The nutritive and perfectly digested portions of food are absorbed partly by the veins of the stomach, entering at once into the circulation, and partly by the intestines. The lining membrane of the intestines folds over and over upon itself, like a ruffle, along the entire edge; this is full of little tubes, or villi, which absorb the chyle.
The blood vessels absorb the nutritive elements from the villi and carry them to the veins in two ways: 1st. Through the portal vein into the liver, where it penetrates every part of the liver, then passes out through the hepatic vein into the veins near the heart; 2d. Through the lacteals, which are attached to the lining membrane of the intestines and empty into the thoracic duct, a tube extending along the spine, and then into the subclavian vein, which lies in the left side of the neck, under the collar bone. The veins also bring with them the lymph, - a thin colorless fluid which comes from the absorbent vessels situated all over the body, and which contains the worn-out particles.
Then the venous blood, supplied from the lacteals with new material, and from the lymphatic vessels with waste materials, enters the heart through the upper door, or right auricle, passes through the valves down into the right ventricle; out through the pulmonary artery into the lungs, where, as purple venous blood, it penetrates to the most remote capillaries.
If the lungs be full of pure fresh air, the oxygen changes the purple blood into red blood, and burns up the impurities. The waste products of the combustion of carbon and hydrogen are expelled from the lungs at every breath in the form of carbonic acid and watery vapor; and not until now can the new elements in the blood, obtained from the food, become in reality food, or perfect blood.
This oxygenized or vitalized blood now returns from the lungs, and enters the heart through the left upper door, or left auricle; the valves open and allow it to pass into the left ventricle, then out through the aorta, or great artery, from which the arteries carry it to the capillaries all over the system.
In the capillaries the new material is deposited wherever needed, and changed by cell growth into new tissue. The lymphatic vessels take up all that is not needed, with the worn-out portions; and the veins then carry this impure blood back again to the lungs and heart.
Thus a continuous circulation is established, the blood coursing over the whole body once in every three to five minutes, the time varying with the amount of exercise and the state of health. During this circulation the combustible compounds are burned by the oxygen received into the blood in the lungs, the carbonaceous products of combustion are expelled through the lungs as carbonic acid gas and watery vapor, and the nitrogenous products through the kidneys in the form of urates.
"This process of digestion and absorption is really a kind of preliminary cooking process, going on from the mouth downwards all the way to the colon; and from every part of the long canal tiny lacteals and absorbing veinlets carry off contributions of food either to the general store of chyle, or to the venous blood which is hurrying back to the heart."