The left ventricle, in contracting, pushes the red blood which it contains in the direction of the auriculo-ventricular orifice and toward the orifice of the aorta,—but the mitral valve is so placed that it closes under the impulse of the moving blood, which is thus forced into the aorta and from thence into all the arteries, in which its motion is caused by the triple action of ventricular contraction, and of the elasticity and contractility of the arterial walls. In vessels of a certain calibre this movement is jerking and rhythmical, precisely like that of the heart, and if we place the finger on the course of an artery we feel the shock of the blood, or the pulse. The pulse and the beating of the heart are synchronous, that is, they take place simultaneously, or rather with an interval so short as to be imperceptible. But in proportion as the blood advances through the arterial ramifications, the numerous changes of direction which it undergoes and the friction between it and the walls of the vessels diminish the force of its impulsion; and at last, on Teaching the capillary vessels it flows continuously and without shock.

In examining, under a microscope, a vascular membrane belonging to a living animal, the circulation of the blood through the capillaries is plainly visible. The largest of these canals allow the column of blood to pass rapidly, in the smallest ones its course is slow and the blood-globules can only pass one by one; they float in a transparent fluid, and sometimes a globule becomes entangled obliquely across the calibre of the vessel and stops until another comes to push it forward. Malpighi was the first to verify in this manner the accuracy of Harvey's theory, forty years after it had been propounded by the illustrious English physiologist.

The different causes, therefore, which accelerate or retard the contractions of the heart, influence the motion of the blood in the arteries; and farther, the contractility of these vessels may be influenced by local causes, and the movement of the blood is modified so as to be either retarded or quickened, as they are contracted or relaxed. In the first case the afflux of blood is not sufficient to excite the organs, which become sluggish and partially paralyzed; in the second it is so great as to cause an abnormal activity of the function. And lastly, we all know that the repose or action of the muscles has the effect of retarding or accelerating the circulation, general or local, which, in the course of time, results in the diminution or increase of the muscular strength.

It is in the capillaries, in fact, that arterial blood yields to the tissues the elements of which it is composed, and which it delivers to them for assimilation, in order to receive in exchange the disassimilated particles which are to be rejected from the system or submitted to a fresh elaboration. A living and nourishing fluid, it carries to the organs life, heat, and the elements of nutrition.