As the valves, however, prevent the blood from re •urning in the course of the arteries, it must be con veyed through other channels back to the lungs to re cive the necessary supply of air. fly many experiments and observations, Harvey discovered, that the last ca Dillary branches of arteries are continued into other ca oillary branches which convey the blood in a different direction; that these last afterwards uniting, formed branches of a larger sia and that these again, also form others still lar;cer, till the whole. by mite l'ated unions, form •r° truks, \which, conveying the olood from all the upper and Lea er extremities, meet at ,Lc right side of the heart. am; d•charge their contents :nto a thin muscular sac, cal:ed auricle. by con tracting. propels it onwa”d to the right ventricle, from which it is again throv.ii into tilt lungs by the pulmonary artery. This artery, as already mentioned, separates in to numberless capillary branches, which being conti nued into others similar to those which we have been -lc scribing convey the blood into another auricle placed on the It side cf the heart. From this auricle it is transmitted to the left ventricle, and from thence driven again into the artery which nourishes the system. Such is the regular course of the blood, which is called its etheunaTmat ; and the discovery of which has confer fed so much honour upon Harvey,--a man not less dis tinguished by his unassuming modesty, than by the tran scendent e;:cellence of his genius.
The vessels which convey the blood in a retrograde course have been called veins. They are still more nu merous than the arteries ; are usually found lying by their sides, and are desclibecl by nearly similar names. They have a much less muscular power, and most of them are assisted by a nomber of valves, situated at small distances in their course.
But this, it may be said, is no explanation whatever of +he manner in which the system is nourished ; for if the arteries be continued into veins, and none of the blood escape from the course of circulation, how is nourishment imparted to the various organs ? To ac count for this, we must suppose that sonic %essels break off from the course of circulation, and terminate other wise than in venous branches. In eflect, we find that various series of branches belong to the arteries ; that the ultimate series arc so minute as not to admit the red or thicket• part of the blood ; that many or them, there lore, are destined to convey only the tom and pellucid part of the circulating fluid, and that of these many wander from the circle, and empty themselves into bones, muscles, ligaments, nerves, and the other parts of which the animal fabric is composed ; and that then, each part, by a vital process termed as..yimilaticn, con verts this Iluid into a substance of its own specific cha racter and properties. We observe, besides, that these vessels pour out their fluids into all the cavities, and many of them into bodies of various shapes, sizes, and structurcs, which afterwards convert them into other fluids of a dille•ent quality. To these bodies anatomists have given the name of glands. Some of them prepare a particular fluid to lubricate the joints, and the parts in motion ; some furnish fluids to promote digestion, and assist in the preparation of aliment ; some yield fluids to protect the skin, and to preserve it in a proper state for performing its several offices ; some a fluid which is employed as one of the means of generating the species ; and some a fluid which serves to nourish the offspring for some time after its birth.
But though we have thus provided for the nourish ment of the system, by allowing the fluids to escape front the course ol circulation, still it may be asked, what have we gained by this explanation ? Is not the difficulty now rather increased ? for having allowed the fluids to escape from the course of circulation, to be lodged in bones, ligaments, muscles, and close cavities, how are they returned ? How shall we dispose of them ? And by what means, when the body is wasting, shall the decayed parts be carried off? As these fluids do not ac ceintilate in the healthy state of a living body, there if.u,t, of course, be some way by which they get out, or some other system of vessels besides veins, by which the v return to the mass of blood. For the solution of this difficulty we are chiefly indebted to Rudbeck and Bartholint, ho discovered about the same time, a sys tem of vessels that convey back to the blood all the de cayel parts, and all those thin and pellucid fluids which wander from the course of circulation. From the clear aid watery appearance of the fluid which they contain, they were named lyinIthatic8 ; they arise from all the in ternal cavities, and from every organ in the whole body. Still more numerous than the veins, they branch out in a similar manner, and terminate in two trunks. whose con tents are emptied into the veins, a little before they enter the heart.
Haying thus conducted the lymphatic fluid, and all the decayed parts of the system, back to the blood, our next concern must be how to dispose of them. It is very evident that these fluids are conveyed hack, either that they may undergo new preparations to fit them again for the situations which they formerly occupied, or to be thrown by some evacuants out of the system. If, after undergoing new preparations, they are to return to the situations which they formerly occupied, what oc casion is there for constant and regular supplies of ali ment ? For if the supplies of aliment be necessary, it will be difficult to assign a reason why the decayed parts of the system should be retained. Supplies of aliment, however, are necessary, and therefore it lollows, that the decayed parts of the system must be thrown out. This conclusion involves us in new embarrassment ; For by what channels are we to discharge them ? For this pur pose we must resort again to the vessels which wander front the course of circulation ; for we observe, that many of these open externally upon the skin, and, on the internal surface of the lungs, into two large glands kidnie8, and into that winding canal which begins at the mouth and ends at the orifice, which is named a0z8. By these outlets, then, we can explain how, along with the vapour, from the lungs, the perspirable matter Of the skin, the fecal discharge, and the urine, all the decayed parts arc evacuated.