The continuity of the circulation of blood through the capillaries is not maintained by the heart alone : the elastic pressure of the arteries on the blood within them plays a most important part, and indeed during the cardiac diastole the circulation is maintained entirely by this elastic pressure.

If the arterioles or capillaries through which the arterial system empties itself into the veins are much contracted, so that the blood can flow only slowly through them, the heart may stop, and yet the blood-pressure may remain for many seconds almost unchanged. But if the arterioles or capillaries are dilated, the arteries quickly empty themselves into the veins, arterial pressure rapidly falls, and circulation soon stops.

Fig. 83.   Diagram to illustrate the effects of the horizontal and vertical position on the circulation of the frog in shock, a, normal circulation in the upright position, b, circulation after dilatation of the veins has been produced by a blow on the intestines.

Fig. 83. - Diagram to illustrate the effects of the horizontal and vertical position on the circulation of the frog in shock, a, normal circulation in the upright position, b, circulation after dilatation of the veins has been produced by a blow on the intestines. The blood does not reach the heart, and it beats empty, so that the circulation stops, c shows the circulation in a horizontal position after the veins have been dilated, as in b. The veins are still dilated, but the blood reaches the heart, and the circulation is carried on. Fig. c is perhaps too diagrammatic, as it appears to show an empty space or air in the veins. In reality the veins, being very thin-walled, collapse. Fig. b is open to the same objection, but if we suppose ourselves to be looking at the vein from the front instead of in section, b represents almost exactly what I have myself seen in repeating Goltz's experiment.

I use the words arterioles and capillaries as synonymous, because it is almost certain that the capillaries do contract. In most cases where contraction has occurred in the peripheral vessels, it is difficult or impossible to say whether its seat is in the capillaries or arterioles.

The action of the heart is to pump the blood out of the veins into the arteries, and this it can only do when the blood reaches it. If the veins are much dilated and the animal is in an upright position, no blood may reach the heart, or so little blood that its pulsations are practically useless. This is seen in the frog when dilatation of the large veins has been reflexly produced by striking the intestines (Fig. 836). When the animal is laid flat, the blood flows into the heart, and then it works normally. It is probable that a similar condition occurs in man, as one of the factors in shock; and in this condition, as well as in fainting, or failure of the heart's action from the effect of drugs, as chloroform, or other causes, the person should be laid flat, with the limbs raised so that the blood may flow out of them into the heart, and with the head low (either perfectly level with the body or depressed below it), in order to permit of an increased supply of blood to the intra-cranial nerve-centres.