Cilia. Coloured transmission electron micrograph (TEM) of a cross section through cilia (circles), from the lining (epithelium) of the human trachea, or windpipe. Cilia in the trachea are hair-like projections that beat rhythmically to move mucus away from the gas-exchanging parts of the lungs, up towards the throat where it can be swallowed or coughed up. They project in parallel rows, with 300 on each cell, measuring up to 10 micrometres in length. Each cilium contains a central core (axoneme), which consists of twenty microtubules arranged as a central pair, surrounded by nine peripheral doublets (as seen).