It appears probable that the internal surface of the bronchi and pulmonary ve sides is an absorbing surface. For when a person breathes air loaded with the va pour of turpentine, that substance very speedily shews itself in the urine, al though the skin will not take it up. If the body really increase in weight in damp air, it might be accounted for by means of pulmonary absorption. It must probably in this way, or by the skin, that contagious matters affect the constitution.
The absorbents are concerned in pro ducing changes in the different secreted fluids : they remove the aqueous portions of the bile and urine, and often take up even the colouring parts of the former, and convey them into the blood, whence they are deposited in all our organs, and produce the yellowness of jaundice. They introduce various diseases into the human frame, as syphilis, hydrophobia, inoculated small-pox, &c.; and in other instances act in a curative manner by tak ing up extravasated blood, by reducing swollen parts, &c.
The circulation, is the motion by which the blood, setting out from the heart, is constantly carried to all parts of the body in the arteries, and returns to the same point in the veins. The uses of this cir culatory motion are, to submit the blood altered by the mixture of lymph and chyle to the contact of the atmosphere in the lungs (respiration), to convey it to several organs in which various animal fluids are separated from it, (secretion) ; and to every part of the body, for sup plying its growth and repairing its losses, by means of its nutritive particles when completely assimilated (nutrition). The conveyance or transport of our fluids, ra ther than their elaboration, is the office of the organs of circulation. In this view they may be compared to those labour ers, who in a large manufactory, from which various products issue, carry the materials to the workmen employed in the actual fabrication. As among the htt ter there are sonic who purify and bring to perfection the materials furnished by others, so the lungs and secretory glands are constantly employed in separating from the blood all those hethrogenous matters which could not be assimilated to the substance of our organs.
The word circulation, when used abso lutely, comprehends the whole course of the blood, as well in the lungs, as in the arteries and veins of the body at large. The greater circulation is the passage of' the blood from the left side of the heart, through the arteries, to the extremities of the body, and its return through the veins to the right side of the same viscus. The lesser circulation is the transmission of the blood from the right to the left side of the heart, through the lungs.
The course, which the blood takes, has been already explained in the article Axeyrour. We subjoin the proofs and ex. periments, by which the facts there stat ed are supported.
The passage of the blood through the heart, i, e. from the right auricle to the left ventricle, by the medium of the lungs, is manifest from the structure of the heart itself. The valves, which are plac
ed at its various apertures, actually will not admit of the blood's motion in any other direction than what we have des cribed. That this fluid passes from the heart into the trunk of the aorta, thence into its branches, and so on to the most minute ramifications, is evinced ; 1. By the effect of ligatures on these vessels ; the artery becomes turgid between the heart and the ligature, and empty be tween the ligature and its distribution. 2. By opening an artery when tied, above and below the ligature ; the blood in this case flows from that opening only, which is nearest to the heart. 3. By ocular testimony ; the passage of the blood can be seen with the aid of glasses in frogs, fishes, &c. The passage of the blood through the veins, in a contrary course to that in which it flows along the arteries, i. e. from the minute ramifications towards the trunks, and thence to the heart, is proved. 1. By the structure and dispo. sition of the valves, which afford an invin: cible impediment to all retrograde motion, 2. By ligatures on these vessels, which make the vein turgid between the extre mities of the body and the ligature, and empty in the rest of its course. 3. mr, opening a vein, when tied, above and be. • low the ligature. 4. By microscopical ob servation in animals.
The passage of the blood from the ar teries into the veins seems to flow as a corollory, from what we have stated con cerning the proofs of its course in these two systems of vessels. We have shewn that the ultimate arteries are continuous with the origins of the veins ; that the bldod moves from the heart to the extre mities In the former Vessels, and that it passes from the extremities to the heart in the latter. The intermediate passage is a direct consequence of these facts. But it may be demonstrated by direct proofs independently of this argument. If we tie the artery of a part, its correspondent vein receives no blood ; if we take off the ligature the vein is again filled. The quantity of blood expelled from the aor tic ventricle is so considerable, that the i supply can only be kept up by its return to the heart. We calculate that two ounces are sent into the aorta at each pul sation ; if we suppose 80 pulsations in a minute, 9,600 ounces will be thrown out in an hour and 14,400 pounds in a day. The same blood, therefore, which the Aorta received from the heart, must be returned to this viseus; and the only pas. sage, by which it can return, is through the veins. Lastly, the passage of the blood from the arteries into the veins, may be proved by the direct testimony of the senses in living animals. The use of the microscopy affords this proof in the transparent parts of cold blooded animals, as the mesentery and web of the foot in frogs, the tail of fishes, &c.