Passive and active hyperæmia. Causes: _passive_: obstacles to return of blood: anæmia: _active_: brain excitement, sun-stroke, violent exertion, fear, abdominal tympany, ptomaines, narcotics, lead, darnel, millet, leguminous seeds partly ripened, tumors, parasites. Symptoms: _horse_: variable, vertigo, stupor, convulsions, apoplexy, irritability, disorderly movements, strong, hard pulse, congested mucosæ, heat of head, dulness, drowsiness, lethargy, coma, alternating periods of violence, aggravated by what tends to increase vascularity of brain, congested optic disc: _cattle_: parallel, with special heat of horns: _dogs_: similar, with desire to move, or wander, or has nausea, howls, snaps. Treatment: cold to head, derivation to limbs and bowels, chloral, bromides, ergot, bleeding, darkness, coolness, non-stimulating food. Congestion of the encephalon is treated here as a pathological entity, though it cannot always be distinguished clinically from some forms of vertigo on the one hand and from the milder types of apoplexy or encephalitis on the other. It has been divided into _passive_ or _venous hyperæmia_ and _active_ or _arterial hyperæmia_. _Passive hyperæmia_, as shown under vertigo and apoplexy is a common result of a tight collar, a tight strap used for cribbiting, a too short bearing rein, dilation or valvular disease of the right heart, or disease of the lungs, violent efforts in running, draught, etc. It tends to be associated with arterial anæmia on the principle that the closed cranial cavity can only admit a certain amount of blood and if an excess accumulates in the veins and capillaries, this must be compensated first by the movement backward to the spinal canal of the cerebro-spinal fluid, and second by the diminution of the blood in the cerebral arteries.