・「通常は弾性のある状態だが、加齢とともに気管軟骨は石灰化して硬化しやすくなる。」（Healthline.com）
・「加齢とともに2個（またはそれ以上）の軟骨が癒合することがある。」（Healthline.com）
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気管（前面） |
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気管（横断面） |
気管・食道（横断面） |
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以下は「Healthline.com」の解説文となる。
In the trachea, or windpipe, there are tracheal rings, also known as tracheal cartilages. Cartilage is strong but flexible tissue. The tracheal cartilages help support the trachea while still allowing it to move and flex during breathing.
There are generally sixteen to twenty individual cartilages in the trachea, which varies from person to person. These C-shaped cartilages are stacked one on top of the other and are open at the area where the trachea is nearest the esophagus, which leads from the throat to the stomach. Each one is about one to two millimeters thick, with a depth of around four to five millimeters.
【語句】
・esophagus：食道
The tracheal cartilages have an order, starting with the peculiar tracheal cartilages. These are the first and last rings in the trachea. The first cartilage is broad and divided at the end. It sometimes blends in with the following cartilage, depending on the structure of the trachea. The last cartilage is broad in the middle and thick, with a triangular shape for a lower border. This piece curves downward, extending into the two bronchi — the main passageways to the lungs — forming an imperfect circle that encloses each bronchus.
When a person ages, the tracheal cartilages tend to calcify, or stiffen. Their normal state is one of elasticity. Two or more cartilages often fuse together in the trachea, as part of the aging process.
【語句】
・peculiar：固有の ・bronchi：bronchus（気管支）の複数形 ・calcify：石灰化する ・stiffen：固くなる ・elasticity：弾性 ・fuse：一緒になる
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