I. =Active= or =Arterial Determination of blood=. In this form the arteries are dilated under a direct nervous influence. _Causes._ In all the regular functions of the body, the flow of blood is under the direct control of the vaso-motor nerves which proceed from the spinal cord, through the branches of the sympathetic to be distributed with the blood vessels. The hard pulse of pleurisy is due to rigid contraction of the constrictor muscles under the action of the vaso-motor nerves, and the blush of shame is due to their relaxation. Some claim an active dilatation of the arterial muscular coats, others look more simply upon the dilatation as a mere yielding of the coats under the blood pressure, when the constrictor muscles are relaxed. This vaso-motor paresis may be induced: 1st, by any lesion of the spinal cord. 2d, by the cutting of a sympathetic trunk, that of the abdomen, for example, which leads to active congestion of the abdominal viscera, or the section of the cervical sympathetic which leads to watering of the eye, sweating, congestion, and scabbing on the corresponding half of the face. 3d, by reflex irritation through the sensory nerves, as in congestion through friction, heat or cold to the skin, or that resulting from excessive use of an organ such as the mammary gland. 4th, by causes acting directly through the brain as in emotional blushing or the facial congestion of violent rage. Physiologically we see the operation of this nervous control in the congestion of the gums during dentition, of the salivary glands during mastication, of the stomach and bowels during digestion, of the womb during gestation, of the mammæ at parturition, and of erectile organs in copulation. Medicinal agents act in the same way, opium or alcohol producing active dilatation, and belladonna and ergot causing active contraction of the arterial walls. Bacteridian poisons act in the same way, tuberculin and a number of others causing active dilatation. The obstruction of one artery by thrombus, tumor, or ligature, causes increased tension in the collateral branches coming off just above and an active congestion in the parts to which these are distributed. While this is directly due to increased local pressure, it is also an instance of the lack of balance between the blood pressure and the resistance of the vascular walls. In this case there is increase of pressure, in the other a diminished resistance. If there is a superficial anæmia, as from cold or chill, there is of necessity, an internal hyperæmia. This contributes to the production of internal congestions and inflammations, though the seat of election of such inflammation is usually determined by the nervous sympathy between the part chilled and the deeper organ affected.