**3. Chest wall: Muscle layers, bone, and pleura**

Because air cannot be visualized by US, the normal lung parenchyma cannot be detected by US theoretically. The image of chest US in chest wall including muscle, fascia, bone, and pleura (Figure 2). The soft tissue echogenicity with multiple layers means muscles and fascia. The normal ribs appear hyperechoic surfaces with prominent acoustic shadows beneath the ribs. Approximal 0.5 cm below the ribs shadows, the visceral and parietal pleura appear as an enchogenic bright line. During respiratory movement, the two pleural lines glide with each other which is referred to as the "Gliding sign". Therefore, the "Gliding sign" means normal parietal and visceral pleura slide over each other during respiration and the loss of "Gliding sign" can be seen in pneumothorax or diffuse pleural thickening.
