_Of Compression of the Brain._—Compression is produced by extravasation within the cranium of blood or other fluid, by the lodgement of a foreign body on the surface of the brain, or in its substance, or by displacement inwards of portions of the cranial bones; and these causes are usually the effects of external injury. It may either follow the injury instantaneously, or supervene some time thereafter. Many examples have occurred of a patient, at first insensible, with symptoms of concussion, having had the functions of the brain restored almost entirely, and again having relapsed very quickly into a comatose state, in consequence of extravasation of blood. The whole circulation is at first lowered by the shock of the commotion, and the blood scarcely flows in the cerebral vessels; but on its restoration, blood is poured out from the lacerated vessels, or from those which have been so injured in their coats as to be unable to withstand the increasing impulse of their contents. As was already observed, the symptoms of compression are often mixed up with those of commotion, but, when an interval of sensibility has occurred, mistake in diagnosis can scarcely occur. Compression is attended with slow, stertorous breathing; a distinct slow pulse; a relaxed state of the limbs, features, and sphincters; and dilated pupil. Total insensibility to external impressions attends compression of the brain, whatever the cause of it may be. These symptoms may, and do sometimes, gradually disappear after a time. But they may continue unabated, and the patient may gradually sink under them. Or, again, his dissolution may be preceded by excited circulation and furious delirium, the vital powers recovering from their first depression, only to become roused into violent and destructive action, again to sink to a still lower ebb, and be ultimately annihilated. Extravasation is most commonly met with on the lateral parts of the brain in the situation here indicated; the coagulum is perhaps extensive, reaching to the base of the skull, in consequence of rupture of the middle meningeal artery, with or without fracture of the parietal bone. [Illustration]