There are small black lines in the nails, which are considered to be subnail bruises, nevus of the nail, nail dystrophy and so on. 1. Bruising under the nail: light bleeding and bruising under the nail caused by local trauma, collision, extrusion and other external factors can show small short black lines in the nail; specifically, light bleeding under the nail bed can be shown as a bleeding point that can form a small black broken line along the texture of the nail and the crevice; a larger amount of bleeding under the nail bed will appear as a bruise spot. 2. Onychomatous nevus: small short black lines in the nail can also be considered as onychomatous nevus. The internal cause of this disease is the growth of pigmented nevus in the nail matrix; when stimulated by external causes (trauma, fungal infection, bleeding under the nail, drugs, etc.), the activity of melanocytes will be activated, and the synthesis of melanin will be excessive, which will result in the symptoms such as black lines on the nails. 3. Nail dystrophy: Nail dystrophy can also be considered as the presence of small short black lines in the nail. This disease is caused by a variety of factors (genetics, skin diseases, nail unit tumors, systemic diseases, physical injury, drugs, etc.) by affecting the structure and function of the nail matrix, resulting in nail plate formation disorders; often manifested in a variety of nail plate shape, texture, color abnormalities. This disease needs to pay attention to the possibility of deterioration, if this black line in a short period of time has become wider, deeper in color, pain and other symptoms, it is necessary to go to the hospital in time for pathological examination. It is recommended that patients go to the dermatology department of regular hospitals, with the help of doctors, to clarify the cause of the disease and choose the appropriate treatment plan, so as not to delay the condition.