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wave from the larger to the smaller arteries, takes longer to reach and distend the more distant branch.
17. Fourthly, when an artery is cut, the outflow of the fluid which it contains is increased by jerks, the intervals of which correspond with the intervals of the beats of the heart. The cause of this is plainly the same as that of the pulse; the force which would be employed in distending the walls of the artery, were the latter entire, is spent in jerking the fluid out when the artery is cut.
18. Fifthly, under ordinary circumstances, the pulse is no longer to be detected in the capillaries, or in the veins. This arises from several circumstances. One of them is that the capacity of the branches of an artery is greater than the capacity of its trunk, and the capacity of the capillaries, as a whole, is greater than that of all the small arteries put together. Hence, supposing the capacity of the trunk to be 10, that of its branches 50, and that of the capillaries into which these open 100, it is clear that a quantity of fluid thrown into the trunk, sufficient to dilate it by one-tenth, and to produce a very considerable and obvious effect, could not distend each branch by more thanth, and each capillary byth of its volume, an effect which might be quite imperceptible.
19. But this is not all. Did the pulse merely become indistinguishable on account of its division and dispersion among so many capillaries, it ought to be felt again when the blood is once more gathered up into a few large venous trunks. But it is not. The pulse is definitely lost at the capillaries. There is, under ordinary circumstances, no pulse whatever in the veins, except sometimes a backward pulse from the heart along the great venous trunks; but this is quite another matter.
This actual loss, or rather transformation of the pulse, is effected by means of the elasticity of the arterial walls, in the following manner.
In the first place it must be borne in mind that, owing to the minute size of the capillaries and small arteries, the amount of friction taking place in their channels when the blood is passing through them is very great; in other words, they offer a very great resistance to the passage of the blood.
The consequence of this is, that the blood cannot get
through the capillaries, in spite of the fact that their total area is so much greater than that of the aorta, into the veins so fast as it is thrown into the arteries by the heart. The whole arterial system, therefore, becomes over distended with blood.
Now we know by experiment that under such conditions as these, an elastic tube has the power, if long enough and elastic enough, to change a jerked impulse into a continuous flow. If a syringe (or one of the elastic bottles now so frequently in use) be fastened to one end of a long glass tube, and water be pumped through the tube, it will flow from the far end in jerks, corresponding to the jerks of the syringe. This will be the case whether the tube be quite open at the far end, or drawn out to a fine point so as to offer great resistance to the outflow of the water. The glass tube is a rigid tube, and there is no elasticity to be brought into play. If now a long indiarubber tube be substituted for the glass tube, it will be found to act differently, according as the opening at the far end is wide or narrow.
If it is wide, the water flows out in jerks, nearly as distinct as those from the glass tube. There is little resistance to the flow, little distension of the india-rubber tube, little elasticity brought into play.
If, however, the opening be narrowed, as by fastening to it a stopcock or a glass tube drawn to a point, or if a piece of sponge be thrust into the end of the tube; if, in fact, in any way resistance be offered to the outflow of the water, the tube becomes distended, its elasticity is brought into play, and the water flows out from the end, not in jerks but in a stream, which is more and more completely continuous the longer and more elastic the tube.
Substitute for the syringe the heart, for the stopcock or sponge the capillaries and small arteries, for the indiarubber tube the whole arterial system, and you have exactly the same result in the living body. Through the action of the elastic arterial walls the separate jets from the heart are blended into one continuous stream. The whole force of each blow of the heart is not at once spent in driving a quantity of blood out of the capillaries; a part only is thus spent, the rest goes to distend the elastic arteries. But during the interval between that beat
and the next the distended arteries are narrowing again, by virtue of their elasticity, and so are pressing the blood on into the capillaries with as much force as they were themselves distended by the heart. Then comes another beat, and the same process is repeated. At each stroke the elastic arteries shelter the capillaries from part of the sudden blow, and then quietly and steadily pass on that part of the blow to the capillaries during the interval between the strokes.
The larger the amount of elastic arterial wall thus brought into play, i. e. the greater the distance from the heart, the greater is the fraction of each heart's stroke which is thus converted into a steady elastic pressure between the beats. Thus the pulse becomes less and less marked the farther you go from the heart; any given length of the arterial system, so to speak, being sheltered by the lengths between it and the heart.
