The pleura is divided into a dirty pleura and a wall pleura, which move relative to each other when breathing. Normally there is a small amount of mucus between the two to lubricate them, so there is no pain, but when there is a chest lesion such as tuberculous pleurisy, abscess chest, pneumothorax, haemopneumothorax or malignant tumour of the pleura, the pleura will adhere and pull, so there is pain when breathing, and this pain can be radiated to the scapula.