The succeeding engravings, will enable the reader to form a more definite idea of this important function of the system,--the circulation of the blood. The heart, in man, and in all warm-blooded animals, is double, having two auricles and two ventricles. In animals with cold blood, (as fishes,) the heart is single, having but one auricle and one ventricle. Fig. 13, represents the double heart as it appears when the two sides are separated, and also the great blood-vessels; those on the left of the figure being on the right side of the body, and _vice versa_. The direction of the blood is represented by the arrows. A, represents the _lower vena cava_, returning the blood from the lower parts of the body, and L, the _upper vena cava_, returning the blood from the head and arms. B, is the _right sinus_, or _auricle_, into which the returned blood is poured. From this cavity of the heart, the blood is carried into the _right ventricle_, C; and from this ventricle, the _pulmonary arteries_, D, convey into the lungs the blood which is returned from the body. These five vessels, A, B, C, D, and L, belong to the right side of the heart, and contain the venous or dark-colored blood, which has been through the circulation, and is now unfit for the uses of the system, till it has passed through the lungs. [Illustration: Fig. 13.] When the blood reaches the lungs, and is exposed to the action of the air which we breathe, it throws off its impurities, becomes bright in color, and is then called arterial blood. It then returns to the left side of the heart, (on the right of the engraving,) by the pulmonary veins E, E, (also seen at _m_, _m_, Fig. 15,) into the left auricle F, whence it is forced into the ventricle, G. From the left ventricle, proceeds the _aorta_, H, H, which is the great artery of the body, and conveys the blood to every part of the system. I, J, K, are branches of the aorta, going to the head and arms. [Illustration: Fig. 14.] Fig. 14, represents the heart, with its two sides united as in nature; and will be understood from the description of Fig. 13. On the opposite page, Fig. 15, represents the heart, with the great blood-vessels, on a still larger scale; _a_, being the _left ventricle_; _b_, the _right ventricle_; _c_, _e_, _f_, the _aorta_, or great artery, rising out of the left ventricle; _g_, _h_, _i_, the branches of the aorta, going to the head and arms; _k_, _l_, _l_, the _pulmonary artery_, and its branches; _m_, _m_, _veins of the lungs_, which bring the blood back from the lungs to the heart; _n_, _right auricle_; _o_, _vena cava inferior_; _p_, veins returning blood from the liver and bowels; _q_, the _vena cava superior_; _r_, the _left auricle_; _s_, the left _coronary artery_, which distributes the blood exclusively to the substance of the heart. [Illustration: Fig. 15.] ORGANS OF DIGESTION AND RESPIRATION.