Because the brain stem is located in the back of the head. The brain stem is regarded as the "center" of the nerve center. The function of brain stem is mainly to maintain individual life, including heartbeat, respiration, digestion, body temperature, sleep and other important physiological functions, all related to the function of brain stem. Brain stem injury is a serious and even fatal injury. About 10%-20% of severe craniocerebral injury is accompanied by brain stem injury. Simple brain stem injury happens occasionally, but it is rare. The brain stem, including the midbrain, pons and medulla oblongata, is located at the bottom of the central axis of the brain, with the dorsal side connected with the cerebellum and the ventral side with a hard skull base, just like a snail lying on a slope. When external force acts on the head, whether it is direct violence or indirect violence, it will cause the collision and movement of brain tissue. The brain stem is not only bruised on the hard skull base, but also injured by pulling, twisting, squeezing and impact of the brain and cerebellum, among which whipping, rotating or occipital violence is the most harmful. Usually, a blow to the forehead can cause the brain stem to collide with the slope; When the lateral head is injured in twisting motion, the brain stem can be twisted and pulled due to the rotation of the brain or cerebellum; When the occipital part is stressed, the brain stem can directly hit the slope and the foramen magnum; In whiplash injury caused by sudden tilt of the head, the medulla oblongata is more likely to be injured; Occipital depressed fracture can directly damage the medulla oblongata when the foot or hip is forced; In addition, when the head is hit, the skull is severely deformed, and the shock wave passing through the cerebrospinal fluid of the ventricle will also cause damage around the aqueduct of the midbrain or the bottom of the fourth ventricle. The primary brain stem injury caused by head violence accounts for about 2-5% of craniocerebral injury and 10-20% of severe craniocerebral injury. In the brain stem, besides the sensory and motor conduction bundles of the brain nuclei and the body, there are also reticular structures and life centers such as respiration and circulation. Therefore, its disability rate and mortality rate are high. Generally speaking, the violence leading to primary brain stem injury is often very serious. Therefore, single brain stem injury is rare, but it exists at the same time as brain contusion, so the injury is more serious than general brain contusion. If the primary brain stem injury is accompanied by intracranial hematoma, brain edema or diffuse brain swelling caused by brain contusion, it will increase the secondary brain stem injury on the basis of the primary brain stem injury, and if the treatment is delayed, it will lead to ischemic necrosis of the trunk, with extremely serious consequences.