Cold-blooded Animals.--In animals which are called cold-blooded, the blood has no fixed temperature, but varies with the temperature of the medium in which the animal lives. Frogs, in the summer, may sit for hours in water with a temperature of almost 100 deg. In winter, they often endure freezing so that the blood and lymph within the spaces under the loose skin are frozen into ice crystals. This change in body temperature is evidently an adaptation to the mode of life. Circulation of the Blood in Man.--The blood is the carrying agent of the body. Like a railroad or express company, it takes materials from one part of the human organism to another. This it does by means of the organs of circulation,--the heart and blood vessels. These blood vessels are called _arteries_ where they carry blood away from the heart, _veins_ where they bring blood back to the heart, and _capillaries_ where they connect the larger blood vessels. The organs of circulation thus form a system of connected tubes through which the blood flows. The Heart; Position, Size, Protection.--The heart is a cone-shaped muscular organ about the size of a man's fist. It is located immediately above the diaphragm, and lies so that the muscular apex, which points downward, moves while beating against the fifth and sixth ribs, just a little to the left of the midline of the body. This fact gives rise to the notion that the heart is on the left side of the body. The heart is surrounded by a loose membranous bag called the _pericardium_, the inner lining of which secretes a fluid in which the heart lies. When, for any reason, the pericardial fluid is not secreted, inflammation arises in that region. [Illustration: Diagram showing the front half of the heart cut away: _a_, aorta; _l_, arteries to the lungs; _la_, left auricle; _lv_, left ventricle; _m_, tricuspid valve open; _n_, bicuspid or mitral valve closed; _p_ and _r_, veins from the lungs; _ra_, right auricle; _rv_, right ventricle; _v_, vena cava. Arrows show direction of circulation.] Internal Structure of Heart.--If we should cut open the heart of a mammal down the midline, we could divide it into a right and a left side, _each of which would have no internal connection with the other_. Each side is made up of an upper thin-walled portion with a rather large internal cavity, the _auricle_, which opens into a lower smaller portion with heavy muscular walls, the _ventricle_. Communication between auricles and ventricles is guarded by little flaps or _valves_. The auricles receive blood from the veins. The ventricles pump the blood into the arteries.