Crawford was the first who proposed any consistent ex planation of the method in which the blood is supplied with the carbon which is discharged from the longs. He observed that the matter of which the body is composed has a constant tendency to change : after some time, it ap pears to become incapable of serving for the purposes of the animal economy, and is therefore removed, while at the same time fresh matter is deposited in its room. The arteries are supposed to be the instruments by which this interchange is effected. They convey the nutritious matter to all parts of the body, so as to repair the necessary waste ; while, at the same time that the blood loses its nutritive particles, it receives the effete matter, which is now be come useless to the system, is conveyed to the lungs by the veins, and is there discharged. It is to this change of matter, which is supposed to be carried on in the capil laries of the systemic circulation. that the blood is con verted from the venous to the arterial state, while the op posite change, from venous to arterial blood, is produced in the capillaries of the lungs. Hence it follows, upon this hypothesis, that the matter which is received by the systemic veins, contains a greater proportion of carbon than that which is employed in the growth and nutrition of the body.
Crawford's hypothesis accords with many well known facts, and seems to afford a simple and natural explanation of them ; yet there are several parts of it that are decidedly objectionable. We 'rave no evidence of any set of vessels by which carbon can enter the veins at their extremities, while, on the other hand, there is an obvious source from which this matter is conveyed into the blood in a different part of the circulation We hate every reason to sup pose, that the only supply which the blood receives is by the thoracic duct, which pours its contents into the sys temic veins, just behave they arrive at their termination in the right auricle, previous to the transmission of the blood through the lungs. as it appears, we are neces sarily reduced to the conclusion, that arterial blood be comes venalized, merely in consequence of the loss of what is removed from it by the capil;ary arteries for the purpose of nutrition and secretion, and not from any addi tional matter that it receives when it undergoes the change.
Sonic observations have been made upon this change, which must materially influence our hypothesis respecting the mode in which it is effected. It has been found, that, in certain cases, the blood has been converted from the scarlet to the purple colour, while it has remained in the same system of vessels; thus seeming to prove, that the alteration depends, not upon any thing which it receives or discharges, but upon some change which is effected by the action of its constituent parts upon each other.
It has been remarked in surgical operations, that after the blood has been confined by the tourniquet in a large arterial trunk, when the instrument is first removed, the fluid has acquired the venous appearance; and Hun ter found, that the same change was effected when a portion of an artery inclosed between two ligatures. It was also found, that arterial blood, when extravasated into the cavities of the cellular substance, gradually acquir ed the venous character, and that the same change took place when a portion of scarlet blood was confined out of the body in glass tubes. From these facts, together with the supposed disproportion between the oxygen absorbed and the carbonic acid produced, as well as from certain considerations to be hereafter mentioned, respecting the equable diffusion of animal heat, the second hypothesis of respiration was proposed by M. Lagrange, and illustrated by the experiments of M. Hassenfratz, in which the oxygen is supposed to be absorbed in the lungs, to be gradually united with the carbon in the course of the circulation, and to be afterwards expired in the form of carbonic acid. In some respects, this hypothesis affords an easy expla nation of the leading phenomena; but as it rests upon facts which are of an equivocal nature, and is much less simple than the old theory, it has not been generally re ceived.
To those who are acquainted with the doctrines of the modern physiologists, it will be obvious that many of their favourite speculations must fall to the ground, if we reject the absorption of oxygen by the lungs. The very exten sive influence which this element has been found to pos sess over the operations of chemistry, and more particu larly the part which it seems to act in the function of respiration, induced many persons to conceive, that its agency must extend to every part of the animal economy. Thus oxygen has been thought by one, to be the imme diate cause of contractility ; by another, of sensibility ; by some, it has been assumed as the chief agent in digestion and secretion ; and it has even been identified with the vital principle itself. In pathology it has acted a no less conspicuous part. Some diseases have been attributed to its excess, others to its deficiency ; some medicines have been supposed to operate, by imparting this element to the body ; others by abstracting it : so that we find the terms oxygenation and deoxygenation of the system fami liarly employed, as operations which were quite within our power to accomplish. But we think it may be safely asserted, that all these speculations are completely without foundation ; the arguments alleged in their favour are de rived from loose analogies, the experiments inapplicable, and the facts of questionable authority.