|Ligaments of the larynx. Posterior view. (Rings visiblea at bottom.)|
|Gray's||subject #237 1086|
The cartilages of the trachea (or tracheal rings) vary from sixteen to twenty in number. Each forms a semicircular ring of hyaline cartilage, which occupies the anterior two-thirds or so of the circumference of the trachea. The posterior one-third of the trachea is completed by fibrous tissue and smooth muscle fibers.
The cartilages are placed horizontally above each other, separated by narrow intervals.
They measure about 4 mm. in depth and 1 mm. in thickness.
Their outer surfaces are flattened in a vertical direction, but the internal are convex, the cartilages being thicker in the middle than at the margins.
Two or more of the cartilages often unite, partially or completely, and they are sometimes bifurcated at their extremities.
They are highly elastic, but may become calcified in advanced life.
The peculiar tracheal cartilages are the first and the last.
The first cartilage is broader than the rest, and often divided at one end; it is connected by the cricotracheal ligament with the lower border of the cricoid cartilage, with which, or with the succeeding cartilage, it is sometimes blended.
The last cartilage is thick and broad in the middle, in consequence of its lower border being prolonged into a triangular hook-shaped process, which curves downward and backward between the two bronchi. It ends on each side in an imperfect ring, which encloses the commencement of the bronchus. The cartilage above the last is somewhat broader than the others at its center.
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