In our physiological illustrations we have, upon all occasions, sought to establish general principles for the solution of the various problems of forensic medicine. It has been said that “it is not so much the knowledge of the laws of physiology, as that of the exceptions to which they are liable, that is required in elucidation of abstruse medico-legal questions.” If this were admitted, the propriety of such scientific applications might be altogether doubted. “_Leges fiunt de his quæ vulgo, non de his quæ raro eveniunt_”; but, in truth, the exceptions of Nature are but apparent—the mere illusions arising from our imperfect view of her phenomena; and will diminish as our knowledge increases, just as the motions of the heavenly bodies cease to appear irregular as soon as their orbits are submitted to a more extended field of observation. The second volume of our work commences with a physiological research into the “Causes and Phenomena of Sudden Death.” To the views developed in this chapter we are the more particularly anxious to direct the attention of the student, as they may be said to constitute the centre, and master-key of forensic physiology; while the obvious importance of their applications, in directing the treatment of asphyxia and cases of poisoning, will convey a striking rebuke to those who still deny the _practical_ utility of such researches. We might even extend this remark to the more ordinary duties of the surgical practitioner, and in support of its truth, maintain, that he can neither fully comprehend, nor successfully treat the more important symptoms which attend injuries of the head, without an acquaintance with those mutual relations which subsist between the functions of the brain and heart, and those of the organs of respiration. To an ignorance of such views we may trace the origin of those discordant opinions which have existed with regard to the proper mode of treating concussion, or compression of the brain. Some practitioners, from having observed that the action of the heart frequently becomes enfeebled on these occasions, have unconditionally insisted upon the necessity of cordials; while others, reasoning upon the state of the brain, have with equal confidence advocated the propriety of immediate and copious depletion by the lancet. Let us see how far a knowledge of the physiological doctrines to which we have alluded will reconcile such conflicting opinions, and point out the proper plan which ought to be pursued in such cases of difficulty.