Adding additonal template files.

I have created an additional template file called ‘mid.php’ I would like it to insert it in between index.php, and footer.php. Unfortunately when I put <?php get_mid(); ?> at the bottom of my index.php file I get an error.

Fatal error: Call to undefined function get_mid() in C:\xampp\htdocs\spv2\wp-content\themes\viewport\index.php on line 46

What can I do about this?