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.
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.
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 blood, from which the _gastric juice_ is secreted by minute vessels in the coat of the stomach. This mixes with the food, and reduces it to a soft pulpy mass, called chyme. It then passes through the lower end of the stomach, into the intestines, which are folded up in the abdomen, and the upper portion, only, of which, is shown in the engraving, at A, A. The organ marked L, L, is the liver, which, as the blood passes through its many vessels, secretes a substance called _bile_, which accumulates in the gall-bladder, marked B. After the food passes out of the stomach, it receives from the liver a portion of bile, and from the _pancreas_ the _pancreatic juice_. The pancreas does not appear in this drawing, being concealed behind the stomach. These two liquids separate the substance which has passed from the stomach, into two different portions. One is a light liquid, very much like cream in appearance, and called _chyle_, of which the blood is formed; the other is a more solid substance, which contains the refuse and useless matter, with a smaller portion of nourishment; and this, after being further separated from the nourishing matter which it contains, is thrown out of the body. There are multitudes of small vessels, called _lacteals_, which, as these two mixed substances pass through the long and winding folds of the intestines in the abdomen, absorb the chyle, and convey it to the _thoracic duct_, which runs up close by the spine, and carries the chyle, thus received, into a branch of the _vena cava superior_, at _t_, whence it is mingled with the blood going into the heart. In this engraving, the _lacteals_ and _thoracic duct_ are not shown; but their position is indicated by the dotted lines, marked X, Y; X, being the lacteals, and Y, the thoracic duct.