at 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