marrow, pith
膸 centers on the soft, vital inner substance of bones and plants: marrow and pith.
膸 combines the flesh radical ⺼ with 遀, which likely contributes the sound. The character is a variant form of 髓, where 骨 (bone) is replaced by ⺼ (flesh), both pointing to the marrow inside bones.
The flesh radical ⺼ beside 遀 suggests the soft, vital tissue deep inside: marrow and pith.
For ズイ, imagine a zoo where the zookeeper extracts marrow from a bone: zoo -> ズイ, and the marrow is the soft pith inside.