Plural of epiphysis; the end sections of long bones that are initially cartilaginous and separate from the shaft, then eventually fuse into one solid bone during adulthood.
From Greek 'epi-' (upon) + 'physis' (growth, nature), plural form. The anatomical term was adopted into medical terminology in the 18th century.
When you're born, your epiphyses are separate pieces of bone connected by cartilage—by age 25, they've all fused into solid bone, which is why you stop growing and why broken bones heal differently in kids versus adults.
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