116. CHAPTER II. THE ARTERIES AND VEINS, THEIR TUNICS, AND THE CIRCULATION OF THE BLOOD.
VERHEYEN. "The tunics of the larger arteries are four in number; but the parietes of the minute branches are so thin that they appear to be composed rather of a single tunic than of four.... The external tunic of the arteries ... has nervous twigs ramifying and winding over it in different directions, and is covered by an extremely dense and as it were retiform texture of vessels of every description, but the chief of which are blood vessels; as is best shown in the bodies of those who have recently died without loss of blood. To this retiform or vascular tunic succeeds a second, which is properly termed the glandular tunic, because it is beset throughout by minute whitish glands. This second tunic is very thick, and may be readily separated into several layers, and exhibits also in the larger arterial trunks a number of small vessels. The third tunic, reckoning from without, is muscular or fibrous, consisting of annular fibres set thickly together. These fleshy and motive fibres surrounding the arterial tube, are not disposed in a thin and single series, as in the venous coat, but aggregated and superimposed one upon another, so as to constitute a membrane of considerable thickness. The fourth and internal tunic is the thinnest, end is almost entirely membranous, or if you prefer the term, nervous. Its fibrillae extending longitudinally, out the annular fibres of the preceding tunic at right angles. In the neighborhood of the heart these fibres are thicker and as it were fleshy.. ..
"The tunics of the veins are almost of the same structure with those of the arteries, but thinner, and arranged in a different order. Thus the membranous tunic, formed of various fibres running lengthwise, is the first; but its fibres do not lie parallel, as in the fourth arterial tunic, but often intersect each other. The second tunic is vascular, like the first tunic of the artery. The third is glandular, like the second of the artery. The fourth and inmost tunic, like the third tunic of the artery, consists of annular fibres, but thinner, and arranged in a very simple series....
"The vena portae has thicker coats, so as to appear in a measure to be an intermediate between the other veins and the arteries....
"The veins are much larger and more capacious than the arteries.... Nevertheless a much greater quantity of blood flows through the arteries than through the veins, if you except that part of the vena cava that lies between the insertion of the [thoracic or] chyliferous duct, and the right ventricle of the heart. In dead subjects almost all the blood is found collected in the veins, while the arteries are usually empty.... In the cavity of the veins, at different intervals, there are certain very thin, light pellicles, called valves, which at one extremity, namely, toward the smaller twigs, are continuous with the interior venous coat, while at the other [namely, toward the heart] their edges are free....They are frequently found near the divisions of the veins, or the junction of the branches; and sometimes are single, sometimes in pairs, sometimes in triplets, and company. The latter are placed anteriorly and posteriorly with respect to the body, while the former [or the single] have a lateral position; or vice versa. (Corp. Hum. Anat., lib. i., tr. i., cap. iv., p. 13, 15; Bruxell., 1710.)
"If the crural vessels be exposed and tied (for instance, in a living dog), the vein swells up in its narrower part, that is to say, on the side of the ligature farthest from the heart, while the part that is broader and nearest to the heart collapses. The contrary happens with the arteries.
"As it is the office of the venous twigs to receive the blood from the little arteries, so we find that these two sets of vessels constantly either accompany or meet each other: but the larger venous branches are sometimes bound up with the arteries, and sometimes separate from them and indeed not unfrequently the veins mount up over the arteries.... Moreover, not only are blood vessels of different kinds connected with each other, but also blood vessels of the same kind; frequently, that is to say, veins with veins, and arteries with arteries; so much so, indeed, that there are scarcely any considerable twigs running from the same trunk to the same region, but after their division again unite, at any rate by minute anastomoses. Most anatomists have hitherto considered such anastomoses to be confined to the regions of the head and uterus;... but they are beautifully seen externally in the skin of a foetus of six or seven months. (Ibid., p. 12, 13.)
"The circulatory motion or circulation of the blood is as follows. The blood passes from the trunk of the vena cave, and from certain of its branches, into the right ventricle of the heart, in part immediately, in part mediately through the right auricle. From the right ventricle it is expelled by the contraction of the heart into the pulmonary artery, by the little branches of which, distributed through the lungs, it is conveyed into the twigs of the pulmonary vein, and passes along the trunk of this vein to the left cavity of the heart, in part immediately, and in part mediately through the left auricle. From the left ventricle it is expelled by the contraction of the heart into the aorta, and by the ramifications of this vessel is distributed all over the body. That portion of the blood that is neither converted into solid substance, nor into any other matter extraneous to the circulation, passes from the little branches of the aorta into the little branches of the veins, and at length comes back through the trunk of the vena cava to the right ventricle of the heart, from which it started....
"If an artery of a dog be tied, and punctured with a lancet above the ligature on the side next the heart, provided the animal be healthy, the blood will spirt out in a strong jet to the height of several cubits; and if the artery be left open for any length of time, the dog will die from loss of blood. If, however, the puncture be made on the other side of the ligature, very little if any blood will escape. But if a vein be punctured, the contrary will happen: on the side of the ligature next the heart no blood will escape, while on the other side a copious stream will be emitted, although not with such force as when the blood escapee from the arteries....
"The sagacious and penetrating Leeuwenhoek endeavors to prove (Epist. 67) that the blood does not flow more rapidly through the large vessels than through the small. To demonstrate this position he constructed two tubes, and made them communicate with each other by means of smaller tubes, so that all together formed a continuous pipe. He does, in fact, prove that the liquid sent through one of the larger tubes moves with equal rapidity through the other. With respect to the smaller tubes, it is evident, that unless collectively they are of a calibre not inferior to that of the larger tube, the liquid must run through them more rapidly then through the larger....The same most curious inquirer watched through a microscope the circulation of the blood in an eel (Epist. 67), and found it so rapid, that in the space of one hour it amounted to a distance of 288 inches ... or 24 feet.... [From this] he concludes, that in a man the blood circulates from the heart to the extremities of the feet and back again, only twice and two thirds in an hour. And that in the same time it completes four times and a little more than one third of the whole circle, through the extremities of the fingers; eight times to the extremities of the head;... and that within the hour, 14 times the quantity of blood in the body is forced from the heart," and company. (Ibid., lib. ii., tr. iv., cap. iv., p. 263, 269.)
