The annular ligaments of the trachea help hold the trachea together between the C ring hyaline cartilages. They are visible as the pink, thinner areas of the trachea between the whiter, thicker rings (the hyaline cartilages) in the picture below. They are also for support.

21_trachea_cartilage%20x750.jpg

In this picture below they are labeled:

trachea_large.jpg