Blood

[AS.] The vital fluid of animals, which circulates through tubular ve els 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-look¬ ing, and destructive of life until purified by the lungs (g.v.) From the extremities of small arte¬ ries the blood enters the thread-like capillaries, where nutrition takes place. These capillary tubes convey the blood to every part of the mus¬ cles 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.). Bio jd, though red in appearance, is a clear liquid without color, in which are particles or corpus¬ cles so minute that the aid of a powerful micro¬ scope is required to examine them. Some cor¬ puscles 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 ti ue, 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 pa es 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 pa through the heart every minute. Every time the heart contracts it sends a fresh telegraph lines, and provided with signal boards or lights. The rule is that no train shall pa a block station while a train is on the section in advance, and until word comes back that the section is clear. Thus, if operated perfectly, there can be only one train on a section of three or four miles at a time, and collisions would be impo ible. But men are not always to be trusted, and an automatic block system, in which the trains themselves work the signals, through electric attachments, is being introduced. 1 he block system was first introduced in 1851, and is now much more common in Europe than in the United States.