The most important function of the muscular tissue in the arteries is to adjust the size of the vessels to the amount of blood which each part of the body requires. and also to adapt the calibre to the quantity of blood that the arteries contain at any moment. The force of the muscu lar contraction in the arteries is probably a very slight factor in propelling the blood. The im pulse of the blood, when it enters the arteries at the moment of ventricular contraction, can be felt in all the superficial arteries of any con siderable size: and we designate this as the pulse. For clinical purposes, it is usually felt in the radial artery at the outer side of the wrist.
In the mesentery of the frog, we can trace the circulation in the capillaries by means of the microscope; and our observations Ilene can prob ably, without error, be applied to the condition in man. The red blood-corpuscles are seen mov ing along with considerable rapidity in the middle of the blood-eurrent, while the white cor puscles advance more sluggishly along the walls of the capillaries. The capillaries present a far larger surface with which the blood comes in con tact than the other blood-vessels, and therefore offer the greatest resistance to the progress of the blood. This resistance largely depends on
the vital capillary force, so that the capillaries of themselves greatly influence the circulation. On the other hand, the of the arteries and veins exerts a decided control over the cir culation in the capillaries which connect them. The propelling force in the veins is due to the impulse from the ventricles, and, to some extent, to the pressure exerted upon the veins by the muscular movements of the body. The suction of the ice: is also a factor to he con sidered. The presence of valves along the veins enables them to take advantage of all the force applied. It is estimated that a portion of the blood can traverse the entire circuit of the cir culation in half a minute. The circulation be gins at a very early period in ftetal life, and presents important modifications, which will lie described under ECETUS (IV.). See also EM III' \IAN.
Our present knowledge of the circulation is of comparatively recent date. It was first de scribed by Ilarvey, in his celebrated work, Exer eilatio do Motu Cordis tt Nanguinis. published in 108.