[AS.] The vital fluid of animals, which circulates through tubular vessels known as arteries and veins. This fluid is largely water, but contains the nutriment derived from the food we eat. The arteries carry the bright-red blood which feeds the system. The veins bring back dark-colored blood, filled with waste sub-stance from the body. Arterial blood is bright red and life-giving. Venous blood is black-looking, and destructive of life until purified by the lungs (q.v.) From the extremities of small arteries the blood enters the thread-like capillaries, where nutrition takes place. These capillary tubes convey the blood to every part of the muscles and bones, to the root of every hair and every part of the brain, and throw it into the veins, so that it may go back to the heart {q.v.). Blood, though red in appearance, is a clear liquid without color, in which are particles or corpuscles so minute that the aid of a powerful micro-scope is required to examine them. Some corpuscles are red, others are white, but the red are so numerous as to tinge the fluid red. Oxygen from the blood unites with particles of tissue, and burns them, causing both the heat and the motion of the body. Exercise makes us warm, because the air is inhaled more rapidly, and the blood passes more rapidly through the lungs in contact with it, and so more oxygen is introduced into the body. The blood circulates through the body once in about two minutes, or about 12 lbs. of blood pass through the heart every minute. Every time the heart contracts it sends a fresh' supply of blood to the blood-vessels, and the motion gives a pulsation to the system. This is distinctly perceived at the pulse in the wrist, because there a rather large artery lies near the surface. The temperature of the human body is 980, that of birds is 104°, and that of fish is 85°.