The little canals in which the blood circulates are provided with muscular fibre. These may relax and the calibre of the vessels is increased, or they contract and the calibre is reduced. The pallor, so characteristic of fear, arises from a contraction of the vessels; the beautiful blush of modesty, most eloquent of all the revelations of psychic facts, is nothing else but a dilatation of the blood-vessels. These two opposite phenomena do not depend on the heart, since we know that the heart beats more forcibly and rapidly during the emotion of modesty as well as during fright. From the nerve-centres innumerable filaments branch off which are distributed to all the ramifications of the blood-vessels. These are the so-called vaso-motor nerves, which, without our noticing it, act on the muscular fibre of the small arteries and veins, increasing or diminishing the calibre of the little canals in which the blood flows. The effects of the passions are far more evident on the countenance, with its blushing and sudden pallor, than elsewhere, because in no other part of the body are the blood-vessels so sensitive as in the face. There are two reasons for this, firstly, the nerve-centres act more powerfully on these vessels; secondly, they are more delicate, sooner growing tired, and relaxing at the slightest disturbance of nutrition. Indeed, if we inhale the vapour of a substance which, like that of nitrite of amyl, paralyses the blood-vessels, the face immediately becomes of a vivid red, and anyone making this experiment feels his face aflame in a few seconds. This is the simplest method which we possess for artificially producing the external phenomena of shame. At different ages and in different persons considerable differences are noticeable with regard to the greater or lesser facility with which they blush or grow pale. I made a long series of investigations in order to see at what degree of temperature the paralysis of the blood-vessels of the hands appears when we dip them into hot water, and at what degree, and after what lapse of time, the hands begin to redden when we hold them in ice-water or in snow, the differences being found to be very considerable. An old lady does not blush under those moral emotions which used to betray her feelings as a girl; and this, not because age has overcome the timidity of youth, or because the hard struggles of life have blunted her sensibility, but because the blood-vessels of the face have, in course of time, become less yielding. On long walks taken in the sun, one always notices that the faces of babies are redder than those of bigger children, and these, in their turn, are more flushed than those of their parents. Even persons of the same age do not respond in the same way to the internal or external stimuli which tend to dilate or contract the blood-vessels. It is well-known that all girls do not blush equally at a pleasantry directed to them.