If we reject the hypothesis of the spiritualists, we must, with the same severity, banish from the borders of experimental science those who, in our time, wish to explain, by means of materialistic doctrines, the mechanism generating thought. Anatomy and physiology, the knowledge of structure and of cerebral functions, have scarcely lisped their first words, and dense darkness reigns over the nature of nervous processes, over the physical and chemical movements animating the hidden parts where consciousness has its throne. Let us speak neither of spirit nor of matter; let us candidly acknowledge our ignorance. We trust to the future of science and persevere in the search after truth. CHAPTER V _PALLOR AND BLUSHING_ I Man has, on the average, four kilograms of blood, and this fluid flows incessantly in a system of tubes, in the centre of which the heart is situated. The arteries carrying the blood from the heart to the surface divide into many branches, separate, extend, and visit all parts of the body, feeding and irrigating them. When the ramifications of the arteries become so small that the eye can no longer see them, as, for instance, in the lips, the finger-tips, the cheeks, the ears, or any part of the skin, they take the name of capillaries. This is meant to indicate that these little arteries are as fine as a hair, but in reality they are very much finer. These last closely connected capillary nets give the skin its beautiful rosy colour. But however much they diminish, dividing and subdividing _ad infinitum_, they still form a system of canals, with walls and closed on all sides. There must be a wound, a cut, or a contusion, before the blood oozes out of these little vessels. Out of the capillaries the blood passes into larger canals called veins. Several veins flowing into each other form a bigger vein; in the same way as a brook is formed by springs, as the brooks, running into each other, form a rivulet, and the rivulets, a river; so the veins gradually receive the blood in larger streams, until at last they carry it in the great trunk-veins to the heart, which drives it again into the arteries.