* * * * * [Illustration: CIRCULATION OF THE BLOOD. [Heart, Lungs, Arteries & Veins.] {101} CHAPTER VII. THE CIRCULATION. _The Blood--Its Plasma and Corpuscles--Coagulation of the Blood--The Uses of the Blood--Transfusion--Change of Color--The Organs of the Circulation--The Heart, Arteries, and Veins--The Cavities and Valves of the Heart--Its Vital Energy--Passage of the Blood through the Heart--The Frequency and Activity of its Movements--The Pulse--The Sphygmograph--The Capillary Blood-vessels--The Rate of the Circulation--Assimilation--Injuries to the Blood-vessels._ [Sidenote: 1. What is required by every living organism? In plants? Insects? Reptiles? Man?] 1. THE BLOOD.--Every living organism of the higher sort, whether animal or vegetable, requires for the maintenance of life and activity, a circulatory fluid, by which nutriment is distributed to all its parts. In plants, this fluid is the sap; in insects, it is a watery and colorless blood; in reptiles and fishes, it is red but cold blood; while in the nobler animals and man, it is the red and warm blood. [Sidenote: 2. Importance and abundance of blood? Dependence of life? Abel? Mosaic law? In what part of the body is blood not found? Quantity of blood in the body?] 2. The blood is the most important, as it is the most abundant, fluid of the body; and upon its presence, under certain definite conditions, life depends. On this account it is frequently, and very properly, termed "the vital fluid." The importance of the blood, as essential to life, was recognized in the earliest writings. In the narration of the death of the murdered Abel, it is written, "the voice of his _blood_ crieth from the ground." In the Mosaic law, proclaimed over thirty centuries ago, the Israelites were forbidden to eat food that contained blood, for the reason that "the life of the flesh is in the blood." With the exception of a few tissues, such as the hair, the nails, and the _cornea_ of the eye, blood everywhere pervades the body, as may be proven by puncturing any part with a {102} needle. The total quantity of blood in the body is estimated at about one-eighth of its weight, or eighteen pounds. [Sidenote: 3. Color of blood? Its consistence? Odor?] 3. The color of the blood, in man and the higher animals, as is well known, is red; but it varies from a bright scarlet to a dark purple, according to the part whence it is taken. "Blood is thicker than water," as the adage truly states, and has a glutinous quality. It has a faint odor, resembling that peculiar to the animal from which it is taken. [Sidenote: 4. What is stated of the blood as viewed under the microscope?]