Translation guide
The cranium is the part of the skull that encloses the brain. In everyday English, people usually say 'skull' instead of 'cranium', which is a technical term. In Japanese, the distinction is similar: use 頭蓋骨 for the whole skull and 頭蓋 for the cranium in medical contexts, but in daily conversation, 頭の骨 or simply 頭蓋骨 is more natural.
skull (everyday)
To refer to the bony structure of the head in general, non-technical contexts.
The most common word for 'skull' in Japanese, used in both everyday and medical contexts. It literally means 'cranium bone'.
The skull protects the brain.
A more colloquial way to say 'skull', literally 'head bone'. Used in casual conversation.
I fell and hit my skull.
To refer specifically to the part of the skull that encloses the brain, in technical or medical contexts.
The precise anatomical term for 'cranium'. Used in medical and scientific contexts.
We will take a CT scan of the cranium.
A more specific anatomical term meaning 'neurocranium', the part of the skull that directly encases the brain. Rare outside of specialized medical texts.
頭蓋骨 (ずがいこつ) is the general word for 'skull' and is used in daily life. 頭蓋 (ずがい) is the medical term for 'cranium' and is rarely used outside of professional contexts. If you're not sure, use 頭蓋骨.
Using 頭蓋 in everyday speech sounds overly technical and unnatural. Stick to 頭蓋骨 or 頭の骨.
The neurocranium has a different embryological origin.