You can view either all records that contain selected heading or click on any subheadings and use the combination in the query.
Browsing the MeSH tree.
Medical Subject Headings (MeSH) is a controlled hierarchical vocabulary of biomedical terms/headings used for the description of document’s content and for more precise retrieval. Headings are grouped into 16 categories and in addition every heading can be supplemented by one or more subheadings, e.g. diagnosis, therapy, psychology etc.
More information about MeSH usage in NML
If you want to search the thesaurus, use either the Quick search for Subject or go to Subjects in the upper menu.