A Treatise on Domestic Economy; For the Use of Young Ladies at Home and at School

X. In the upper part, V S, is the _vena cava superior_, which receives the blood from the head and arms; v, v, v, are the tributaries of the upper _vena cava_, which bring the blood back from the head and arms; _d_, _d_, represents the course of the _thoracic duct_, a delicate tube by which the chyle is carried into the blood, as mentioned on page 89; _t_, shows the place where this duct empties into a branch of the _vena cava_. It thus appears, that wherever a branch of the _aorta_ goes to carry blood, there will be found a tributary of the upper or lower _vena cava_, to bring it back. 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.]