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The tracheal rings, (tracheal cartilages or C-shaped cartilaginous rings) vary from sixteen to twenty in number. Each forms an incomplete 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 and smooth muscle tissue. ==Middle tracheal cartilages== 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「tracheal rings」の詳細全文を読む スポンサード リンク
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