The MeSH Tree Structure
MeSH vocabulary is organized by 16 main branches:
This example displays part of the hierarchy that includes the term, Face. Notice the narrower terms indented under the broader terms.
When PubMed searches a MeSH term, it will automatically include narrower terms in the search, if applicable. This is also called "automatic explosion."
Some terms occur in more than one place in the hierarchy. For example, "Eye" appears under the Anatomy branch, but also under the Sense Organs branch. Automatic explosion will include narrower terms from all instances of the term in the hierarchy.