[OS X TeX] Helvetica Italic in plain TeX using TexShop and XeTeX?

[Mark London]

Hi - Thanks to Alan Munn who showed me how to use Arial fonts in plain 
Tex in TexShop, by using XeTex:

\font\arial="Arial"
\arial Some text in Arial
\bye

However, on the mac, there is no Helvetica Italic, or Bold Italic. 
Instead, there is Helvetica Neue Italic and  Bold Italic.  Is there a 
way to get true Helvetica Italic fonts?  Just curious.  Thanks! - Mark