As solid food requires to be broken up or ground before it passes into the stomach, the mouth is provided with teeth firmly implanted in the jaws, while the lower jaw is moved by strong muscles in two directions, one vertical, the other lateral. Man is supplied with two sets of teeth; the first adapted to the jaws of childhood; the second larger, which replace the former, are designed to last through life. The rudiments of each set are found in the jaws before birth. During the grinding or mastication of the food it is moistened and softened by a fluid called saliva. This also acts chemically upon it, changing the starch into sugar. The food is carried from the mouth to the stomach in a long tube called the æsophagus, by means of the muscular contraction of this tube.
The stomach is a flask-shaped organ consisting of a double wall of tissue, the outer one being muscular, the inner one vascular. This latter membrane has a large supply of blood vessels, which convey the blood out of which the gastric juice is manufactured and secreted by the little glands of this membrane. The gastric juice is a fluid which contains an active ingredient called pepsin. This, aided by the acid which this fluid contains, effects a remarkable change on the albumen of the food, making it a liquid. From the stomach the food passes into the small intestine, where it receives from a small tube the pancreatic juice which changes the fat into an emulsion.
The intestine is a tube, about twenty-five feet in length, which, like the stomach, has a double wall. Its inner coat contains multitudes of little projections called “villi.” These contain blood vessels which absorb and carry off the liquified food. It also secretes a fluid called intestinal juice, which acts upon the unchanged starch, making it into sugar. Its muscular wall by a continuous contraction produces a motion known as the “peristaltic motion,” which carries the food onward in its course. The intestine also receives the bile, a fluid produced by the liver.
The liver is the largest gland in the body. It receives the portal vein which conveys the blood from the intestines. This vein, after dividing and sub-dividing, thus bringing blood into communication with all parts of this organ, is again collected into a main trunk which passes on toward the heart. During its passage through the liver the bile is eliminated from the blood by the little cells of which this gland is composed. From the liver the bile is carried toward the intestine, into which it is poured to assist in digesting the food, and to be itself changed.
Besides the little blood vessels in the villi of the intestines there is another set of vessels called the lacteals, which aid in taking up the digested food and pouring it into the circulation; also, throughout the body a set of similar vessels collect the waste material and pour it into the great veins, returning to the heart, in order that it may be renovated or cast off; these are the lymphatics.
The kidneys are great excretory organs, and are similar in shape to those of a sheep, but are somewhat larger. They are glands, and excrete urea, as well as other salts and waste materials, all of which are highly poisonous if not removed from the blood.
Blood is the life-giving fluid of the body: it is the source from which all tissues are built, and it is the workman that carries the waste material away from the tissues. In order to accomplish its work it must circulate, and this requires a separate set of organs. First, the impure blood must be carried back from the different parts of the body; then it must be distributed again. It must also be made to flow onward in a continuous current. Blood is a viscid fluid, of a red color, containing over seventy per cent. water, with solid matter. Its color differs on each side of the heart. When it returns from the body it is blue, but when it leaves it is red. Under the microscope it is seen to contain minute globules, or disk-like cells; to these the blood owes its color. It possesses the remarkable property of spontaneous coagulation when drawn from the veins. It is forced on by the heart.
The heart is a muscle and consists of four cavities; two called auricles having weak walls, and two ventricles with strong muscular walls. The blood returning from the body is poured into the right auricle, thence into the right ventricle; from this cavity it is forced through the lungs and returns again to the heart, being poured into the left auricle, which empties into the left ventricle. This ventricle forces the blood throughout the body. The blood received by the heart from the body is impure, and is sent to the lungs, where it gives up part of its impurities and receives oxygen from the air.
Arteries are those vessels that distribute the blood, while the veins collect it and return it to the heart, thus all streams of blood leaving the heart are conveyed in arteries, and those pouring into it are carried in veins.
The lungs are the organs that purify the blood, and in order that this be thoroughly done, the blood is distributed throughout their substance in minute capillary vessels. The lungs themselves are vascular; being made up of a multitude of air cells, their surface is greatly increased; hence their power of absorption. The diaphragm is a muscular partition lying below the lungs. It is dome-shaped, and when its fibers are shortened it enlarges the cavity in which the lungs are situated. This creates a partial vacuum, causing the air to rush into the lungs. The blood absorbs the oxygen from the air and gives up carbonic acid gas. When the muscles of the diaphragm are relaxed the elastic force of the air cells in the lungs expels the remaining gases from the lungs. The diaphragm is assisted by the action of muscles situated between the ribs; these lift the ribs and enlarge the cavity of the thorax. The lungs also act as a reservoir for the air used in the production of vocal sounds. They communicate with the atmosphere by means of a tube called the trachea; this terminates in the pharynx, with which the nostrils also communicate, thus completing a passage to the outside air.