taken by the nerve impulses, for any rhythmical sympathy existing between the respiratory and vasomotor nerve centres in the medulla cannot well influence the vessels when the cord is cut.

Thus we seem forced to fall back upon the muscular coats of the arteries for an explanation of the respiratory variation in the blood pressure, and to accord to this tissue automatic rhythmical contractility.

The blood pressure in the capillaries cannot be directly measured by the means above described; it is difficult to estimate, and very variable. The slightest change of pressure in the corresponding veins or arteries causes the pressure in the capillaries to rise or fall. Thus, variations in pressure are constantly occurring in the capillaries, which cause an alteration in the rate of flow, or even a retrograde stream in some parts of the network.

The regulation of the blood supply, and, therefore, of the pressure in the capillaries, is under the control of the small arterioles which supply them; a slight relaxation of the muscle of the arterioles causes great increase in the amount of blood flowing through the capillaries, as can readily be seen with the micro

scope.

The blood pressure in the veins must be less than that in the capillaries, and, as has been said, must diminish as the heart is approached, where in the great veins (superior cava) the pressure is said to be rather below that of the atmosphere (-3 to -5 mm., mercury). During inspiration the minus pressure may become further lowered, while, on the other hand, it is only by very forced expiration that it ever becomes equal to or at all above that of the atmosphere.

This is a most important fact, as the suction considerably helps the flow of blood from the veins, and also the current of fluid from the thoracic duct that bears the chyle from the intestines and the fluid collected from the tissue drainage back to the blood.

The pressure of the blood in the veins may be said to be generally nil, since the veins are nowhere overfilled with blood.

The pressure, on the other hand, that can be registered and measured depends upon forces communicated from without, namely: (1) gravity; (2) the elastic pressure of the surrounding tissue; and (3) the pressure exerted by the muscle during contraction. This pressure is increased by any circumstance which impedes the flow of blood through the right side of the heart, through any large vein, or through the pulmonary circulation; but when no abnormal obstacle exists in the venous blood current the pressure in those vessels can never attain any great height, for, as we have seen, the large trunks are constantly being emptied by the heart's action.

Most circumstances which tend to lower arterial pressure also tend to raise the pressure in the veins, so that when the heart's action is weak or its mechanism faulty the venous pressure rises.

In the veins of the extremities the pressure greatly depends on the position of the limb, as it varies almost directly with the effect of gravity.

In the pulmonary circulation the direct measurement of the intra-vascular pressure is rendered extremely difficult, and possibly erroneous, by the fact that to ascertain it the thorax has to be opened. It has been found in the pulmonary artery to be in a dog 29.6 mm., in a cat 17.6 mm., and in a rabbit 12 mm. of mercury.

THE ARTERIAL PULSE.

Each systole of the ventricle sends a quantity of blood into the aorta, and thus communicates a stroke to the blood in that vessel. The incompressible fluid causes the tense arterial wall to distend still further, and the shock to the column of blood is not transmitted onward directly by the fluid, but causes the elastic walls of the arteries to yield locally, and thus it is converted into a wave which passes rapidly along those vessels. This motion in the walls of the vessel can be felt wherever the artery can be reached by the finger, but best, as is the case in the radial and temporal arteries, where the vessel is superficial and lies on some unyielding structure, such as bone.

This motion of the vessel wall is called the arterial pulse. It consists of a simultaneous widening and lengthening of the artery. The arteries near the heart are more affected by the pulse wave than those more remote, the wave becoming fainter and fainter as it travels along the branching arteries. In the smallest arteries it is hardly recognizable, and under ordinary circumstances is quite absent in the capillaries and veins.

The diminution in the pulse wave in the smaller arteries chiefly depends upon the fact that the force of the wave is used up in distending the successive parts of the arteries. In the small arteries the extent of surface to which the pulse wave is communicated is great, and thereby the wave is much decreased. It is probable that reflected waves pass from the peripheral end of the arterial tree-the contracted arterioles-and meeting the pulse wave in the small arteries help to obliterate it. So long as the arterioles are contracted to the normal degree no pulsation is communicated to the capillaries, because the wave, reaching the arterioles, is reflected by them.

