up the arterial pressure. No doubt the great increase of surface over which the blood has to move in the capillaries, and the pressure exercised upon them by the surrounding elastic tissues, have influence in impeding the emptying of the arteries. But the contractility of the arterioles is the most important item, as may be seen from the following consideration. The resistance offered by the capillaries is insignificant when compared with the arterial blood pressure, for the increase of friction accompanying their greater extent of surface is counterbalanced by the decrease of friction dependent upon the great total capacity of the capillaries in comparison with that of the small arteries. The

capillary resistance alone is therefore not sufficient to restrain the blood from rushing into the veins. This is seen when the arterioles are paralyzed by the destruction of the nervous mechanism controlling them; the blood then flows readily through the capillary network, the veins become engorged, the arterial blood pressure falls, and the circulation comes to a standstill, in spite of the heart's more rapid beats. We know that beyond the arterioles the pressure falls suddenly, and in the capillary network it is always very low.

The four great factors in keeping up the arterial blood pressure may be thus enumerated: 1, the heart beat; 2, perfect

aortic valves; 3, the elastic resiliency of the large arteries; 4, the resistance offered by the contraction of the muscular arterioles.

Heart Beat.-If any factor fail, the mechanism of the circulation is at once impaired. For example, the heart's beat may be stopped by the stimulation of the inhibitory nerve fibres of the

Mercurial Manometer for measuring and recording the blood pressure. (a) Proximate limb of manometer. (6) Union of two limbs of manometer. (c) The rod floating on mercury and carrying the writing point. (d) Stop-cock through which the sodium bicarbonate can be introduced between the blood and mercury of manom

eter.

vagus, in which case the blood pressure rapidly falls, as shown by the curve taken by the graphic method. Or weakness of the heart beat may arise from disease (fatty degeneration) of the muscle, when signs of low arterial tension can be recognized in the human subject.

Valves. Any insufficiency of the aortic valves that permits

the blood to flow backward into the ventricle, allows the arterial pressure to fall between each ventricular systole, and gives rise to the characteristic "pulse of unfilled arteries," as it is called by the physician.

Elasticity of Arteries.—The resiliency of the arterial coats may also be destroyed to a certain extent by degeneration of the tissue, in which case the large arteries become greatly distended, and unable to exert their normal steady pressure on the blood.

FIG. 132.

R

A

The ordinary modern form of rotating blackened cylinder (R), which is moved by clockwork in the box (A) by means of the disc (D) pressing upon the wheel (n), which can be raised or lowered by the screw (L), so as to come in contact with any part of the disc more or less near the centre, and thus rotate at different rates. The cylinder can be raised by the screw (v), which is turned by the handle (U). (Hermann.)

Contractile Arterioles.—Injuries of the nervous centres are often associated with paralysis of the muscular arterioles and fall of blood pressure; but the effect upon the blood pressure of dilatation of the small arteries can be best seen by experimenting on the nerves that control their contraction. If paralysis or inhibition of the vasomotor mechanisms be experimentally produced,

the result on the arterial pressure is the same, a sudden fall, which may reach that of the atmosphere. The chief opposition to the outflow of blood from the arteries being removed, they cease to be tense, even though the ventricle continue to beat and pump the blood into them.

MEASUREMENT OF THE BLOOD PRESSURE.

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 a horse 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

FIG. 133.

20000
Mof

T

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.

general idea of the average permanent differences that exist in the different vessels of large animals and man.

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

Capillaries of Finger
Small Veins of Arm
Large Vein of Neck

66

+38 mm.,
+ 9 mm.,

46

64