Cushing and Crile have studied the subject exhaustively in animals. They have shown that certain injuries are likely to be followed by well-marked reduction of blood pressure; for example, those of the brain, the interior of the larynx, the abdomen and testicle, are often followed by a marked reduction of pressure without any preliminary rise. In other words, vasomotor paralysis is sometimes an almost instantaneous effect of certain injuries. When most of the blood is collected in the venous system and the central nervous system fails, because of lack of blood supply, to respond to those normal stimuli which are essential to heart action and respiration, the heart weakness or heart failure is due, not alone to failure of its innervation, but to its reduced output and its diminished content of blood on the left side. Crile has shown that the more abundant the blood supply to a given part the more it contributes to production of shock; hence, the value of cocainizing the interior of the larynx and the nerve trunks. Aside from emotional causes--which are sometimes inseparable from even surgical cases--the principal agencies in the production of surgical shock and collapse are those which make a sudden and deep impression through the medium of the sensory nerves upon the central nervous system or the large sympathetic ganglia; ultimately upon the latter in all serious cases. _Loss of blood, then, need not play a very important role._ Weakened, anemic, or neurotic patients are predisposed by virtue of these conditions, and also the young and aged. Fright combined with injury increases the degree of the effect. Crile has shown that trifling lesions of the interior of the larynx will cause symptoms which do not occur in animals if the superior laryngeal nerves be divided or if the parts be cocainized. The upper portions of the abdomen are more sensitive in this respect than are the lower, testicles particularly. The skin is more impressible than the muscles or tissues beneath, save the nerve trunks, which are very sensitive; the bones and large joints slightly so. After shock has been once produced further injury causes a disproportionate lowering of blood pressure. So-called “concussion of the brain” is essentially a condition of shock following injury to this particular part of the body. (See Head Injuries.) _Shock and hemorrhage are often closely associated_, and loss of blood is doubtless a powerful factor in the production of the former, especially in those already reduced or whose blood contains a lowered percentage of hemoglobin. There are, therefore, great advantages in entrusting an assistant with the duty of watching blood pressure during serious operations.