Numerous theories have been advanced to account for the symptoms seen in shock. Until recent years it was customary to suppose the vaso-motor centres had failed, being overcome by exhaustion consequent upon excessive stimulation by a greatly increased number of afferent impulses from the periphery of the body. It was suggested that as a result there was a general dilatation of the vascular system, especially in the abdominal veins, and therefore a general impairment of the circulation. Various hypotheses were, in addition, formulated, to account for the vaso-motor failure. These included the ideas of deficient carbon dioxide in the blood, exhaustion of the adrenal secretion, and exhaustion of nerve-cells in the higher centres. All these theories found their supporters and much experimental evidence was brought forward, but none was susceptible of final proof. The whole theory of vaso-dilatation and the idea that the patient “bleeds into his own abdominal veins” were eventually disposed of by observation of the clinical facts. Many extensive abdominal operations have been performed upon shocked patients, but the accumulation of blood in the splanchnic area has never been demonstrated. It has, on the other hand, been found that in the limbs the arteries and arterioles are strongly contracted. It is also by no means unusual to meet with the condition known as venospasm; the veins are collapsed and their walls contracted, so that it becomes necessary to use a considerable positive pressure before any fluid can be induced to flow into them. It has, in addition, been shown that the vaso-motor system is still active, and the heart, although beating rapidly, still responds to reflex stimulation and to increase of intracranial tension.