_Active hyperæmia_ may be brought about by any excitement which especially affects the brain. This has been already noted in connection with insolation (sun-stroke). It may result from severe exertion during hot weather, in a violently contested race, in drawing a heavy load up hill, or in harsh training. Violent exertion just after a meal is especially injurious. Also the excitement of travelling by rail, or that caused by proximity to locomotives, to discharges of firearms and to other causes of great fear; encreased blood tension in the cerebral vessels in connection with hypertrophy of the left ventricle, or obstruction in other vessels (of the limbs) so as to direct the force of the current into the carotids, the expulsion of blood from the splanchnic cavities by gastric or intestinal tympany, or overloading of the paunch, and irritation of the brain by ptomaines and toxins in certain infectious diseases (rabies, canine distemper, etc.). In the same way vegetable narcotics (opium, etc.) produce congestion. Among the most common causes of congestion are lead, poisoning by lolium temulentum, partially ripened lolium perenne, millet, Hungarian grass, and partially ripened seeds of the leguminosæ (chick vetch, vicia sativa.). Other causes are the presence of tumors (cholesteatoma) and parasites (cœnurus, cysticercus) in the brain. _Symptoms._ Cerebral hyperæmia, like other brain disorders may give rise to a great variety of symptoms, according to the condition of the animal and the susceptibility of its nerve centres. Some cases have the characteristic seizures of vertigo, others the manifestations of heat stroke, and others, epileptic explosions or apoplectic symptoms. For these see under their respective headings. In other cases the symptoms are those of encephalo-meningitis but moderate in its type and often tending to a transient duration, or to prompt resolution and recovery.