Can you explain how are filters populated in the search results page of Magento?

Specifically,

My filters that should be showing up on the search page are not doing so, and I am wondering if anyone was aware of the location in code that they are attached to the Layer object.

eg: on app/code/core/Mag开发者_开发百科e/Catalog/Block/Layer/View.php they are loaded up with the call to $this->getLayer()->getFilterableAttributes() in the _getFilterableAttributes() function. But beyond that I am confused as to where they come from.