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 (1/3 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 systole, and when the wave has passed from the arch of the aorta, its summit has just reached the arterioles.

Marey's Sphygmograph.

Fig. 139. Marey's Sphygmograph.

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.

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), 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.

Tracing drawn by Marey's Sphygmograph.

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.

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 (i) 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.

2. There are many reasons why it cannot be a wave of reflection from the periphery of the arterial tree; viz., (i) Its curve is not found to be nearer the primary wave when the peripheral vessels are approached. (2) The arterioles which form the peripheral resistance are at too irregular distances to give one definite wave of reflection. (3) It is seen in the spurting of an artery cut off" from the periphery. (4) It increases with greater elasticity and low tension, which cause the reflected waves to diminish.

3. The dicrotic notch then most probably depends upon a negative centrifugal wave, caused by the sudden stoppage of the inflow and the momentary reflux of blood during the closure of the valves; and the dicrotic crest is, no doubt, produced by the completion of their closure, at which moment the sudden check given to the reflux of the blood column causes a positive centrifugal wave to follow the primary wave of the pulse.

The view that the reflux of blood and the closure of the valves produce the dicrotic wave is supported by the fact that the conditions which increase the dicrotism - viz. (1) sharp, strong systole, (2) low tension, and (3) perfect resiliency - promote the recoil and closure; and, on the other hand, the conditions which interfere with the closure of the valves also diminish the dicrotic wave in the most marked degree, viz. (1) inefficiency of the aortic valve, and (2) a rigid calcareous condition of the arteries.

It can be shown in an elastic tube, fitted with a suitable pump and sphygmographs, that when its outlet is closed a positive wave is reflected from the distal end back to the pump, and when the outlet is opened a negative centripetal wave is reflected. This fact assists in explaining the variations in the character of the pulse curve of the radial artery where the equidistance of the derived arterioles enables the reflected waves to have considerable effect. When the arterioles are constricted (a condition corresponding to the closure of tube) a positive centripetal wave is reflected, and is added to the pulse wave so as to diminish the dicrotic notch, and give the curve known as characteristic of the "high-tension" pulse seen in Bright's disease. (Fig. 141, 11.) On the other hand, when the arterioles are widely dilated (corresponding to the open condition of the tube) a negative wave is reflected, and is subtracted from the force of the pulse wave so as to exaggerate the dicrotic notch, and give the tracing characteristic of the "low-tension" pulse seen in fever, etc. (Fig. 141 III)

The mean rate of the pulse varies in different individuals, seventy-two per minute being a fair average for a middle-aged adult. It varies also with many circumstances, which, though purely physiological, must be borne in mind in taking the pulse as a clinical guide.

1. Age. At birth it is about 140 per minute, and is, generally speaking, quicker in young than in old people, commonly falling to 60 in aged persons.

I. Scheme of Normal Pulse Curve: a, Entrance of ventricular stream into the aorta.

Fig. 141. I. Scheme of Normal Pulse Curve: a, Entrance of ventricular stream into the aorta, the lever is jerked too high, reaching*; ab shows real summit of waves; b, point at which stream from ventricle ceases; c, negative wave caused by (i) sudden cessation of inflow and slight reflux of blood; d, point of closure of aortic valves; e, positive wave from valves (dicrotic wave). The time may be measured on abscissa at a' b' d'.

II. Scheme of High Tension Pulse Curve (constricted arterioles). A. Curve of radial pulse, which is the resultant of positive reflected wave C added to the primary curve B.

III. Scheme of Low Tension Pulse Curve (dilated arterioles). A. Radial pulse curve, which is the resultant of the negative reflected wave C subtracted from the primary wave B. {After Grashey).

2. Sex. It is more rapid in females than in males.

3. Posture. It is quicker standing than lying, particularly if a patient who has been lying down, stand or sit up, the pulse becomes more rapid.

4. The time of day. At its minimum at midnight, it gains in rapidity till 9 o'clock in the morning; falls in the daytime, and rises in the evening till 6 o'clock.

5. Muscular exercise quickens it.

6. It is quicker during inspiration than expiration.

7. It increases with increase of temperature.

8. It is variously affected by emotions.