From what I can tell (and from what Wikipedia can tell), the Pubmed database forms a superset of the Medline database, throwing some extra stuff in there. However, is there any way to make a search in the Medline database only?
There are several MedLine search engines, some of them requiring an (academic) account. You may want to use
By searching PubMed with "all databases" selected, you will notice all the different databases PubMed is linked with, including the "Literature" section (PubMed database alone). Searching with the PubMed database alone will usually give the exact same results as the engines mentioned above.