Veins. The blood vessels and their branches through which blood flows from all parts of the body back to the heart. All the veins except the pulmonary veins carry impure (venous) blood; the pulmonary veins carry arterialized (oxidated) blood from the lungs. Ve'na ca'va. Either of the two large veins discharging into the right auricle of the heart. Por'tal vein. The large, short vein that drains the liver and adjacent parts. Ven'tri cles. The two chambers of the heart that receive blood from the auricles and force it into the arteries. V. RELATING TO THE RESPIRATORY SYSTEM AND ORGANS OF EXCRETION Al ve'o li ([)a]l v[=e]'o l[=i]). (Plural of _alveolus_). Air cells. The cells, or cavities, that line the air passages and air sacs at the ends of the bronchial tubes. Breath. Air taken in or sent out in respiration; that breathed out containing carbon dioxid, watery vapor, and various impurities. Bron'chi (br[)o]n'k[=i]). (Plural of _bronchus_). The two main branches of the trachea. These branch into numerous smaller branches, called the bron'chi al tubes. Car'bon di ox'id. A gas formed of carbon and oxygen; colorless and odorless; has a somewhat acid taste, and is used for aerating soda water and other beverages; is present naturally in mineral and spring waters. It is present largely in the fissures of the earth and makes the choke-damp of mines. Called also car bon'ic acid. Ep i glot'tis. The valve-like cover that prevents food and drink from entering the larynx. Ex cre'tion. A waste substance thrown out, or rejected, from the system; for example, carbon dioxid, sweat, ur'ine, the fe'ces. Lar'ynx. The enlargement of the windpipe, near its upper end, across which are stretched the vocal cords. Lungs. Two spongy organs in the thorax, entered by the bronchi with their bronchial tubes; they contain in the walls of their air cells the capillaries through which the blood passes from the branches of the pulmonary artery to the branches of the pulmonary veins. Rec'tum. The lowest and last section of the alimentary canal, being the discharge pipe of the large intestine, and excreting the solid wastes in the form of the feces. Res pi ra'tion. Breathing; the action of the body by which carbon dioxid is given off from the blood and a corresponding amount of oxygen is absorbed into the blood. Skin. The continuous outer covering of the body, in the deeper layer (der'ma) of which are located the sweat glands, which secrete sweat (a watery, oily substance containing impurities from the blood) and excrete it through the sweat ducts and their openings (pores) in the surface of the skin. Tra'che a (or tr[=a] ch[=e]' [.a]). The windpipe between the larynx and the bronchi.