meters high; or, in the parlance of the laboratory, has been found equal to from 24 to 54 millimeters of mercury.'

Summary of the Capillary Flow.-Whether in the lungs or in the rest of the body, the general characters of the capillary flow, as learned from direct inspection and from experiment, may be summed up as follows: The blood moves through the capillaries toward the veins with much friction, continuously, slowly, without pulse, and under low pressure. To account for these facts is to deal systematically with the mechanics of the circulation; and to that task we must now address ourselves.

C. THE PRESSURE OF THE BLOOD IN THE ARTERIES, CAPILLARIES, AND VEINS.

Why does the blood move continuously out of the arteries through the capillaries into the veins? Because there is continuously a high pressure of blood in the arteries and a low pressure in the veins, and from the seat of high to that of low pressure the blood must continuously flow through the capillaries, where pressure is intermediate, as already stated.

Method of Studying Arterial and Venous Pressure, and General Results. Before stating quantitatively the differences of pressure, we must see how they are ascertained for the arteries and veins. The method of obtaining the capillary pressure has been referred to already. If, in the neck of a mammal, the left common carotid artery be clamped in two places, it can, without loss of blood, be divided between the clamps, and a long straight glass tube, open at both ends, and of small calibre, can be tied into that stump of the artery which is still connected with the aorta, and which is called the "proximal" stump. If now the glass tube be held upright, and the clamp be taken off which has hitherto closed the artery between the tube and the aorta, the blood will mount in the tube, which is open at the top, to a considerable height, and will remain there. The external jugular vein of the other side should have been treated in the same way, but its tube should have been inserted into the "distal" stump-that is, the stump connected with the veins of the head, and not with the subclavian veins. If the clamp between the tube and the head have been removed at nearly the same time with that upon the artery, the blood may have mounted in the upright venous tube also, but only to a small distance. To cite an actual case in illustration, in a small etherized dog the arterial blood-column has been seen to stand at a height of about 155 centimeters above the level of the aorta, the height of the venous column about 18 centimeters above the same level. The heights of the arterial and venous columns of blood measure the pressures obtaining within the aorta and the veins of the head respectively, while at the same time the circulation continues to be free through both the aorta and the venous network. Therefore, in the dog above referred to, the aortic pressure was between eight and nine

1 N. v. Kries: "Ueber den Druck in den Blutcapillaren der menschlichen Haut," Berichte über die Verhandlungen der k. sächsischen Gesellschaft der Wissenschaften zu Leipzig, math.-physische Classe, 1875, S. 149.

times as great as that in the smaller veins of the head. As, during such an experiment, the blood is free to pass from the aorta through one carotid and both vertebral arteries to the head, and to return through all the veins of that part, except one external jugular, to the vena cava, it is demonstrated that there must be a continuous flow from the aorta, through the capillaries of the head, into the veins, because the pressure in the aorta is many times as great as the pressure in the veins. Obviously, such an experiment, although very instructive, gives only roughly qualitative results.

Two things will be noted, moreover, in such an experiment. One is that the venous column is steady; the other is that the arterial column is perpetually fluctuating in a rhythmic manner. The top of the arterial column shows a regular rise and fall of perhaps a few centimeters, the rhythm of which is the same as that of the breathing of the animal; and, while the surface is thus rising and falling, it is also the seat of frequent flickering fluctuations of smaller extent, the rhythm of which is regular, and agrees with that of the heart's beat. At no time, however, do the respiratory fluctuations of the arterial column amount to more than a fraction of its mean height; compared to which last, again, the cardiac fluctuations are still smaller. It is clear, then, that the aortic pressure changes with the movements of the chest, and with the systoles and diastoles of the left ventricle. But stress is laid at present upon the fact that the aortic pressure at its lowest is several times as high as the pressure in the smaller veins of the head. Therefore, the occurrence of incessant fluctuations in the aortic pressure cannot prevent the continuous movement of the blood out of the arteries, through the capillaries, into the veins.

