The influence of the arterioles upon the blood-pressure in a living animal can be to a great extent ascertained by the rapidity or slowness of the fall of the blood-pressure during the diastole of the heart. When the heart is beating slowly the diastole may be long enough to show distinctly the curve which the blood-pressure describes during its descent; but if the heart is beating quickly the diastole may be so short that this curve cannot be exactly obtained. It is then necessary to prolong the diastole artificially by stimulation of the vagi.

The reason why the part which the arterioles play in maintaining the blood-pressure can be ascertained by the way in which it falls during cardiac diastole, natural or artificial, is that in the healthy heart the aortic valves close during the diastole so as to separate the aorta completely from the ventricle.

In considering the blood-pressure during the diastole, we may therefore disregard the heart entirely, and look upon the aorta and its branches as an elongated elastic bag closed at its cardiac end, but open at its capillary end. This bag is distended with blood, which in consequence of the elastic pressure exerted upon it by the arterial walls tends to flow out into the veins. The rate at which it does this will depend 1st, on the elastic pressure or arterial tension; and,

2ndly, on the size or degree of contraction of the arterioles or capillaries.

If we connect a manometer with this elongated bag as in Fig. 90, and place on the mercurial column a float by which its diastole from b, the arteries. c, the capillaries. d, the veins. e, mercurial manometer. f, a float. g, a recording cylinder.

height can be recorded on a revolving cylinder, it is evident that the pressure-curve will fall more quickly to zero when the capillaries are dilated, and more slowly when they are contracted. With capillaries of the same size, the rate of flow will vary

Fig. 90.   Diagram of the circulation. a, the heart, completely shut off by the valves during with the arterial pressure. If the pressure be high the curve will fall more rapidly than when it is low, for the greater blood pressure will drive the blood more rapidly through the open arterioles.

Fig. 90. - Diagram of the circulation. a, the heart, completely shut off by the valves during with the arterial pressure. If the pressure be high the curve will fall more rapidly than when it is low, for the greater blood-pressure will drive the blood more rapidly through the open arterioles. If we find that with a normal pressure the pressure-curve falls more slowly than usual during the diastole, we may conclude that the arterioles are contracted; and if we find that the fall is slower, notwithstanding that the pressure is higher than usual, the proof that the arterioles are contracted is so much the stronger.

This is what Meyer and Il observed in the case of digitalis, where we found, as in the accompanying figure (Fig. 91), that the fall of the blood-pressure during the cardiac diastole in a dog is much slower after than before the injection of digitalis into the circulation.

In observations of this sort it must always be borne in mind that a great difference exists between the vessels of the intestines on the one hand, and those of the muscles on the other. The former are readily controlled by the vaso-motor centre, and when this is stimulated they contract greatly. Those of the muscles appear to be but slightly influenced by the vaso-motor centre, so that when it is stimulated they hardly contract at all, and indeed the flow of blood through them becomes accelerated on account of the contraction of the vessels elsewhere. When the vaso-motor centre is stimulated at the same time that the vagus is irritated, the blood-pressure appears to fall nearly as quickly as when the vagus alone is irritated. It seems possible, however, that this result may be really due to some extent to actual dilatation of the vessels in the muscles, for stimulation of the motor nerves of muscle appears to produce a vaso-dilating effect on their blood-vessels (Gaskell and others).

Effect Of The Arterioles On Pulse Curves 128

Fig. 91. - Tracing showing the blood-pressure and form of the pulse-wave before and after the injection of digitalis in the dog. The thin line shows the blood-pressure before, and the thick one after, the injection. The curve sinks more slowly after the injection, notwithstanding the greater pressure in the vessels.

The want of power of the vaso-motor centre over the vessels of the muscles is probably of considerable pathological importance. John Hunter l noticed, when he was bleeding a lady from a vein in the arm, that the blood, which previously had been dark and venous, became bright scarlet, like arterial blood, when she fainted, and remained so during the continuance of the faint. This seems to indicate that during syncope, although the superficial vessels are empty and contracted, the arterioles of the muscles are dilated like those of an actively secreting salivary gland.

1 Brunton and Meyer, Journal of Anatomy and Physiology, vol. vii. 1872, p. 134. The experiments described in the paper were performed in 1868.

If we find, then, that after the injection of a drug the blood-pressure remains constantly high, during stoppage of the heart, we may conclude that the vessels of the muscles are contracted as well as those of the intestine. Such a condition occurs after the injection both of digitalin and of erythrophlceum, in which the pressure sometimes remains high for many seconds, or even for a minute or more, after the heart has finally ceased to beat (Fig. 89).