The pulse wave can easily be shown to take some time to pass along the vessels. Near the orifice of the aorta the arterial distention occurs practically at the same time as the ventricular systole, but even with comparatively rough methods the radial pulse can be observed to be a little later than the heart beat. The difference of time between the pulse in the facial and the dorsal artery of the foot has been estimated to be one-sixth of a second, and the difference in the distance of these vessels from the heart is about 1500 mm., so that the rate at which the pulse wave travels is nearly 10 metres per second. The velocity of the wave is said to depend upon the degree of elasticity of the walls of the vessels, and it would appear to be quicker in the lower than in the upper extremities.

The time that the wave takes to pass any given point must be equal to the time taken to produce it, that is to say, the time the ventricle occupies in sending a new charge of blood into the aorta, which is about one-third of a second. Knowing the rate at which the wave travels (10 m. per sec.) and the time it takes to pass any given point (sec.), its length may be calculated to

be about three metres, or about twice as long as the longest artery. Thus the pulse wave reaches the most distant artery in one-sixth of a second, or about the middle of the ventricular

The frame (B, B, B) is fastened to the wrist by the straps at B, B, and the rest of the instrument lies on the forearm. The end of the screw (v) rests on the spring (R), the button of which lies on the radial artery. Any motion of the button at R is communicated to v, which moves the lever (L) up and down. When in position, the blackened slip of glass (P) is made to move evenly by the clockwork (H) so that the writing point draws a record of the movements of the lever.

systole, and when the wave has passed from the arch of the aorta, its summit has just reached the arterioles.

Numerous instruments have been invented for the demonstration and graphic representation of the pulse in the human being. Of these the one in general use is Marey's Sphygmograph (Fig. 139),

FIG. 140.

Tracing drawn by Marey's Sphygmograph. The surface moved from right to left. The vertical upstrokes show the period when the shock is given by the systole of the ventricle. The upper wave on the downstroke shows when the blood has ceased to enter the aorta. Then comes the dicrotic depression, which is a negative wave produced by the momentary backflow in aorta, and the dicrotic elevation caused by the closure of the valves.

by means of which a graphic record of the pulse is made, in the form of a tracing of a series of elevations and depressions (Fig. 140). The elevations correspond to the onset of a wave, and the

depressions to its departure, or to the temporary rise and fall of the arterial pressure. In the falling part of the curve an irregularity caused by a slight second wave is nearly always seen. This is called the dicrotic wave. Sometimes there are more than one of these secondary waves, the most constant of which is a small wave preceding the dicrotic, called predicrotic; but the dicrotic is always more marked than any other. Several waves of oscillation can be seen as a gradually decreasing series in tracings taken from elastic tubes, but we cannot say positively that they occur in the arteries. When several secondary waves exist in the pulse curve, the smaller ones probably depend on oscillations caused by the lever of the instrument.

The dicrotic wave does not depend on the instrument, because in most cases the skilled finger laid on the radial artery at the wrist can easily detect it, and it can be directly seen in the vessel when the pulsation in the arteries is visible, or when a jet of blood escapes from an artery.

When a new charge of blood is shot into the aorta the elastic wall of the vessel is suddenly stretched. At the same time a shock is given to the column of blood, and the fluid next the valves is moved forward with great velocity. Owing to its inertia the fluid tends to pass onward from the valves, and thus allows a momentary fall in pressure which is at once followed by a slight reflux of the blood and the forcible closure of the valves.

The first crest or apex of the pulse curve corresponds to the shock given by the systole, and is greatly exaggerated by the inertia of the lever. The crest of the predicrotic wave marks the moment when the blood ceases to flow from the ventricle, and, therefore, it is the real head of the pulse wave.

The dicrotic wave has been explained as (1) a wave of oscillation, (2) a wave reflected from the periphery, or (3) a wave from the aortic valves.

1. If the first, it should be less marked than the predicrotic, which by this theory is said to be the first wave of oscillation, for each succeeding oscillation is less than its forerunner. But, as already mentioned, the dicrotic is invariably the larger.