The _Capillaries_ form an extremely fine net-work, and are distributed to every part of the body. They vary in diameter from 1/3500 to 1/2000 of an inch. They are so universally prevalent throughout the skin, that the puncture of a needle would wound a large number of them. These vessels receive the blood and bring it into intimate contact with the tissues, which take from it the principal part of its oxygen and other elements, and give up to it carbonic acid and the other waste products resulting from the transformation of the tissues, which are transmitted through the veins to the heart, and thence by the arteries to the lungs and various excretory organs. The blood from the system in general, except the lungs, is poured into the right auricle by two large veins, called the superior and the inferior _vena cava_,' and that returning from the lungs is poured into the left auricle by the _pulmonary veins._ During life the heart contracts rhythmically, the contractions commencing at the base, in each auricle, and extending towards the apex. Now it follows, from the anatomical arrangement of this organ, that when the auricles contract, the blood contained in them is forced through the auriculo-ventricular openings into the ventricles; the contractions then extending to the ventricles, in a wave-like manner, the great proportion of the blood, being prevented from re-entering the auricles by the tricuspid and mitral valves, is forced onward into the pulmonary artery from the right ventricle, and into the aorta from the left ventricle. When the contents of the ventricles are suddenly forced into these great blood-vessels, a shock is given to the entire mass of fluid which they contain, and this shock is speedily propagated along their branches, being known at the wrist as the _pulse_. On inspection, between the fifth and sixth ribs on the left side of the chest, a movement is perceptible, and, if the hand be applied, the impulse may be felt. This is known as the throbbing, or beating of the heart. If the ear is placed over the region of the heart, certain sounds are heard, which recur with great regularity. First is heard a comparatively long, dull sound, then a short, sharp sound, then a pause, and then the long, dull sound again. The first sound is caused mainly by the tricuspid and mitral valves, and the second is the result of sudden closure of the semilunar valves. No language can adequately describe the beauty of the circulatory system. The constant vital flow through the larger vessels, and the incessant activity of those so minute that they are almost imperceptible, fully illustrate the perfectness of the mechanism of the human body, and the wisdom and goodness of Him who is its author.