In all other Vertebrates a more rapid movement of the blood is rendered possible by the insertion of a second heart, quite similar in every respect to the other heart, in the course of the blood between the respiratory organs and the body. The first heart drives the blood through the lung capillaries, and therefore corresponds to the fish-heart; from these capillaries the blood returns to the auricle of the second heart, and from the ventricle of that heart travels to the various parts of the body. When it has completed this course, it returns to the auricle of the first heart. Although these two structures work independently, they lie close together and make up a single organ. We do not therefore speak of two individual hearts, but of one heart with two halves. The first half, which receives the blood, poor in oxygen, that is returned from the body, and sends it on to the lungs, lies on the right, and is termed the _right_ half. The second half, which receives the richly oxygenated blood from the lungs, and pumps it to the various parts of the body, is termed the _left_ half (Fig. 9 and explanation). In the arrangement just described, which is found in Mammals and Birds, the blood returning from the lungs is propelled with new force through the body, and therefore circulates very quickly, so that the various parts receive a relatively large amount of oxygen in a short time. It is therefore intelligible that Birds and Mammals develop more warmth than Fish. They possess a special, constant body temperature, somewhat different in different species, but usually lying between 98° and 104° Fhr., and they are called _warm-blooded_ animals. [Illustration: FIG. 9.—Diagram of the Mammalian Heart. 1, right, 2, left ventricle; 3, right, 4, left auricle; 5, superior, 6, inferior vena cava; 7, pulmonary artery forking into branches for right and left lungs; 8, the four pulmonary veins; 9, the great body-artery (aorta); the arrows indicate the direction of the blood-stream. ] [Illustration: FIG. 10.—Diagram of the Heart of a Reptile. Between the right (_r.K._) and left (_l.K._) ventricles is a perforated partition. _r.V.K._, right auricle; _l.V.K._, left auricle; _H.v._, vena cava, carrying back the blood which has traversed the body into the right auricle; _L.art._, pulmonary artery; _L.v._, pulmonary vein; _Ao._, aorta. ]