Trachea lining. Coloured scanning electron micrograph (SEM) of the lining of the trachea (wind pipe). The trachea links the larynx to the lungs. At centre is a mucous-secreting gland, it is surround by epithelial cells that have hair- like cilia (yellow) on their surface. Mucous secreted by the gland traps debris, such as dust particles or bacteria, in the inhaled air, while the beating of the cilia moves the mucous and particles upwards out of the respiratory tract. This helps to keep the lungs and airways clear and prevent infection.