where does initramfs get the /init script?

[Richard Jenniss <richard@promodus.net>]

Hello,

I extract the /boot/initrd.img-2.6.18-3-486 and I see the following, all 
directories:
/bin    /conf    /etc    /lib    /modules    /sbin    /scripts

I have grub set to go no further than (initramfs)
Once I have booted to that point I see the file /init script.
How is the /init script created when I don't see this within the initrd 
file: initrd.img-2.6.18-3-486?

<background info>
I boot into (initramfs) by having the following in my grub menu.1st:

title test
root   (hd0,0)
kernel   /boot/vmlinuz-2.6.18-3-486 root=/ ramdisk_size=1048576 rw 
init=/bin/sh
initrd   /boot/initrd.img-2.6.18-3-486
</background info>


Anyone?

Thanks and best regards,
Richard Jenniss.