A question still remains to be considered, whether, when the air enters the pulmonary vesicles, it is absorbed in its whole substance, that proportion of each of its constituents which is necessary for the wants of the system being retained, while the excess of each is rejected, or whether the quantity only be absorbed which is afterwards employed, consisting of a large proportion of oxygen and a small proportion of nitrogen. Sir H. Davy thinks that the whole of the air is absorbed, and that the surplus quantity of cach of the constituents is afterwards discharged; he remarks that air has the power of acting upon blood through a stratum of serum, and he conceives it probable, that in this case the whole mass must be absorbed before it can arrive at the red particles, upon which its action is specifically exercised.1 But although this view of the subject appears to be the most probable, yet we must not consider it as resting on any very decisive evidence.2

and Orfila's Toxicologie, t. i. p. 531. et seq. In this case it has been supposed by Dr. Prout more probable that the union of the phosphorus and oxygen should take place in the pulmonary vesicles than in the course of the circulation; See Ann. Phil. v. xiii. p. 278; but the effect is of so peculiar a nature that it seems scarcely possible to reason from it to what takes place under ordinary circumstances.

Researches, p. 447.

2 Sir H. Davy's experiments on the respiration of nitrous oxide have been adduced in favour of this opinion, because they have been thought to prove that nitrogen was generated by this process, which it has been supposed could only have taken place by the decomposition of the nitrous oxide after it had been

Upon what Part of the Blood does the Air act?

139

I have already made some observations on the supposed discharge of hydrogen from the lungs. The experiments and arguments that were employed by Lavoisier, to prove that the water contained in the expired air is generated by the union of oxygen and hydrogen, appear to be totally inadequate to the purpose, and accordingly the hypothesis itself, although at one time so very generally adopted, is at present, I conceive, entirely abandoned.3

As the blood is a very compound fluid, composed of various substances that are loosely combined together, and possess different chemical properties, it has been a subject of inquiry, upon which of its constituents does the air more particularly act. According to the hypothesis which supposes the lungs to be the seat of the operation, the inquiry will be, from what part does the oxygen procure the carbon, and according to the other hypothesis, by what part is

previously absorbed by the blood; Researches, p. 412. et seq.; and an argument was drawn from this in favour of the absorption of atmospheric air by the blood. There are, however, several points in these experiments, with respect to the capacity of the lungs in their different states of distention, as well as the relation which they bear to the quantity of air inspired, which require to be re-considered, before we can admit the conclusion that is deduced from them. See note 50 of the Essay on Respiration.

3 I may observe, that upon either hypothesis concerning the mode in which the oxygen unites with the carbon, the water was equally supposed to be generated by the union of oxygen and hydrogen, although they differ in the one being a rapid union effected in the lungs, the other a more slow process carried on during the course of the circulation.

4

the oxygen attracted. Of the two substances into which the blood separates by its spontaneous coagulation, the crassamentum and the serum, the latter appears to be similar, in its chemical relations, to many other parts of the body, and has not been found to possess any specific or peculiar chemical properties, whereas the former, when employed separately, has the power of acting upon the air in the same manner with the entire mass of blood. Hence therefore we infer that the crassamentum is the great agent in bringing about the change which is effected by respiration. The crassamentum itself is, however, composed of fibrine and red particles, and as the former of these has precisely the same chemical properties with the muscular fibre, which does not appear to possess any relations peculiar to itself, we naturally regard the red globules as that part of the crassamentum on which the air more particularly acts." Their organization is peculiar to themselves, they are the only part of the blood which is known to possess any specific chemical characters; we have reason to suppose that they are easily decomposed, and are more readily acted upon than either the serum or the

4 Berzelius observes, that "serum absorbs very little oxygen;" Med. Chir. Tr. v. iii. p. 232.

5 Young's Medical Literature, p. 503. The curious discovery of Messrs. Dumas and Prevost, that the temperature of an animal is in exact proportion to the quantity of red globules which exist in its blood, may afford an indirect proof of this opinion; Ann. Chim. et Phys. t. xxiii. p. 64. et seq. Sce also Dr. Prout, Ann. Phil. v. xiii. p. 270.

fibrine, and it is principally by their change of colour that we are enabled to form our judgment respecting the action of the air upon the blood. The nature of this action is, however, obscure, and we know nothing more than that they appear to have a strong attraction for oxygen, for although it has been shown that they contain a small quantity of iron, there appears no foundation for the opinion, which at one time prevailed, that the iron is the part by which the oxygen is attracted."

Some other circumstances have been pointed out, in which arterial differs from venous blood; it has been stated, for example, that it contains less water and crassamentum. But even, if we admit the facts, which are perhaps not very completely established, this difference might be attributed rather to the effects of secretion and transudation, than to what is to be regarded as the proper action of the lungs. The consideration of these differences between the two states of the blood will therefore be better

The opinion that the iron in the blood is the constituent on which the air more specifically acts, was generally adopted by the physiologists of the last century, see Haller, El. Phys. vi. 3. 18.; and at one time appeared to be proved by the experiments of Fourcroy and Vauquelin, who pointed out the state of combination in which it exists, and the nature of the change which was effected upon it by the air; Fourcroy's System by Nicholson, v. ix. p. 207..0; but notwithstanding the high authority of these chemists, there appears to have been some inaccuracy in their statement. See the experiments and reasoning of Wells, Phil. Trans. for 1797, p. 427. et seq.; also my remarks on the iron in the blood in vol. i, p. 460. et seq.

understood, when we have considered the nature of the secretions, as well as of the substance from which the blood itself is produced, and have compared, as far as is in our power, the chemical relation which these bodies bear to each other. I shall conclude this section by recapitulating the changes which, according to the present state of our knowledge, the blood appears to undergo by respiration, after premising that our information upon this subject is still in á very imperfect state, and that, in most cases, we arrive at our conclusions, rather by indirect inferences, than by any direct experiments that can be madé upon the blood itself.

1. The blood, when it leaves the right side of the heart, is of a purple colour; during its passagé through the lungs it is converted into a bright scarlet, and again acquires the purple colour when it arrives at the venous part of the circulation. 2. This change from purple to scarlet is effected by the oxygen of the atmospheric air, which is received into the vesicles of the lungs. 3. The same change of colour may be produced upon the crassamentum of the blood out of the vessels by exposing it to atmospheric air, or still more to oxygen, while, on the contrary, scarlet blood is rendered purple by exposure to hydrogen, nitrogen, or carbonic acid. 4. The blood, in passing through the lungs, discharges a quantity of carbon, which is expired in combination with oxygen, under the form of carbonic acid gas. 5. A quantity of aqueous vapour is discharged from the lungs, but this is rather to be considered as the