Where is the function, class where the_title is defined ?

I would like to modify the core of the function the_title() to use custom fields if present and render the title, links differently. So I “grepped” the_title in all the files, and still am not sure I have the right one.

Can someone tell me where this function the_title is defined ?

I see some stuff in \wp-includes\classes.php, is this the right place to start ?