The normal blood pressure in healthy adults is 130 to 140 Mm. of mercury in the tube. In children it ranges from 90 to 110 Mm. Females have an average lower pressure of 10 Mm. Excitement or slight stimuli will send the pressure up thirty or forty points. It is also higher than the above average in arteriosclerosis. In uremia it is always high. In cases of intracranial tension it is also high, as the brain alone of all the organs of the body has no complete vasomotor apparatus of its own; when it needs more blood this has to be contributed from the general supply. When pressed upon by a clot, depressed bone, or foreign body it becomes anemic, and on effort to furnish the needed blood from other parts the vascular tonus is increased. Cushing has shown the value of these estimations in cases of _head injury_, for the rise of blood pressure may be regarded as an indication for operation. In typhoid fever a sudden rise of pressure is associated with perforation, or perhaps with the peritonitis which is its immediate result. On the other hand, in this disease a sudden fall of pressure is an indication of hemorrhage. The course of events in surgical shock is about as follows: Injury to afferent sensory nerves acts as a vasomotor stimulus after it reaches the centre in the fourth ventricle; a reflex impulse is then sent out which produces arterial contraction and raises the blood pressure. When the abdomen is concerned the opening and handling of its contents produce the same result through the splanchnic centres. If, however, the stimulus is excessive, too often repeated, or too prolonged the vasomotor grip is lost, the arterioles dilate, and the blood pressure is reduced. A severe injury to any part of the body may produce this effect without the preliminary rise. The popular impression that a patient “bleeds to death into his own veins” has this to justify itself, that the arterial tonus is lost and the blood is pumped through the arterioles to accumulate in the capillaries and veins, especially the abdominal, thus overloading the right side of the heart and giving it a disproportionate amount of work. Accompanying these circulatory disturbances are others, secondary and unavoidable, as of respiration, which becomes rapid and enfeebled in proportion to the degree of shock. Any factor which tends to weaken the heart’s force favors both phenomena. So important is the respiratory action that patients die from cessation of respiration rather than from impairment of the circulation. This shows the importance of maintaining artificial respiration in cases of severe shock.