slight modification of Without A Footer's .htaccess

This is .htaccess code Shawn provided and Id like to alter it for one of my sites:

<IfModule mod_rewrite.c>
	RewriteEngine On
	RewriteBase /
	RewriteCond %{REQUEST_FILENAME} ^(.*).htm [NC,OR]
	RewriteCond %{REQUEST_FILENAME} ^(.*).html [NC]
	RewriteRule ^(.*)       /passthru.php?file=$1
</IfModule>

Code (markup):

I plan to put .htaccess in main root folder (public_html) but as structure of all my (visible, online, not protected, crawable or however you want) pages is like this:

public_html/abcd/index.html
public_html/efgh/index.html

i want to sent all subfolder/index.html's to public_html/passthru.php (or www.site.com/passthru.php, if relative path is impossible to use), nothing else.

My question is how .htaccess should look to exclude anything that doesnt fit to that subdomain/index.html structure (no sending of any files sitting in root to passthru.php, just the ones sitting in subfolders and having exact name of index.html)?