Bio * 關於氣管軟骨的問題 *

You can imagine that esophagus is a canal locates nearly behind trechea. When food passes through the esophagus it swells and presses towards trechea. If the rings of carilage of the trechea is in full O-shaped we may feel pain during swallowing food. But it is unnecessary to bronchi since the esophagus is not near to them. Such contradiction is the result of evolution that lasts for serveral million years.