In quantity, the blood is from 3 to 4 per cent of the total weight of the human body. It is contained in the heart and in two sets of vessels, one set called the arteries, leading from the heart by various ramifications to all parts of the body, and the other set tailed the veins, leading from all parts of the body back to the heart.

136. Circulation

Through these vessels the blood is moving in a constant stream, which we call the circulation. It does not move of itself, but is forced along by a very powerful pump, the heart. This is a highly muscular organ divided into four chambers, which are separated by valves and partitions, the two upper chambers being called the right and left auricles, and the two lower, the right and left ventricles. The right auricle is above the right ventricle, and is separated from it by a valve, and the same is true of the left auricle and ventricle. Out of the left ventricle, the blood is pumped into the arteries, and after reaching the arterial

Fig. 10. - Diagram of circulation. 1, heart; 2, lungs; 3, head and upper extremities; 4. spleen; 6, intestine; 6, kidney; 7, lower extremities; 8, liver. (Collins.) capillaries throughout the entire body, it passes from these into the smallest divisions of the veins and comes back to the heart along the venous system, entering the right auricle. It is then carried to the lungs by way of the right ventricle and is returned to the left auricle to be sent to the left ventricle, and from there to again start on its journey through the body. As we shall see, the arterial blood carries to the body food nutrients and oxygen, and the venous blood brings back the wastes. The principal facts pertaining to the blood and its circulation have been reviewed in this simple manner as an aid to the discussing of other considerations somewhat pertinent to our subject.

136. Entrance Of Nutrients

The nutrients, as prepared for use by digestion, enter the blood on its return flow to the heart, coming into the venous cavity by way of the hepatic (liver) vein and the thoracic duct as previously described. When, therefore, the right side of the heart is reached, a new accession of food material is on its way to sustain the various functions of nutrition.

We are more interested in the object of blood circulation than we are in its mechanism. Somehow the digested food disappears into these constantly moving blood currents, and the only evidence of its effect which comes to us from ordinary observation is the warmth, motion, and perhaps growth, of the animal that is nourished.