Chlo'ro phyll. The green coloring matter of plants, formed by the action of sunlight on the plant cells. It is a necessary part of the plant's digestive system, since without it the plant could not break up the carbon dioxid of the air into the carbon which it uses in preparing its starch food, and the oxygen which it gives off as waste. Fer men ta'tion. A chemical change in plant or animal substance, produced usually by the action of bacteria, in the process of which the substance is broken up (decomposed), and new substances are formed. Nar cot'ic. Any substance that blunts the senses, or the body's sensibility to pain or discomfort. Ni'tro gen. A tasteless, odorless, colorless gas, forming nearly four-fifths of the earth's atmosphere; and constituting a necessary part of every plant and animal tissue. Pro'te ins. Foods containing a large amount of nitrogen; such as meat, fish, milk, egg, peas, beans. IV. RELATING TO THE BLOOD AND THE CIRCULATORY SYSTEM A or'ta. The main artery of the body; it leads out from the left ventricle of the heart, carrying arterialized blood (blood that has been acted upon by oxygen) to all parts of the body except the lungs. Ar'te ries. The blood vessels and their branches that carry blood from the heart to all parts of the body. The pul'mon a ry artery carries impure (ve'nous) blood to the lungs. Au'ri cles (o'r[)i] klz). The two chambers of the heart that receive blood from the veins. Cap'il la ries. The minute blood vessels which form a network between the ends of the arteries and the beginnings of the veins. Cir cu la'tion. The passage of the blood from the heart into the arteries, and from them through the capillaries into the veins, and through the veins back into the heart. Cor'pus cles (cor'p[)u]s'lz). Minute jelly-like disks or cells. These are of two kinds, red and white, the red (the oxygen carriers) being about 350 times as many as the white, and giving the blood its color. Heart. A muscle-sac located in the thorax between the lungs, its lower point, or a'pex, being tilted somewhat to the left; the centre and force-pump of the circulatory system. Ox i da'tion. Combining with oxygen. Ox'y gen. A colorless, odorless, tasteless gas, which forms about one-fifth of the earth's atmosphere. It is found in all animal and vegetable tissues. When it combines with other substances, a certain amount of heat is produced; and if the process is sufficiently rapid, a flame is seen. Pulse. The regularly recurring enlargement of an artery, caused by the increased blood flow following each contraction of the ventricle of the heart.