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Yet even with the freest use of water, what can we make of such cases upon any chain of chemical facts at present discovered? What can we make of it, even in conjunction with the use of air? The weight and solid contents of the animal body are derived chiefly from that principle which modern chemists denominate carbone: yet neither water nor air, when in a state of purity, contains a particle of carbone. Again, the substance of the animal frame is distinguished from that of the vegetable by its being saturated with nitrogene, of which plants possess comparatively but very little; yet though the basis of atmospherical air consists of nitrogene, water has no more of this principle than it has of carbone; nor is it hitherto by any means established, that even the nitrogene of the animal system is in any instance derived from the air, or introduced by the process of respiration: for the experiments upon this subject, so far as they go, are in a state of opposition, and keep the question on a balancefactis contraria facta.
Shall we then suppose, with others, that the circle of perpetual mutation, which is imposed upon every other species of visible matter, is in these cases suspended, and that the different organs of the system are, so long as the anomaly continues, rendered incorruptible? But this is to suppose the intervention of a miracle, and without an adequate Let us, then, rather confess our ignorance, than attempt to be wise upon the basis of conceit. All that we do know is, that bodies of every kind are reducible to a few elementary principles, which appear to be unchangeable, and are certainly invisible; and that from different combinations and
modifications of these proceeds every concrete and visible form hence air itself, and water; hence mineral, vegetable, and animal substances. Air, therefore, and water, or either separately, may contain the rudimental materials of all the rest. We behold metallic stones, and of large magnitude, fall from the air, and we suppose them to be formed there we behold plants suspended in the atmosphere, and still, year after year, thriving and blooming, and diffusing odours: we behold insects apparently sustained from the same source; and worms, fishes, and occasionally man himself, supported from the one or the other, or from both. These are facts, and as facts alone we must receive them, for we have at present no means of reasoning upon them. There are innumerable mysteries in matter as well as in mind; and we are not yet acquainted with the nature of those elementary principles from which every compound proceeds, and to which every thing is reducible. We are equally ignorant of their shapes, their weight, or their measure.
ON THE CIRCULATION
OF THE BLOOD, RESPIR
ATION, AND ANIMALIZATION.
THE progress of science is slow, and often imperceptible; and though, in a few instances, it has been quickened by an accidental discovery or an accidental idea, that has given a new turn, or a new elasticity to the chain of our reasoning, still have we been compelled in every instance to follow up the chain, link after link, and series after series, and have never leaped forward through an intermediate space without endangering our security, or being obliged to retrace our career by a painful and laborious re-investigation.
It required a period of three thousand six hundred years to render the doctrine of a vacuum probable, and of five thousand six hundred to establish it upon a solid foundation. For its probability we are indebted to Epicurus, for its certainty to Sir Isaac Newton. The present theory of the solar system was very imperfectly suggested by Pythagoras and his disciples five centuries before Christ, and greatly advanced though not completed by Copernicus fifteen centuries after Christ. Archimedes was the first who invented the celebrated theorem for squaring the parabola, which was upwards of two hundred years before the Christian era; yet a complete solution of the problem of squaring the circle is even now a desideratum. The simple
ON THE CIRCULATION OF THE BLOOD, ETC. 305
knowledge of the magnet was familiar to the Romans, Greeks, and some of the oriental nations while in their infancy; its polarity has been employed by the mariner for nearly six centuries in Europe, and for a much longer period by the Chinese, in their own seas; yet at this moment we are only becoming acquainted with its general laws, and have recently but begun to appropriate it to other purposes than that of the compass.
The circulation of the blood in the animal system is our first subject of inquiry for the present lecture, and it is a subject which has laboured under the same difficulties, and has required as long a period of time as almost any of the preceding sciences, for its complete illustration and establishment. Hippocrates guessed at it; Aristotle believed it; Servetus, who was burnt as a heretic in 1553, taught it; and Harvey, a century afterwards, demonstrated it.
I shall not here enter into the various steps by which this wonderful discovery was at length effected; the difficulty can be only fairly appreciated by those who are acquainted with the infinitely minute tubes into which the distributive arteries branch out, and from which the collective veins arise but every one is interested in the important fact itself, for it has done more towards establishing the healing art upon a rational basis, and subjecting the different diseases of mankind to a successful mode of practice, than any other discovery that has emblazoned the annals of medicine.
In our last lecture we traced the action of the digestive organs: we beheld the food first comminuted by means of jaws, teeth, or peculiar
muscles or membranes; next converted into a pulpy mass, and afterwards into a milky liquid; and in this state drunk up by the mouths of innumerable minute vessels, that progressively unite into one common trunk, and convey it to the heart as the chief of the system, for the use and benefit of the whole.
But the new-formed fluid, even at the time it has reached the heart, has by no means undergone a sufficient elaboration to become genuine blood, or to support the living action of the different organs. It has yet to be operated upon by the air, and must for this purpose be sent to the lungs, and again returned to the heart, before it is fitted to be thrown into the general circulation.
This is the rule that takes place in all the more perfect animals, as mammals, birds, and most of the amphibials; and hence these classes are said to have a double circulation. And as the heart itself consists of four cavities, a pair belonging to each of the two circulations, and each pair is divided from the other by a strong membrane, they are also said to have not only a double circulation, but a double heart a pulmonary and a corporeal heart.
The blood is first received into the heart on the ⚫ pulmonary side, and is conveyed to the lungs by an artery which is hence called the pulmonary artery, that soon divides into two branches, one for each
*Cuvier seems to ascribe a double heart to the class of amphibia, without any limitation. See Lawrence's additional note E. chap. xii. of his translation of Blumenbach's System of Comparative Anatomy. Blumenbach himself has remarked, that many of the frogs, lizards, and serpents have a simple heart, consisting of a single auricle and ventricle, like that of fishes. Sect. 162.