The C ring cartilages and the annular ligaments of the trachea. It helps bridge the gap between the ends of the C ring cartilages on the dorsal side. Its primary function is the construction and dialation of the trachea while air moves through it. It helps stabilize the hyaline (C ring) cartilages. It is smooth muscle, so it is involuntary.

Kozui SLimming Suit

In this picture, the entire structure would be covered by the trachealis muscle