The upright tubes employed in the foregoing experiment are called "manometers." They were first applied to the measurement of the arterial and venous blood-pressures by a clergyman of the Church of England, Stephen Hales, rector of Farringdon in Hampshire, who experimented with them upon the horse first, and afterward upon other mammals. He published his method and results in 1733.2 The height of the manometric column is a true measure of the pressure which sustains it; for the force derived from gravity with which the blood in the tube presses downward at its lower opening is exactly equal to the force with which the blood in the artery or vein is pressed upward at the same opening. The downward force exerted by the column of blood varies directly with the height of the column, but, by the laws of fluid pressure, does not vary with the calibre of the manometer, which calibre may therefore be settled on other grounds. It follows also that the arterial and venous manometers need not be of the same calibre. Were, however, another fluid than the blood itself used in the manometer to measure a given intravascular pressure, as is easily possible, the height of the column would differ from that of the column of blood. For a given pressure the height

1 From μavós, rare. The name was given from such tubes being used to measure the tension of gases.

2 Stephen Hales: Statical Essays: containing Hamastaticks, etc., London, 1733, vol. ii. p. 1.

of the column is inverse to the density of the manometric fluid. For example, a given pressure will sustain a far taller column of blood than of mercury.

The

The Mercurial Manometer.The method of Hales, in its original simplicity, is valuable from that very simplicity for demonstration, but not for research. clotting of the blood soon ends the experiment, and, while it continues, the tallness of the tube required for the artery, and the height of the column of blood, are very inconvenient. It is essential to understand next the principles of the more exact instruments employed in the modern laboratory.

In 1828 the French physician and physiologist J. L. M. Poiseuille devised means both of keeping the blood from clotting in the tubes, and of using as a measuring fluid the heavy mercury instead of the much lighter blood. He thereby secured a long observation, a low column, and a manageable manometer.1 The "mercurial manometer" of to-day is that of Poi

T

RT

P

D

R

B

L

F

M

FIG. 15.-Diagram of the recording mercurial manometer and the kymograph; the mercury is indicated in

seuille, though modified (see Fig. deep black: M, the manometer, connected by the leaden

pipe, L, with a glass cannula tied into the proximal stump of the left common carotid artery of a dog; A,

15). In an improved form it consists of a glass tube open at both ends, and bent upon itself to the shape of the letter U. This is held upright by an iron frame. If mercury be poured into one branch of the U, it will fill both branches to an equal height. If fluid be driven down upon the mercury in one branch or "limb" of the tube, it will drive some of the mercury out of that limb into the other, and the two surfaces of the mercury may come to rest at very unequal levels. The difference of level, expressed in millimeters, 1 J. L. M. Poiseuille: Recherches sur la force du cœur aortique, Paris, 1828.

the aorta; C, the stop-cock, by opening which the manrubber tube, with a pressure-bottle of solution of sodium carbonate; F, the float of ivory and hard rubber; R, the

ometer may be made to communicate through RT, the

light steel rod, kept perpendicular by B, the steel bearing; P, the glass capillary pen charged with quickly drying ink; T, a thread which is caused, by the weight of a

light ring of metal suspended from it, to press the pen

obliquely and gently against the paper with which is covered D, the brass "drum" of the kymograph, which drum revolves in the direction of the arrow. The sup

ports of the manometer and the body and clock-work

The aorta and its branches are drawn disproportionately large for the sake of clearness.

of the kymograph are omitted for the sake of simplicity.

measures the height of the manometric column of mercury the downward pressure of which in one limb of the tube is just equal to the downward pressure of the fluid in the other. In order to adapt this "U-tube" to the study of the blood-pressure, that limb of the tube which is to communicate with the artery or vein is capped with a cock which can be closed. Into this same limb, a little way below the cock, opens at right angles a short straight glass tube, which is to communicate with the blood-vessel through a long flexible tube of lead, supported by the iron frame, and a short glass cannula tied into the blood-vessel itself. Two short pieces of india-rubber tube join the lead tube to the manometer and the cannula. Before the blood-vessel is connected with the manometer, the latter is filled with fluid between the surface of the mercury next the bloodvessel and the outer end of the lead tube, which fluid is such that when mixed with blood it prevents or greatly retards coagulation. With this same fluid the glass cannula in the blood-vessel is also filled, and then this cannula and the lead tube are connected. The cock at the upper end of the "proximal limb" of the manometer is to facilitate this filling, being connected by a rubber tube with a "pressure bottle," and is closed when the filling has been accomplished. The fluid introduced by Poiseuille and still generally used is a strong watery solution of sodium carbonate. A solution of magnesium sulphate is also good. If, in injecting this fluid, the column of mercury in the “distal limb" is brought to about the height which is expected to indicate the blood-pressure, but little blood will escape from the blood-vessel when the clamp is taken from it, and coagulation may not set in for a long time.

