It is important in this respect to remember that it is only so long as blood is in the arteries that it is available for the nutrition of cells. Once in the veins it is useless for nutrition; and were it not that it readily passes from the veins into the arteries again, it might as well be outside the body for any purposes of nutrition.

The veins are very capacious, and when dilated to their utmost, they can alone hold all the blood the body contains, and more. During life they are constantly kept more or less in a state of contraction by the action of the nervous system, but when they become completely dilated, as after death, all the blood flows into them, leaving the arteries empty. It is therefore possible, as Ludwig has well expressed it, to bleed an animal into its own veins. Schiff has shown that when the blood-vessels relax as they do after section of the medulla oblongata, the whole of the blood of another animal as large as the one experimented upon must be introduced in addition to its own, in order to raise the pressure within the vessels to the normal. Even this is insufficient to keep up the pressure, for the vessels go on still dilating, and the pressure falls, notwithstanding the large quantity of blood which is present in them. It is therefore evident that the normal action of the vaso-motor centres is more than equivalent, for the purposes of circulation, to as much blood again as the animal possesses. Weakened power of these centres is to a certain extent equivalent to bleeding, and increased power has a similar effect to an increase in the quantity of blood in the vessels.