After the head is hit, there will be a “big bag”, mostly subcutaneous hematoma, while other parts of the body will not, because of the different subcutaneous structure. Generally speaking, the fat layer under the scalp is thin, rich blood supply, under the scalp is the hard skull, and the subcutaneous tissue and the skin layer and the capillary tendon membrane layer connects closely. After trauma, the scalp capillary rupture, outflow of blood can not be diffused, will be gathered between the scalp and skull, while being hit by the part of the soft tissue swelling, inter-cellular fluid will also accumulate, it will form a “big bag”. In other parts of the body, there are usually layers of fat and muscle under the skin, which act as a cushion to reduce capillary breakage when hit, and therefore produce relatively less blood and tissue swelling. In addition, the blood produced by the breakage can also spread to the skin, fat and muscle to form bruises, so it will not form a “big bag”. After the head was hit out a “big bag”, do not use your hands to rub, also should not be immediately hot compresses, otherwise it will aggravate the bleeding, the bulge is getting more and more serious. In the 48 hours after the injury can be wrapped in a towel with ice or ice packs for local cold compresses, 48 hours after the change to hot compresses to promote hematoma absorption. In general, the “big bag” will be absorbed within 1-2 weeks. If accompanied by symptoms such as dizziness, nausea, vomiting, etc., should immediately go to the hospital for examination to avoid adverse events.