Apple Mail app not showing headers

The Apple mail app includes a preview pane below the summary of inbox contents. It used to show a message with header fields called out by To:, From:, CC:. Now it is not showing the field names, and not showing the To or CC info at all. I can't find anything under View tab to change the display format. Same problem if I display a message in its own separate window, there are no header field names and no CC field shown at all. How can I get back to the old view format showing field names? I am running OS 10.9.5. Is there a way to reset the App to default settings, or to download a fresh copy of just Mail (not the entire OS)?