When arteries inosculate, two currents of blood, moving in opposite directions, must come together, and retard each other's motion. This probably is the rea son, why larger arteries, in which the blood flows with rapidity, so seldom con join ; whilst the smaller ones, in which the blood's motion is more tardy, commu nicate in surprising numbers, and with a frequency proportionate to their minute ness. The very frequent communications of the minute arteriesprevent the preju dicial consequences of obstruction of the trunks almost as effectually, as if those ar teries themselves communicated by more direct and larger channels.
All these minute arterial tubes are capa ble of enlargement ; and it is an ascertain ed fact, that even the aorta itself may be gradually obstructed at some distance from the heart, without the parts which it sup plies being deprived of nourishment. From an attentive • consideration of all these circumstances, it has been conclud ed, that the moderate increase of the area of the branches of large arteries; the acute angles at which they divide ; their dearly rectilinear course ; and the rare oc currence of inosculation between them ; are designed to facilitate the rapid motion of the blood in them, so that it may arrive unchanged, and in the same state that it MS in when projected from the heart, at that part of the body, for the nourishment of which it was intended : on the contrary, the great increase of the area of the smaller vessels, the variety oftheir an gles, 'Weir tortuous course, and their fre quent communications, were designed to check the velocity of the blood's motion, when it has arrived at that part, where secretion is to be performed, and nutrition is to take place. Contrary opinions have indeed been maintained; and for the fur ther discussion of this subject, we must refer the reader to the remarks on the circulation in the article PUTRIOLOGT.
Termination of the arteries.—When these vessels have become very minute, they terminate in two ways; they either turn back again, and become veins, and return the blood to the heart, or they send oft' fine vessels, which abstract something from the circulating blood, and are there fore called secerning arteries. Though none but minute arteries are ever reflect ed to become veins, yet many of them are of sufficient magnitude to admit common waxen injection ; and when this experi ment succeeds, the continuity of the arte ries and veins is very manifest It seems therefore to follow from this facility of communication, that the mass of the blood is constantly and freely circulating, in or der to undergo that change which is ef fected in the lungs, whilst but a small part of it proceeds into the very minute arte ries, for the purpose of having secretions made from it. For these arteries, however
minute, must be considered large, in com parison with the exility of others, which cannot be injected with wax, and even reject the red globules of the blood, or admit them in such small proportion, that they do not impart the red colour to the fluid which moves in those vessels. Now, we may venture to affirm that these glo bules do not much exceed in diameter the 150,000th part of an inch, which cir cumstance sufficiently shows the minute ness of the lesser arteries.
The sccerning arteries are in general too minute to admit of demonstration ; they arc however evident in some glands; in the kidney, for instance, they may be seen continued into the excretory vessels. Subtile injections, when thrown into the larger arterial trunks, ooze out on the sur faces of membranes, and into the cellular substance,ancl they are generally supposed to be poured forth from the open orifices of secerning arteries. Analogy, therefore, rather than actual demonstration, leads us to believe, that the secerning arteries ab stract the particles of nutrition, or the ma terials which compose the fabric of the body, from the circulating fluids, and dc posit them from their open mouths, so as by this means to build up and keep in re pair the structure of the body.
Distribution of the griat artery, whose branches supply the whole of the body, is named the aorta. It arises from the upper part of the left ventricle ; and emerges from the heart, between the pulmonary artery and the right auricle. It first ascends in the chest ; opposite the upper edge of the second rib it bends backwards till it reaches the left side of the spine, in which situation it descends from the fourth or fifth dorsal to the last lumbar vertebra.
By the arch of the aorta is meant that part of the vessel which arises from the heart, and bends across the chest It sends off the following branches : the two first arising at right angles close to the heart ; the three following from the con vexity of the arch : 1. Right coronary artery of the heart.