of either oxygen or azote by the lungs, nor is it probable that hydrogen is discharged from them, although experi ments have been adduced in favour of each of these points.