13. Much has been disputed about what is to be accounted the primary part of the body, on which all the rest depend; and one class of disputants have arranged themselves on the side of the blood, and the other on that of the nerves. The dispute is like one about the beginning of a circle. It cannot be decided, because the blood cannot act without nerves, nor the nerves without blood. I speak of the human body, being aware that in some animals the position may be controverted. The following is a concise state of the matter.
14. All the blood in the body passes through the heart; which has four cavities; two called ventricles, and two auricles. These, from their position in the body, are called the right and left. The right ventricle communicates with the right auricle, as does also the left ventricle with the left auricle; but there is no communication between the right ventricle and the left, nor between the right auricle and the left. Through these cavities all the blood passes to every part of the body, and returns from every part; but, as in the former case, we are here at a great loss where to begin its motion; for this is precisely to find the beginning of a circle. As we must begin somewhere, however, we shall do so with the right ventricle of the heart. This receives the blood returning from all parts of the body, and propels it into the right ventricle; not the whole quantity at once, for it cannot contain one half of it; but by degrees. The auricle contracts as soon as it is full; and in the time that the auricle fills, the ventricle contracts, so that it may be empty, and ready to receive the blood from the auricle. By the contraction of the right ventricle the blood is driven into the pulmonary artery, and passes into the lungs. Here the artery branches into an infinite number of small vessels much finer than hairs; and these again, uniting into larger trunks, form at last the pulmonary vein, which brings back the blood to the heart. The pulmonary vein is inserted into the left auricle of the heart, which, as soon as it is filled with blood, contracts, and expels the blood from it into the left ventricle. From the left ventricle issues a large artery called the aorta, which by its branches supplies the whole body with the vital fluid. In all parts of the body the arteries divide themselves into innumerable small branches, which terminate in veins equally small as in the lungs; but it has been disputed whether the arteries and veins actually join each other in the form of vessels, or whether the arteries deposit the blood in small cells, from which the veins suck it up. The dispute is of no consequence, nor can it be absolutely decided, on account of the exceeding smallness of the vessels; though the microscopical observations are rather favourable to the opinion of a continuation of vessels. The veins from all parts of the body unite into larger vessels, and these again uniting with one another, form at last one very large vein called the vena cava, which opens into the right auricle of the heart, from which the circulation goes on as already described. The two ventricles of the heart, and all the veins throughout the body, are furnished with a kind of valves, which allow the blood to proceed in the way of circulation, but prevent its returning in a contrary direction.
15. The lungs, through which all the blood in the body passes, receive likewise the air which we draw in every time we breathe. They consist of two large bodies called lobes; from their situation called the right and left. The air is conveyed into them by the wind-pipe, called also the trachea, and the aspera arteria. On entering the cavity of the breast, the wind-pipe divides into two large branches called the bronchiæ; one of which goes to the right and the other to the left lobe of the lungs. By the further division and subdivision of these vessels the lungs are filled with an innumerable multitude of little tubes, terminating in exceedingly minute bladders or cells, which are the final receptacles of the air sucked in when we breathe. Each of these cells is surrounded with a kind of network of blood-vessels exceedingly small, and consisting of very thin membranes; so that, in passing through the lungs, the blood is exposed as much as possible to the action of the air.
16. It is a matter of great importance to find out what is the use of this exposure of the blood; and great disputes have taken place concerning it. In former times it was supposed that the blood received from the air a vital spirit, without which it would have been totally incapable of performing its offices in the body. Later physiologists endeavoured to explode this notion. Dr. Hales particularly, by shewing that the circulation of the blood through the lungs might be continued by inflating and contracting them alternately by the fumes of burning brimstone, endeavoured to prove that the use of the air was only to give the lungs an opportunity of dilating and contracting alternately, by which means principally he thought the circulation might be carried on. This continued to be the most common hypothesis as late as the time of Dr. Huxham. It was however thought also that by the compression of the air the blood was altered in its texture, its bulk, &c. Accordingly Dr. Huxham tells us in the preface to his treatise on air and epidemic diseases, that “air fit for respiration ought neither to be too hot, nor very cold; for the use of the inspired air is to temperate the blood, which would otherwise grow too hot, and putrefy, as is evident from the experiment of the most excellent Boerhaave made in a hot house; for, if the air is more hot, or even equally hot, as the blood of any animal, it certainly soon dies.”[47]
17. The modern discoveries in the composition of air, have tended greatly to elucidate the use of this fluid in the lungs, and its action on the blood in respiration. Dr. Priestly first determined it to be what he terms a phlogistic process, i. e. a process by which the parts of the blood no longer proper to be retained among the rest, or at least some of them, are carried off. That something is carried off either from the lungs themselves, or from the blood circulating through them, is evident; for the air which is taken into the lungs in a dry state, comes out of them extremely moist, and loaded with vapour. An essential change is also made in the nature of the air itself; for it now assumes in a great measure the nature of what has been called fixed air, or the fume of charcoal, or fermenting liquor, and thus becomes unfit for being breathed a second time. This change is made by the addition of some terrestrial substance to the pure atmosphere, which the latter volatilizes and carries along with it.[48]
