I have already mentioned that we can to a certain extent ascertain whether a rise or fall in blood-pressure is due to the heart or arterioles, by comparing the pressure-curve with the pulse-curve (p. 271 et seq.). If they run parallel the effect may be attributed in great measure to the heart.

But the effect of the heart on the blood-pressure is not so simple as that of the arterioles. In the case of the arterioles we have to consider only the rate at which the blood will flow through them when they are more or less contracted; but in the case of the heart we have to consider not only the rapidity of its pulsations, but the amount of blood which is sent into the arterial system at each beat. We judge of the amount of blood chiefly by the extent to which the blood-pressure oscillates with each pulsation. A large quantity of blood will, as a rule, cause an extensive, and a small quantity only a slight oscillation. When the heart is beating slowly, so that it has time to fill completely during each diastole, the oscillations are large, and when it is beating quickly the oscillations are small.

It is evident that although quick pulsations tend to raise the blood-pressure, they only do so up to a certain point, as beyond that, the heart does not get properly filled, and so sends but little blood into the aorta at each

Causes of Alterations in Blood-pressure and Pulse-rate.

Blood-pressure may be diminished

By slow action of the heart

Irritation, or increased excitability of vagus roots .......

Directly, by the action of the drug on them.

Indirectly, by increased blood-pressure.

„ by accumulation of CO2 in the blood.

Irritation, or increased excitability of vagus ends in the heart

Reflexly, by irritation of some afferent nerve.

Paralysis of sympathetic ends in the heart (?)

Paralysis of the cardiac ganglia.

Weakness of the heart ....

Paralysis of the cardiac muscular fibres.

By smallness in the amount of blood sent into the heart at each systole

Imperfect systole of the heart.

Imperfect diastole of heart.

Contraction of the pulmonary vessels.

Great dilatation of the venous system.

Directly, by the action of the drug.

By dilatation of the small arteries .

Paralysis of the vaso-motor centre. .

Reflexly, through the depressor.

Reflexly, through vagus and sensory nerves, when brain is removed or animal poisoned by opium.

,, „ ,, peripheral ends

„ „ „ fibres .

In operations by division of the cord or of the splanchnics.

Paralysis of the muscular coat of the arterial walls.

Blood-pressure may be increased

By quick action of the heart .

Paralysis of vagus roots.

Paralysis of vagus ends in heart.

Stimulation of sympathetic roots . . .

Directly, by action of drug.

Indirectly, by lowered blood-pressure.

Stimulation of sympathetic ends in heart (?)

Directly.

Stimulation of the cardiac ganglia. . .

Reflexly, by stimulation of the sensory nerves of the endocardium.

Indirectly, by causing increased temperature of body.

By larger amount of blood at each beat

More perfect diastole and systole.

By contraction of the small arteries

Directly, by action of drug on it.

Indirectly, by accumulation of CO2 in the blood.

Irritation of vaso-motor centre . .

Reflexly, through the cervical sympathetic.

„ „ peripheral terminations.

Reflexly, through the vagus, when the brain is present, and the animal is not narcotised.

Direct irritation of muscular coat of vessels.

Reflexly, through sensory nerves.

„ „ „ vaso-motor fibres .

In operations by irritation of the peripheral ends of the divided spinal cord or splanchnics.

Stimulation of nerve-fibres of course frequently occurs in experiments from the application of a faradaic current to the trunk of a nerve, but it probably never occurs from the action of drugs introduced into the general circulation.

beat. But the heart may sometimes be imperfectly filled even when it is beating slowly; this has been shown to occur in the case of the frog by Goltz. When a blow or two is struck on the intestines the veins dilate and the blood accumulates in them, so that the heart, which is also stopped at first, receives no blood when it does begin to beat again. It can therefore send none into the aorta, and the circulation remains completely arrested, although the heart is beating.

If the pulmonary capillaries also are contracted the left ventricle will receive little blood, and so will send little blood into the arteries, although the right ventricle may be much distended. This appears to occur during poisoning with muscarine, which causes the lungs to become blanched,1 the right ventricle distended, and the left ventricle and the arterial system empty: so that little blood flows from a wound.2

Fig. 94.   For description vide p. 263.

Fig. 94. - For description vide p. 263.

It is difficult, however, to estimate precisely the quantity of blood sent into the arteries at each beat, and its relation to the rapidity of the pulse, so as to ascertain directly how much the rise or fall of blood-pressure is due to the heart; and therefore this is sometimes estimated indirectly by ascertaining first how much of the effect of the drug on the blood-pressure is due to the arterioles, and then attributing to the heart what is not accounted for by their action.

Sometimes also we may get useful information by compressing the abdominal aorta as near the diaphragm as possible before and after injection. We thus diminish so greatly the number of capillary outlets by which the blood may flow from the arteries into the veins that we greatly lessen, though we do not quite destroy, the effect of the arterioles on the blood-pressure. We can thus estimate more precisely the action of the heart upon it.

Section of the spinal cord below the medulla oblongata, by destroying the effect of the vaso-motor centre upon the vessels, also aids us in estimating the action of the heart.

Another method of ascertaining what share in alterations of the circula. tion locally is due to the heart and arterioles respectively, consists in the combined use of the manometer and Ludwig's stromuhr or Marey's haemo-dromometer. The manometer shows the general blood-pressure while the haemodromometer shows the rate of circulation in the particular artery experimented upon. If the rate of flow increases while the blood-pressure remains constant or sinks, it is evident that the arterioles of the particular vascular district to which the artery is distributed have become dilated. If, on the other hand, the rate of circulation diminishes while the pressure remains constant or rises, it is clear that the arterioles have become contracted.

This method is only capable of being applied to large arteries such as the carotid or femoral. By placing the stromuhr in the femoral artery, Dogiel and Kowalewsky found that during suffocation the rapidity of the blood-flow diminished while the pressure rose, showing that the peripheral vessels were contracted.3

1 Lauder Brunton, Brit. Med. Journ., Nov. 14, 1874.

2 Schmiedeberg and Koppe, Das Muscarin, p. 57. 3 Pfluger's Archiv, 1870, p. 489.

By the use of the stronmhr, Dogiel1 has found that the rapidity of the flow of blood in the carotid is first increased and then diminished by alcohol, the greatest diminution occurring during complete narcosis.