In order to understand the action of drugs on the circulation it is absolutely necessary to have a clear idea regarding the effect of the heart and capillaries in maintaining the blood-pressure. This is best obtained by using a schema which can be easily made from a spray-apparatus (Fig. 85). By removing the glass or metal tube from one of these, and attaching a nozzle with a small stopcock to the india-rubber tube in its stead, we obtain a very good schema of the circulation; and, by imitating on it the changes which occur in the heart and vessels, we may form a much clearer idea of them than we could otherwise do. The india-rubber ball will represent the heart; the elastic bag, surrounded by netting, will represent the elastic aorta and larger arteries; and the stopcock, which regulates the size of the aperture through which the air escapes, will represent the small arteries and capillaries, whose contraction or dilatation regulates the flow of blood from the arteries into the veins. We may judge of the tension in the arteries by the distension of the bag, or still better, we may connect the tube between it and the stopcock with a mercurial manometer, and estimate the tension by the height of the mercurial column which it sustains. If we turn the stopcock so as to present some resistance to the escape of air, and then compress the india-rubber ball, very little air will issue from the stopcock even while we are squeezing the ball; the greater part of it goes to distend the bag; and, when we cease to compress the ball, very little air passes through the stopcock. At the next squeeze, the bag becomes a little more distended; and a little more air passes through the stopcock, not only while we are compressing the ball, but even when we relax our grasp. At each squeeze of the ball, the elastic bag becomes tighter, till it is so tense, and contracts so strongly on the air inside, that it can press all the extra amount of air, forced into it when the ball was compressed, through the stopcock during the time when the ball is relaxed. When this is the case, every time we squeeze the ball we see the bag become a little fuller, and air issue more quickly from the nozzle. At each relaxation, while the ball is refilling, the bag gets a little slacker, and the air passes out of the nozzle a little more slowly, but never stops entirely. During the time the ball is filling, the valves between it and the bag and nozzle are closed, and cut it off from any connection with them. All this time, then, the stream of air from the nozzle must be entirely independent of the ball; it is produced by the contraction of the elastic bag, and by it alone. The bag may be stretched, and the tension of its walls increased in consequence, in two ways: first, by working the ball more quickly or compressing it more completely; second, by lessening the opening of the nozzle, and thus hindering the passage of air through it. One trial will, I think, be enough to show how much easier it is to alter the pressure by changing the size of the nozzle than by any alteration in the working of the ball, and to prove that alterations in blood-pressure probably depend much more on alterations in the lumen of the small arteries than on changes in the action of the heart.

Fig. 85.   Simple schema of the circulation, consisting of a spray producer, bladder, and mercurial manometer. The elastic ball represents the heart; the elastic bag, covered with netting to prevent too great distension, represents the aorta and arterial system, and the bladder represents the venous system.

Fig. 85. - Simple schema of the circulation, consisting of a spray-producer, bladder, and mercurial manometer. The elastic ball represents the heart; the elastic bag, covered with netting to prevent too great distension, represents the aorta and arterial system, and the bladder represents the venous system.

But our schema, as it at present exists, is not a perfect representation of the heart and vessels; for it draws its air from an inexhaustible reservoir, the atmosphere, and is not obliged each time to use that amount alone which it had previously driven through the nozzle; while the heart can only use the blood which has been forced by it through the capillaries and returned to it by the veins. In order to make our schema complete, we must connect its two ends by tying them into a bladder or large thin caoutchouc bag (such as is used, after inflation, as a toy for children), so that the air shall pass into it from the nozzle and be sucked out of it by the elastic ball. This will represent the veins. If we then repeat the experiment just described, we shall find that, when we begin to work the ball and stretch the elastic bag representing the arteries, the bladder representing the veins becomes empty and collapsed; and just in proportion as we fill the bag do we empty the bladder. If we now stop, the air will gradually escape from the bag to the bladder, till the air in both is of equal tension, as at first.