=Active Hyperemia.=--Active hyperemia may be produced by purely nervous influences, even those of emotional origin. The flushing of the face which is known as “blushing” is, perhaps, the most common illustration of this fact. It is well known also that this is, in some degree at least, the result of division of certain nerves which have to do with the regulation of the blood supply. The cervical sympathetic is the best known and most often studied of these, and the consequences of division of this nerve in the neck are stated in all the text-books on physiology. So also by electrical stimulation of certain nerves the parts supplied by them can be made to show a very active hyperemia, which will subside shortly after discontinuance of stimulation, providing this has not been kept up too long. In active hyperemia there is absolute increase of intra-arterial tension, and under these circumstances pulsation may be noted in those small vessels in which commonly it is not seen nor felt. This is the explanation of the throbbing pain complained of under many actively hyperemic conditions. This hyperemia affords the explanation of the clinical signs to which attention has already been called. The increased heat of the part is the result of greater access of blood, which prevents cooling by radiation and evaporation; the peculiar redness is due to the greater filling of the capillaries with the blood, which gives the peculiar hue to the skin and visible textures; while to the increased pressure upon sensory nerves is also due the pain. The minuter changes occurring within the congested part call for more accurate description. Whether or not there is actual dilatation of capillaries under these circumstances is a matter still under dispute, but of the dilatation of the larger vessels there can be no possible question. As hyperemia is to such a great extent brought about by action of the nervous system, it is well to divide it more accurately into the hyperemia of paralysis, or _neuroparalytic congestion_, which is the result of a paralysis of the constrictor fibers of the vasomotor system, and into the hyperemia of irritation, or _neurotonic congestion_, which is due to the irritation of the dilators (Recklinghausen). Physiologists are fairly well agreed that as between the dilating and the constricting apparatus of the vasomotor system there is ordinarily preserved a certain degree of equilibrium; to which fact is probably due that normal condition of affairs inaugurated after temporary disturbance, since overaction in one direction succeeds reaction in the other. As Warren has illustrated this, our common treatment of frostbite by cold applications is a concession to this fact, since by the cold applications we endeavor to limit the reaction which would otherwise follow after thawing out the frozen part.