The organs of our body are like so many little machines, to which one must furnish fuel if their working power is to be increased. But whereas, in ordinary mechanisms, it is a strange hand which keeps up the fire and directs the movements, our organism is so perfect that in it all apparatus regulate themselves with the greatest harmony of object. In the working muscle the blood-vessels expand, thus more easily to transmit the fuel, and in order that the muscle may convert the chemical force of food into a contraction. In the digesting stomach the circulation is more abundant, because the glands must secrete a greater quantity of juice, the little veins absorb the fluids contained in the stomach, and the muscles contract more quickly in order to mix the food. Our organism, like all working machines, not only consumes and destroys fuel, here represented by those elements which constitute the blood, but through its activity it also wastes those parts of the body which represent the wheels, axles, hinges, and other parts of a mechanism. At every contraction of the muscles, at every sensation in the brain and nerves during any mental work, there is a wasting of the organs. The blood, flowing continually through all parts of the body in order to feed the flame of life, sweeps at the same time the most remote corners of our organism clean of soot, or the remains of combustion. The vessels become relaxed and expand. Nutrition and organic change become more rapid, the nutritive fluid trickles more easily through the walls of the vessels, the blood flows more quickly, and carries everywhere along with it all the waste products in order to bring them to the kidneys. These purify the blood, and expel, with the urine, the scoriæ of the working organism. We have seen how the circulation in the brain is accelerated during mental activity, emotion, and in a waking condition; we shall return to this subject in the next chapter, and study more nearly the mechanism by which such variations are produced in all the other organs of the body. This subject is of great importance to physiologists, because in no other way can the slender link which connects psychological phenomena with the material functions of the organism be rendered more evident.