is deficient. In tracings, on the other hand, from arteries of medium size, e.g., the radial, the upstroke is usually single. In this case the percussion-impulse is not sufficiently strong to jerk.

Fig. 128.-Pulse-tracing of radial artery, with double apex. (Sanderson.) up the lever and produce an effect distinct from that of the systolic wave which immediately follows it, and which continues. and completes the distension. In cases of feeble arterial tension, however, the percussion-impulse may be traced by the sphygmograph, not only in the carotid pulse, but to a less extent in the radial also (fig. 128).

The interruptions in the downstroke are called the katacrotic waves, to distinguish them from an interruption in the upstroke, called the anacrotic wave, which is occasionally met with in cases

Fig. 129.-Anacrotic pulse from a case of aortic aneurism. A, anacrotic wave (or percussion wave). B, tidal or pre-dicrotic wave, continued rise in tension (or higher tidal wave).

in which the pre-dicrotic or tidal wave is higher than the percussion wave.

There is considerable difference of opinion as to whether the dicrotic wave is generally present in health, and also as to its canse. The balance of opinion, however, appears to be in favour of the belief that the dicrotic wave is present in health, although it may be very faint; while in certain conditions not necessarily diseased, it becomes so marked as to be quite plain to the unaided finger. Such a pulse is called dicrotic. Sometimes the dicrotic rise exceeds the initial upstroke, and the pulse is then called hyperdicrotic.

As to the cause of dicrotism, one opinion (1) is that it is due to a recovery of pressure during the elastic recoil, in consequence of a rebound from the periphery. It may indeed be produced on a

schema by obstructing the tube at a little distance beyond the spot where the sphygmograph is placed. Against this view, however, is the fact that the notch appears at about the same point in the downstroke in tracings from the carotid and from the radial, and not first in the radial tracing, as it should do, if this theory was correct, since that

artery is nearer the periphery than the carotid, and as it does in the corresponding experiment with the arterial schema when the tube is obstructed. (2) The generally accepted notion among clinical observers, is that the dicrotic wave is due to the rebound from the aortic valves which causes a second wave; but the question cannot be considered settled, and the presence of marked dicrotism in cases of hæmorrhage, of anæmia, and of other weakening conditions, as well as its presence in cases of diminished pressure within the arteries, would imply that it might, at any rate sometimes, be due to the altered specific gravity of the blood within the vessels, either directly or through the indirect effect of these conditions on the tone of the arterial walls.

Waves may be produced in any elastic tube when a fluid is being driven through it with an intermittent

force, such waves being

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specific gravity of the fluid used, and with the kind of tubing, and may be therefore supposed to vary in the body with the condition of the blood and of the arteries.

Some consider the secondary waves in the downstroke of a normal tracing to be oscillation waves; but, as just mentioned, even if this be the case, as is most likely with post-dicrotic waves, the dicrotic wave itself is almost certainly due to the rebound from the aortic valves.

The anacrotic notch is usually associated with disease of the arteries, e.g., in atheroma and aneurism. The dicrotic notch is called diastolic or aortic, and in point of time indicates the closure of the aortic valves.

Of the three main parts then of a pulse-tracing, viz., the percussion wave, the tidal, and the dicrotic, the percussion wave is produced by sudden and forcible contraction of the heart, perhaps exaggerated by an excited action, and may be transmitted much more rapidly than the tidal wave, and so the two may be distinct ; frequently, however, they are inseparable. The dicrotic wave may be as great or greater than the other two.

According to Mahomed, the distinctness of the three waves depends upon the following conditions:

The percussion wave is increased by:-1. Forcible contraction of the Heart; 2. Sudden contraction of the Heart; 3. Large volume of blood; 4. Fulness of vessel; and diminished by the reversed conditions.

The tidal wave is increased by:-1. Slow and prolonged contraction of the Heart; 2. Large volume of blood; 3. Comparative emptiness of vessels; 4. Diminished outflow or slow capillary circulation; and diminished by the reverse conditions.

The dicrotic wave is increased by:-1. Sudden contraction of the Heart; 2. Low blood pressure; 3. Increased outflow or rapid capillary circulation; 4. Elasticity of the aorta; 5. Relaxation of muscular coat; and diminished by the reversed conditions.

One very important precaution in the use of the sphygmograph lies in the careful regulation of the pressure. If the pressure be too great, the characters of the pulse may be almost entirely obscured, or the artery may be entirely obstructed, and no tracing is obtained; and on the other hand, if the pressure be too slight, a very small part of the characters may be represented on the tracing.

The Pressure of the Blood within the Arteries (producing arterial tension).

It will be understood from all that has been said about the arteries in a normal condition (a) that they are during life continually "on the stretch," even during the cardiac

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diastole, and that in consequence of the injection of more blood at each systole of the ventricle into the elastic aorta, that this stretched condition is exaggerated each time the ventricle empties itself. This state of distension of the arteries is due to the pressure of blood within them, and arises in consequence of the resistance presented by the smaller arteries and capillaries (peripheral resistance) to the sudden emptying of the arterial system between the contractions of the ventricle. It is called the condition of arterial tension. It will be further understood (b) that, as the blood is forcibly injected into the already full arteries against their elasticity, it must be subjected to the pressure of the arterial walls, so that, when an artery is cut across, the blood is projected forwards by this force for a considerable distance. Thus, although the blood distends the arteries and produces tension, yet the elasticity of the arteries re-acts upon the blood, and subjects it to pressure. We have therefore to remember that we have to do with two things related but not identical, viz., the pressure which the blood exerts upon the arterial walls tending to stretch them, and the pressure to which the blood is subject by the arteries tending to drive it on in the direction of least resistance. The only direction in which it can be driven is onwards towards the capillaries, and so the blood-pressure in the arteries is one of the great agents in maintaining the circulation.

Fig. 131.-Diagram of mer curial manometer.

The relations which exist between the arteries and their contained blood are thus so obviously of importance to the carrying on of the circulation, that it becomes necessary to be

able to gauge the alterations in blood-pressure very accurately. This may be done by means of a mercurial manometer in the following way:-The short horizontal limb of this (fig. 131) is connected, by means of an elastic tube and cannula, with the interior of an artery; a solution of sodium or potassium carbonate

Fig. 132.-Diagram of mercurial kymograph. A, revolving cylinder, worked by a clockwork arrangement contained in the box (B), the speed being regulated by a fan above the box; cylinder supported by an upright (b), and capable of being raised or lowered by a screw (a), by a handle attached to it; D, C, E, represent mercurial manometer, a somewhat different form of which is shown in next figure.

being previously introduced into this part of the apparatus to prevent coagulation of the blood. The blood-pressure is thus communicated to the upper part of the mercurial column; and the depth to which the latter sinks, added to the height to which it rises in the other, will give the height of the mercurial column which the blood-pressure balances; the weight of the soda solution being subtracted.

For the estimation of the arterial tension at any given moment, no further apparatus than this, which is called Poiseuilles's