USB Endpoint call

Hi,

I modified USB memory example from Keil and I can enumerate. But I was trying to trace back how are the USB_Endpoint() functions called.
For example usbuser.c file has EndPoint definitions that look like this:

/*
 *  USB Endpoint 1 Event Callback
 *   Called automatically on USB Endpoint 1 Event
 *    Parameter:       event
 */

void USB_EndPoint1 (u32 event) {
        // BulkIn
}


/*
 *  USB Endpoint 2 Event Callback
 *   Called automatically on USB Endpoint 2 Event
 *    Parameter:       event
 */

void USB_EndPoint2 (u32 event) {
  USB_EP_Bulkout();     // Handle data that arrived on this Endpoint
}

I use Endpoint 1(IN) and Endpoint 2(OUT). But when I tried to search for these functions to see where they are called I came up empty.
Is this done in a interrupt routine someplace? IS this function name assembled someplace?
I assumed that as soon as the host sends data to the device it goes to the OUT endpoint and my device executes my function USB_EP_Bulkout().
But I couldn't find out where this UEB_EndPoint2() is called.

Any help is appreciated.
Thanks