public function getRelatedPartByID( $cid )
    {
        $parts = $this->getRelatedParts();
        foreach ( $parts as $part )
        {
            if ( ( $part->getHeader( 'Content-ID' ) !== '' ) &&
                ( $part->getHeader( 'Content-ID' ) == "<$cid>" ) )
            {
                return $part;
            }
        }
        return false;
    }