Before entering the heart, the blood receives a fresh supply of nourishment, by a duct which leads from the stomach. The arteries have their origin from the heart, in a great trunk, called the aorta, which is the parent of all the arteries, as the spinal marrow is the parent of the nerves which it sends out. When the arteries have branched out into myriads of minute vessels, the blood which is in them passes into as minute veins; and these run into each other, like the rills and branches of a river, until they are all united in two great veins, which run into the heart. One of these large receivers, called the vena cava superior, or upper vena cava, brings back the blood from the arms and head, the other, the vena cava inferior, or lower vena cava, brings back the blood from the body and lower limbs.

      In the preceding figure, H, is the heart, which is divided into four compartments; two, called auricles, used for receiving the blood, and two, called ventricles, used for sending out the blood. A, is the aorta, or great artery, which sends its branches to every part of the body. In the upper part, at a, a, a, are the main branches of the aorta, which go to the head and arms. Below, at a, a, are the branches which go to the lower limbs. The branches which set off at X, X, are those by which the intestines are supplied by vessels from the aorta. Every muscle in the whole body, all the organs of the body, and the skin, are supplied by branches sent off from this great artery. When the blood is thus dispersed through any organ, in minute vessels, it is received, at their terminations, by numerous minute veins, which gradually unite, forming larger branches, till they all meet in either the upper or lower vena cava, which returns the blood to the heart. V I, is the vena cava inferior, which receives the blood from the veins of the lower parts of the body, as seen at v, v. The blood, sent into the lower limbs from the aorta, is received by minute veins, which finally unite at v, v, and thus it is emptied through the lower vena cava into the heart: o, o, represent the points of entrance of those tributaries of the vena cava, which receive that blood from the intestines, which is sent out by the aorta at X, 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.

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.

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.

Fig. 15.

      ORGANS OF DIGESTION AND RESPIRATION

      Digestion and respiration are the processes, by which the food is converted into blood for the nourishment of the body. The engraving on the next page (Fig. 16) shows the organs by which these operations are performed.

      In the lower part of the engraving, is the stomach, marked S, which receives the food through the gullet, marked G. The latter, though in the engraving it is cut off at G, in reality continues upwards to the throat. The stomach is a bag composed of muscles, nerves, and blood-vessels, united by a material similar to that which forms the skin. As soon as food enters the stomach, its nerves are excited to perform their proper function of stimulating the muscles. A muscular (called the peristaltic) motion immediately commences, by which the stomach propels its contents around the whole of its circumference, once in every three minutes.

Fig. 16.

This movement of the muscles attracts the blood from other parts of the system; for the blood always hastens to administer its supplies to any organ which is called to work. The blood-vessels of the stomach are soon distended with

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