The exaggerated afferent impulses reaching the brain as a result of trauma, inhibit the action of the nuclei in the region of the fourth ventricle and cerebellum which maintain the muscular tone, with the result that the muscular tone is diminished and there is a marked fall in the arterial blood pressure. The capillaries dilate--the blood stagnating in them and giving off its oxygen and transuding its fluid elements into the tissues--with the result that an insufficient quantity of oxygenated blood reaches the heart to enable it to maintain an efficient circulation. As the sarco-lactic acid liberated in the muscles is not oxygenated a condition of acidosis ensues. The more highly the injured part is endowed with sensory nerves the more marked is the shock; a crush of the hand, for example, is attended with a more intense degree of shock than a correspondingly severe crush of the foot; and injuries of such specially innervated parts as the testis, the urethra, the face, or the spinal cord, are associated with severe degrees, as are also those of parts innervated from the sympathetic system, such as the abdominal or thoracic viscera. It is to be borne in mind that a state of general anæsthesia does not prevent injurious impulses reaching the brain and causing shock during an operation. If the main nerves of the part are "blocked" by injection of a local anæsthetic, however, the central nervous system is protected from these impulses. While the aged frequently manifest but few signs of shock, they have a correspondingly feeble power of recovery; and while many young children suffer little, even after severe operations, others with much less cause succumb to shock. When the injured person's mind is absorbed with other matters than his own condition,--as, for example, during the heat of a battle or in the excitement of a railway accident or a conflagration,--even severe injuries may be unattended by pain or shock at the time, although when the period of excitement is over, the severity of the shock is all the greater. The same thing is observed in persons injured while under the influence of alcohol. _Clinical Features._--The patient is in a state of prostration. He is roused from his condition of indifference with difficulty, but answers questions intelligently, if only in a whisper. The face is pale, beads of sweat stand out on the brow, the features are drawn, the eyes sunken, and the cheeks hollow. The lips and ears are pallid; the skin of the body of a greyish colour, cold, and clammy. The pulse is rapid, fluttering, and often all but imperceptible at the wrist; the respiration is irregular, shallow, and sighing; and the temperature may fall to 96° F. or even lower. The mouth is parched, and the patient complains of thirst. There is little sensibility to pain.