Besides the forces indicated in preceding para graphs, the action of the heart, the elasticity of the arteries, the action of valves in the veins, and the effect of muscular contraction, there are others which aid in the circulation of the blood, and notably the movements of breathing, and the suction action produced by the relaxation of the heart after each contraction.
The Rapidity of the Circulation varies. It is quickest in the arteries and slowest in the capillaries. In the capillaries there is a much greater channel space, and since the quantity of blood entering the heart is always the same as that leaving it, the blood must flow more slowly in the capillaries if the balance is to be maintained. Thus from the aorta onwards to the capillaries the speed diminishes, and from the capillaries through the veins it increases to the heart. In the capillaries the rate is esti mated as 1 to 1 inches in a minute, and in one of the larger arteries 10 to 15 inches in a second. The length of time taken for a portion of blood to travel the whole round of the circulation is, in the horse, about half a minute. It has been estimated by injecting into one of the veins of the -neck a substance—ferrocyanide of potassium —easily detected by chemical tests, and noting how long time elapsed before it was found in the blood of the same vein in the opposite side of the neck.
Nervous Control of the Circulation.—The walls of the blood-vessels are largely muscular. The muscular fibres are beyond the control of the will. Nevertheless they are controlled by the nervous system, by the agency of nerves distri buted among the muscular fibres. The nerves are called vasomotor (rasa, vessels), and they are governed from a centre—the vasomotor centre—situated in the medulla oblongata (p. 150). The influence on the vessel may be briefly stated thus: By the nervous energy continually reaching the muscular walls they are kept in a moderate state of contraction or tone, so that the channels are maintained at an average size. If the vasomotor nerves are stimulated more than usual, the muscular walls contract more, the size of the channel is diminished the quantity of blood flowing through it is correspondingly di minished, and the supply to the part lessened. If the stimulus is less than usual, the amount of contraction is lessened, the vessels dilate by the force of blood within them, the channel becomes widened, and the supply to the part is increased.
Now various influences from various parts of the body, or from the brain itself, may reach the vasomotor centre and excite it to increased activity; the nerves are thereby stimulated and the vessels contract. But there is another curi ous effect that may be produced on the centre, what is called an inhibitory or restraining effect, whereby the ordinary influence of the centre is, for a time, suspended, the tone of the vessels becomes diminished, and their channels imme diately widen. The production of blushing and pallor is thereby explained.
Blushing is caused by some emotion which acts on the vasomotor centre and diminishes its activity. The tone of the vessels being lessened they dilate, more blood rushes along their channels, and the skin becomes redder and hotter by the increased quantity of blood. It is usually in the face that this is manifested.
Pallor is just the reverse condition. Some emotion so acts on the vasomotor centre as to increase its activity. The blood-vessels contract, less blood flows through them, and the part be comes pale. Thus the same emotion that causes redness of one person's face may produce pale ness of another's.
More than this, the heart and the blood vessels are related by nervous communication in a curious .way. Connected with the heart there is a nerve called the depressor nerve. Suppose the blood-vessels in a considerable part of the body to be unduly contracted. Blood flows through them less easily, and the heart has more work in forcing it along. This may occur to such a degree that the heart is unneces sarily burdened. Its labour may become so heavy as to threaten fatigue, and danger of ex= haustion may arise. At this period an influence passes from the heart by the depressor nerve to the vasomotor centre. The action of the centre is restrained, the tone of the vessels is dimin ished, by the pressure of blood within them they dilate, the blood flows along easily, and the heart is relieved. This is just an instance of how all the bodily functions are regulated and controlled by the nervous system, and made to servo the good of the whole body.