The first attempt at direct measurement of blood pressure was made by the Rev. Stephen Hales about the middle of the last century, who, wishing to compare the motion of fluids in animals with that in plants, connected a tube with an artery of a living animal, and found that the blood was ejected with considerable force, and that when the artery of ahorse was brought into union with a long upright tube, the blood reached a height of about three yards.

The column of blood is not now used as a measure, because so much blood leaving the vessels tends to empty them and to reduce the pressure in the arteries; besides, the coagulation of the blood soon stops the experiment. We now employ the mercurial manometer, which consists of a column of mercury in a U-shaped tube. To prevent coagulation, the tube between the mercury and blood is filled with a solution of sodium carbonate, the pressure being regulated to equalize as nearly as possible that of the blood. A rod is made to float upon the mercury, in the open side of the tube, and to the upper extremity of this a writing apparatus can be attached, so that by the movements of the mercury, a graphic record of the blood pressure and its variation can be traced on a regularly moving surface. This instrument, known as Ludwig's Kymograph, is that used in all ordinary measurements and experiments on blood pressure.

In order to overcome the inertia of the mercurial column, another manometer has been devised, which will be mentioned in speaking of the character of the curve (p. 302). When an experiment of long duration has to be made, a recorder with a rolled strip of paper can be employed (Fig. 133).

The modern accurate methods of research have taught us the differences in pressure that exist in the various parts of the vascular system. However, direct measurement can only be accomplished in vessels of such a size as to admit a cannula, hence the pressure in the capillaries in the very minute arteries and veins can only indirectly be estimated. The pressure in all parts of the vascular system is subject to frequent variations to be presently mentioned, but this table may be useful in giving a general idea of the average permanent differences that exist in the different vessels of large animals and man.

Ludwig's Kymograph with continuous paper.

Fig. 133. Ludwig's Kymograph with continuous paper.

The instrument consists of an iron table, above which the recording surface is slowly drawn past the writing points from an endless roll of paper on the left by the motion of the cylinder (C), and rolled up on a spindle next the driving-wheel on the right. The mercurial manometers are so fixed on (D) that the open ends come in front of the firm roller upon which the paper rests. The writing style can be seen rising from these tubes while the other limbs of the manometers lead through the stop-cocks to the tubes which are in communication with the blood vessels. The time is recorded by means of a pen attached to the electro-magnet (M), which by a "breaking" clock is demagnetized every second. The moment at which a stimulus is applied is marked on the zero line by a key to which another pen is attached near the time marker.

Large arteries (Carotid, Horse) + 160 mm., mercury. Medium " (Brachial, Man) +120 mm., " Capillaries of Finger + 38 mm., "

Small Veins of Arm + 9 mm., "

Large Vein of Neck - 1 to - 3 mm., "

If the different parts of the circulation be represented on the base line h. a. c. v., these letters corresponding to heart, arteries, capillaries, and veins respectively, and if the height of the blood pressure be represented on the vertical line in mm. Hg., the curve h, a, c, v, would give about the relative pressure in the various parts of the circulation. This shows that in the receiving chamber of the heart the pressure is negative, while the ventricular pump drives it to the height of the arterial pressure 160 mm. Hg. In the arteries the pressure is everywhere high, while just before the blood reaches the capillaries a sudden fall occurs. The variation after this is merely a gentle descent until the large venous trunks are reached, where the blood pressure is below zero, i. e., below the pressure of the atmosphere.

Diagram showing the relative height of the blood pressure in the different regions of the vessels.

Fig. 134. Diagram showing the relative height of the blood pressure in the different regions of the vessels.

H. Heart, a. Arterioles, v. Small Veins. A. Arteries, c. Capillaries. V. Large Veins. H. V. being the zero line (= atmospheric pressure), the pressure is indicated by the height of the curve. The numbers on the left give the pressure (approximately) in mm. of mercury.

From a purely physical point of view the ventricle may be regarded as pumping the blood up to an elevated high-pressure reservoir of small capacity (the arteries), from which it rapidly falls by numerous outlets into an expansive, low-lying irrigation basin (the wide capillaries), while it slowly trickles back to the well (the auricle), which lies below the surface pressure.

From this diagram the following points can be gathered: i. The great difference between the pressure on the arterial and venous sides of the circulation.

2. The comparatively slight difference in pressure in the different parts of the arterial or of the venous systems respectively.

3. The suddenness of the fall in the pressure between the small arteries and the capillaries, where the great resistance to the outflow is met with.

4. In the large veins the pressure of the blood is habitually below that of the atmosphere, only becoming positive during forced expirations.