#Concussion of the Brain or Cerebral Shock.#--The symptoms associated with concussion of the brain are to all intents and purposes those of surgical shock (Volume I., p. 250), the activity of the vital centres being disturbed by violence acting directly upon the brain tissue instead of by impulses transmitted to it by way of the afferent nerves. Various theories have been put forward to account for the depression of the vital functions in concussion. According to Duret, with whose views we agree, the wave of cerebro-spinal fluid set in motion by the impact of the blow on the skull, passes, both in the ventricles and in the sub-arachnoid space, towards the base, where it impinges upon the pons and medulla, stimulating the restiform bodies and so inducing a fall in the blood pressure and a profound anæmia of the brain. The disturbance of the cerebro-spinal fluid may at the same time produce the microscopic lesions in the brain tissues described on p. 341. The symptoms of shock may be the only evidence of injury, or they may be superadded to those of fracture of the skull, or laceration of the brain. The _clinical features_ vary according to the severity of the violence. In the slightest cases the patient does not lose consciousness, but merely feels giddy, faint, and dazed for a few seconds. His mind is confused, but he rapidly recovers, and, perhaps after vomiting, feels quite well again, save for a slight shakiness in his limbs. In more severe cases, immediately on receiving the blow the patient falls to the ground unconscious. Sometimes he suffers from a general tetanic seizure associated with arrest of respiration, which is usually of short duration and is frequently overlooked, but may prove fatal. The pulse is slow, small, and feeble, and is sometimes irregular in force and frequency. The respirations are short, shallow, slow, and frequently sighing in character. The temperature falls to 97° F., or even lower. The skin is cold and pallid and covered with clammy sweat, and the features are pinched and pale. In uncomplicated cases the pupils are usually equal, moderately dilated, and react sluggishly to light. The patient can be partially roused by shouting or by other forms of external stimulation, but he soon subsides again into a lethargic condition. Although voluntary movement and the deep reflexes are abolished, there is no true muscular paralysis.