Form validation and form_validation.php

In the  How to Save Your Rules section of the Form Validation documentation it states

Your validation rule file will be loaded automatically and used when you call the run() method.

The "validation rule file" is, as I'm sure you all know: application/config/form_validation.php

But I'm not finding how or where this loading occurs. I'm probably suffering from male pattern blindness but I cannot find it. Does anyone know where in the core source code this happens?