=Circulation.=--The blood is the most important and the most abundant fluid of the body. It constitutes about one-twelfth of the entire weight of the person. To the eye it appears as a simple fluid, varying in color from a bright scarlet to a dark purple. Under the microscope, it is seen to consist of a clear, colorless fluid in which float a multitude of corpuscles, or solid discs. These corpuscles are so minute that thirty-five hundred, arranged side by side, will extend only one inch, and fourteen thousand, placed one upon the other, would not exceed one inch in height. There are also white corpuscles which are fewer in number, larger, and globular in form.
The size and shape of the blood corpuscles in man differ from those in animals. So important and well-defined is this difference in point of law that the guilt or innocence of criminals has often hung upon the results of a scientific examination of the blood found upon the garments of the suspected person.
=Coagulation.=--The coagulation, or thickening of the blood, when it leaves the living tissues, is a principle of the greatest importance to life. Without it, the slightest injury might prove fatal. In minor injuries, the blood coagulates, thus closing the mouths of the injured blood vessels, and bleeding ceases spontaneously.
=The Heart.=--The great center of the circulatory system is the heart. With ceaseless energy it drives the blood through the arteries to every part of the body, laden with the life-giving elements absorbed from the food and vitalized by the oxygen in the lungs. In the outward flow of the blood each part of the body appropriates the particular elements which it requires. The return of the blood through the veins brings with it the waste and cast-off particles of the bones, muscles, and tissues, to be expelled through the lungs, except such elements as exude through the pores of the skin. By this unceasing round of waste and repair, the entire body, it is believed, is renewed every seven years, and some parts are replaced several times within that period. From the moment a human being begins to live, he begins to die.
=Action of the Heart.=--The alternate contraction and dilation of the muscles of the heart constitute the heart-beats or throbs of the pulse. These vary with the individual. In the average adult, the number of beats is seventy-two per minute. In the cases of Bonaparte and Wellington the number was less than fifty. Heat, food, and exercise increase its action, as cold, fasting, and sleep diminish it. Emotion of joy, grief, and fear also exert a modifying influence.