The whole quantity of blood in the body is estimated on an average to be about one-ninth of its entire weight. The distinctions between the arterial and the venous blood are marked, since in the arterial system the blood is uniformly bright red, and in the venous of a very dark red color The blood-corpuscles contain both oxygen and carbonic acid in solution. When carbonic acid predominates, the blood is dark red; when oxygen, scarlet. In the lungs, the corpuscles give up carbonic acid, and absorb a fresh supply of oxygen, while in the general circulation the oxygen disappears in the process of tissue transformation, and is replaced, in the venous blood, by carbonic acid. The nutritive portions of food are converted into a homogeneous fluid, which pervades every part of the body, is the basis of every tissue, and which is termed the _blood_. This varies in color and composition in different animals. In the polyp the nutritive fluid is known as _chyme_, in many mollusks, as well as articulates, it is called _chyle_, but in vertebrates, it is more highly organized and is called blood. In all the higher animal types it is of a red color, although redness is not one of its essential qualities. Some tribes of animals possess true blood, which is not red; thus the blood of the insect is colorless and transparent; that of the reptile yellowish; in the fish the principle part is without color, but the blood of the bird is deep red. The blood of the mammalia is of a bright scarlet hue. The temperature of the blood varies in different species, as well as in animals of the same species under different physiological conditions; for this reason, some animals are called _cold-blooded._ Disease also modifies the temperature of the blood; thus in fevers it is generally increased, but in cholera greatly diminished. THE blood has been aptly termed the "vital fluid," since there is a constant flow from the heart to the tissues and organs of the body, and a continual return after it has circulated through these parts. Its presence in every part of the body is one of the essential conditions of animal life, and is effected by a special set of organs, called the _circulatory organs_. * * * * * CHAPTER VII. PHYSIOLOGICAL ANATOMY. CIRCULATORY ORGANS. Having considered the formation of chyle, traced it through the digestive process, seen its transmission into the _vena cava_, and, finally, its conversion into blood, we shall now describe how it is distributed to every part of the system. This is accomplished through organs which, from the round of duties they perform, are called _circulatory_. These are the Heart, Arteries, Veins, and Capillaries, which constitute the _vascular system_.