18. But, whatever may be carried off from the blood, during its passage through the lungs, something is certainly added to it, for the blood in the pulmonary artery is of a dark red, but when it has undergone the action of the air in the lungs, and returns by the pulmonary vein, it is then of a bright scarlet, which colour it retains through all the arteries of the body, but loses it on its return through the veins. This scarlet colour is communicated to blood in all cases when exposed to the air; and Dr. Priestley has observed that it is acted upon by the air even through a bladder; much more then must it be so through those very thin membranes which form the coats of the fine pulmonary vessels. What this subtile matter is which the blood receives, shall be afterwards inquired into; at present it is sufficient to take notice that it is absolutely necessary, for the purposes of life, that the blood should pass through the lungs: for, as Dr. Huxham observes, “we see neither nutrition, nor the motion of the muscles, performed by any blood that hath not passed through the lungs; this is observable from the coronary arteries[49] to the ultimate ramifications of the aorta.” As the previous circulation of the blood through the lungs therefore is absolutely necessary to the growth and life of the body, and as the blood certainly receives something from the air, we must account this a proof, and no inconsiderable one, that the air contains a vital spirit, which it imparts to the blood in the lungs. But, before we proceed farther on this subject, it is proper to take some notice of
19. The nerves. These, which constitute such a remarkable and important part of the human body, are white cords, of a soft pulpy substance, defended by a tough skin which goes along with them as far as they can be traced. All the nerves either originate from the brain, or terminate in it. The former doctrine hath been generally adopted, and in conformity to that doctrine the following account of the nerves is laid down. The brain is enclosed in the cavity of the scull, but not without the intervention of two membranes, called the dura and pia mater, to prevent injury from the hard bones, as well as for other purposes. The brain is divided into two lobes, the right and left. It is composed of two different kinds of substance, the outermost called the cortical, the innermost the medullary substance; the latter seems composed of fine fibres. The whole of the medullary part of the brain terminates in a substance called the cerebellum, very much resembling the brain, but smaller. The cerebellum terminates in another substance resembling the medullary part of the brain, called the medulla oblongata. The cerebellum lies in the back part of the head, and the medulla oblongata under it. The latter terminates in the spinal marrow, extending from the lower and back part of the head to the lower extremity of the back bone, and is enclosed in the hollow of that bone. The nerves proceed from these four substances, viz. the brain, the cerebellum, the medulla oblongata, and spinal marrow. As they pass to all parts of the body they accompany the arteries, dividing with them into innumerable small branches; but they do not return with the veins; so that they seem not to contain any fluid which goes and comes, or which circulates like the blood. The nervous fluid, if any such there be, seems to move constantly one way, either to the brain or from it.
20. Hitherto we have noticed only things which are evident to our senses, and which the industry of anatomists has abundantly evinced; but now our subject renders it necessary to step aside a little into the obscure regions of theory and conjecture. The muscles, as we have formerly said, are the fleshy parts of the body; and by them all the motions of the body are performed. The flesh is distributed into distinct portions, each of which is enclosed in a membrane belonging to itself. Each of these portions is a muscle, and each muscle has a branch of an artery and the branch of a nerve belonging to it. On both these the action of the muscle depends; for, if we cut the nerve belonging to a muscle, it immediately loses all power of action; and if we cut the artery which accompanies the nerve, it does the same. As therefore the blood is found to receive something from the air, and as it loses this when passing through the arteries, and as the nerves lose their power when the communication with the blood is cut off, it seems extremely probable, that what is imbibed by the blood in the lungs is taken up by the fine ramifications of the nerves, and is no other than the immediate principle of life and sensation. Thus we will establish a doctrine directly opposite to that commonly received; for, instead of supposing that the nerves originate from the brain, we are now led to suppose that they terminate in it. Instead of supposing that the sensations originate in the brain, we will be led to suppose that every sensation originates in the organ appointed for that sensation. Thus we are conscious that our eyes, not our brain, are the parts of our body which immediately perceive the light; our fingers, or any other parts of the body, feel what is applied to them; and of consequence we have reason to believe that the animal spirits, nervous fluid, or whatever we please to call it, proceed from the surface of the body inwards to the brain, not outwards from the brain to the surface of the body. The brain itself seems to resemble a large collection or reservoir of water, in which the sensations, like so many small streams from every part of the body, unite, and in which our intellectual faculties reside in a manner totally inexplicable by us. Thus far it seemed necessary to theorise, in order to form some idea, however obscured, of the connexion between the nerves and our sensitive and intellectual, or, if we please to call them so, our spiritual faculties.
21. In consequence of this very intimate connexion between the blood and nerves, it is easy to see that any injury done to the one may very greatly affect the other; and that a very slight, nay, to us imperceptible, change in the organization of either, may produce the most grievous, and even incurable disorders throughout the whole body, or in any particular organ. Let us now consider a little farther the blood-vessels.
22. It hath been a question, whether in the structure of these vessels nature hath observed an exact proportion. For instance, if the blood passes by a kind of starts through four cavities, as we are assured that it does, it seems natural to suppose that these four should be exactly equal. This, however, hath been denied; and some, from its accommodating the human frame to their theory, have fancied that they saw the use of such disproportionate work. Dr. Huxham expresses himself in the following words: “Nor doth the air only refrigerate the blood, but, by preventing its too great ebullition, and condensing it, hinders it from bursting the vessels. This indeed is of exceeding great importance, if, with the very learned Helvetius, we suppose the capacity of the right ventricle of the heart to be greater than that of the left, and that the pulmonary arteries are larger than the correspondent veins; for it thence follows, that the blood ought to be considerably condensed by the inspired air, that an equal quantity of blood may be received, in one and the same time, by the pulmonary veins and left ventricle of the heart, that is thrown off from the right ventricle, and through the more capacious pulmonary arteries. This indeed many deny, asserting quite the contrary. It is necessary, however, that the aorta should receive as much blood from the left ventricle of the heart, as is thrown off from the right ventricle through the pulmonary artery; and that in the very same and equal time, or a fatal deluge would soon overwhelm the lungs, because the contraction of each ventricle is made at one and the same time; we always find therefore the aorta and pulmonary artery, in a natural state, equal on this account; also the capacity of the ventricles ought to be equal, that they may receive, in one and the same space of time, equal quantities of blood,” &c.