chest, hollow space, cavity
膛 centers on an enclosed empty space within the body or an object: the chest cavity or any hollow interior.
膛 combines the flesh radical ⺼ (body part) with 堂, which likely contributes the sound トウ and the idea of a hall or chamber, suggesting a bodily cavity.
The flesh radical ⺼ marks this as a body part, and 堂 (hall) suggests a large open space inside. Picture the chest as a grand hall within the body.
For トウ, imagine a toe poking into the hollow chest cavity: toe -> トウ, and the toe explores the empty space.
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