Smooth muscle fibres in the trachea (400X)

This image has the same magnification as the previous one but is taken from the part of the trachea that is not covered by cartilage.

Instead, we find a thinner, less well-defined light pink layer beneath the epithelium. What could this be?

Well, in the section of the trachea not covered by cartilage, we find strands of smooth muscle extending between the ends of the cartilage. Like all other smooth muscle, this muscle is not under voluntary control but is regulated by the autonomic nervous system and the endocrine system (hormones). When these muscles contract, the diameter of the trachea decreases, increasing the resistance to airflow.