Every inch of the arterial system may, in fact, be considered as converting a small fraction of the heart's jerk into a steady pressure, and when all these fractions are summed up together in the total length of the arterial system no trace of the jerk is left.
As the effect of each systole be omes diminished in the smaller vessels by the causes above mentioned, that of this constant pressure becomes more obvious, and gives rise to a steady passage of the fluid from the arteries towards the veins. In this way, in fact, the arteries perform the same functions as the air-reservoir of a fireengine, which converts the jerking impulse given by the pumps into the steady flow of the delivery hose.
20. Such is the general result of the mechanical conditions of the organs of the circulation combined with the rhythmical activity of the heart. This activity drives the fluid contained in these organs out of the heart into the arteries, thence to the capillaries, and from them through the veins back to the heart. And in the course of these operations it gives rise, incidentally, to the beating of the heart, the sounds of the heart, and the pulse.
It has been found, by experiment, that in the horse it takes about half a minute for any substance, as for instance a chemical body, whose presence in the blood can easily be recognized, to complete the circuit, ex. gr, to pass
from the jugular vein down through the right side of the heart, the lungs, the left side of the heart, up through the arteries of the head and neck, and so back to the jugular vein.
By far the greater portion of this half minute is taken up by the passage through the capillaries, where the blood moves, it is estimated, at the rate only of about one and a half inches in a minute, whereas through the carotid artery of a dog it flies along at the rate of about ten inches in a second.
Of course to complete the circuit of the circulation, a blood-corpuscle need not have to go through so much as half of an inch of capillaries in either the lungs or any of the tissues of the body.
Inasmuch the force which drives the blood on is (putting the other comparatively slight helps on one side) the beat of the heart and that alone, however much it may be modified, as we have seen, in character, it is obvious that the velocity with which the blood moves must be greatest in the aorta and diminish towards the capillaries.
For with each branching of the arteries the total area of the arterial system is increased, the total width of the capillary tubes if they were all put together side by side being very much greater than that of the aorta. Hence the blood, or a corpuscle, for instance, of the blood being driven by the same force, viz. the heart's beat, over the whole body, must pass much more rapidly through the aorta than through the capillary system or any part of that system.
It is not that the greater friction in any capillary causes the blood to flow more slowly there and there only. The resistance caused by the friction in the capillaries is thrown back upon the aorta, which indeed feels the resistance of the whole vascular system; and it is this total resistance which has to be overcome by the heart before the blood can move on at all.
The blood driven everywhere by the same force simply moves more and more slowly as it passes into wider and wider channels. When it is in the capillaries it is slowest; after escaping from the capillaries, as the veins unite into
larger and larger trunks, and hence as the total venous area is getting less and less, the blood moves again faster and faster for just the same reason that in the arteries it moved slower and slower.
A very similar case is that of a river widening out in a plain into a lake and then contracting into a narrow stream again. The water is driven by one force throughout (that of gravity). The current is much slower in the lake than in the narrower river either before or behind.
21. It is now necessary to trace the exact course of the circulation as a whole. And we may conveniently commence with the portion of the blood contained at any moment in the right auricle. The contraction of the right auricle drives that fluid into the right ventricle; the ventricle then contracts and forces it into the pulmonary artery; from hence it passes into the capillaries of the lungs. Leaving these, it returns by the four pulmonary veins to the left auricle; and the contraction of the left auricle drives it into the left ventricle.
The systole of the left ventricle forces the blood into the aorta. The branches of the aorta convey it into all parts of the body except the lungs; and from the capillaries of all these parts, except from those of the intestines and certain other viscera in the abdomen, it is conveyed, by vessels which gradually unite into larger and larger trunks, into either the superior or the inferior vena cava, which carry it to the right auricle
But the blood brought to the capillaries of the stomach and intestines, spleen and pancreas, is gathered into veins which unite into a single trunk-the vena porta. The vena portæ distributes its blood to the liver, mingling with that supplied to the capillaries of the same organ by the hepatic artery. From these capillaries it is conveyed by small veins, which unite into a large trunk-the hepatic vein, which opens into the inferior vena cava. The flow of the blood from the abdominal viscera, through the liver, to the hepatic vein, is called the portal circulation.
The heart itself is supplied with blood by the two coronary arteries which spring from the root of the aorta just above two of the semilunar valves. The blood from