Crile and his co-workers have shown that in surgical shock histological changes occur in the cells of the brain, the adrenals, and the liver, and that these are identical, whatever be the cause that leads to the exhaustion of the energy-transforming mechanism.  These changes vary in degree, and range from slight alterations in the structure of the protoplasm to complete disorganisation of the cell elements.

The influences which contribute to bring about this form of exhaustion that we call shock are varied, and include such emotional states as fear, anxiety, or worry, physical injury and toxic infection, and the effects of these factors are augmented by anything that tends to lower the vitality, such as loss of blood, exposure, insufficient food, loss of sleep or antecedent illness.

Any one or any combination of these influences may cause shock, but the most potent, and the one which most concerns the surgeon, is physical injury, e.g., a severe accident or an operation (traumatic shock).  This is usually associated with some emotional disturbance, such as fear or anxiety (emotional shock), or with haemorrhage; and may be followed by septic infection (toxic shock).

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 anaesthesia 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 anaesthetic, 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.