The Recording Mercurial Manometer and the Graphic Method.— When the arterial pressure is under observation, the combined respiratory and cardiac fluctuations of the mercurial column are so complex and frequent that it is very hard to read off their course accurately even with the help of a millimeter-scale placed beside the tube. In 1847 this difficulty led the German physiologist Carl Ludwig to convert the mercurial manometer into a self-registering instrument. This invention marked an epoch not merely in the investigation of the circulation, but in the whole science of physiology, by beginning the present "graphic method" of physiological work, which has led to an immense advance of knowledge in many departments. Ludwig devised the "recording manometer" by placing upon the mercury in the distal air-containing limb of Poiseuille's instrument an ivory. float, bearing a light, stiff, vertical rod (see Fig. 15). Any fluctuation of the mercurial column caused float and rod to rise and fall like a piston. The rod projected well above the manometer, at the mouth of which a delicate bearing was provided to keep the motion of the rod vertical. A very delicate pen placed horizontally was fastened at right angles to the upper end of the rod. If a firm vertical surface, covered with paper, were now placed lightly in contact with the pen, a rise of the mercury would cause a corresponding vertical line to be marked upon the paper, and a succeeding fall would cause the descending pen to inscribe a second line covering the first. If now the vertical surface were made to move past the pen at a uniform rate,

the successive up-and-down movements of the mercury would no longer be marked over and over again in the same place so as to produce a single vertical line. The space and time taken up by each fluctuation would be graphically recorded in the form of a curve, itself a portion of a continuous trace marked by the successive fluctuations; thus both the respiratory and cardiac fluctuations could be registered throughout an observation by a single complex curving line. Ludwig stretched his paper around a vertical hollow cylinder of brass, made to revolve at a regular known rate by means of clock-work, and the conditions above indicated were satisfied' (see Fig. 15). Upon the surface of such a cylinder vertical distance represents space, and a vertical line of measurement is called, by an application of the language of mathematics, an "ordinate;" horizontal distance represents time, and a horizontal line of measurement is called an "abscissa." The curve marked by the events recorded is always a mixed record of space and time. The instrument itself, the essential part of which is the regularly revolving cylinder, is called the kymograph." It has undergone many changes, and many varieties of it are in use. Any motor may be used to drive the cylinder, provided that the speed of the latter be uniform and suitable.

2

The curve written by the manometer or other recording instrument may either be marked upon paper with ink, as in Ludwig's earliest work; or may be marked with a needle or some other fine pointed thing upon paper black

BL

T

FIG. 16.-The trace of arterial blood-pressure from a dog anæsthetized with morphia and ether. The cannula was in the proximal stump of the common carotid artery. The curve is to be read from left to right.

P, the pressure-trace written by the recording mercurial manometer;

B L, the base-line or abscissa, representing the pressure of the atmosphere. The distance between the base-line and the pressure-curve varies, in the original trace, between 62 and 77 millimeters, therefore the pressure varies between 124 and 154 millimeters of mercury, less a small correction for the weight of the sodium-carbonate solution;

T, the time-trace, made up of intervals of two seconds each, and written by an electro-magnetic chronograph.

ened with soot over a flame. The trace written upon smoked paper is the more delicate. After the trace has been written, the smoked paper is removed from the kymograph and passed through a pan of shellac varnish. This

1 C. Ludwig: "Beiträge zur Kenntniss des Einflusses der Respirationsbewegungen auf den Blutlauf im Aortensysteme," Müller's Archiv für Anatomie, Physiologie, und wissenschaftliche Medicin, etc., 1847, S. 242. 2 From кuμа, a wave.