Lady Cassandra, the last human from the "The End of the World" episode, has had over 700 surgeries. She has become nothing more than a face on a piece of skin stretched onto a frame and connected to a brain in a designer jar filled with a preserving solution. When Rose walks behind her, what do we see on Cassandra's back?
We see very little, really.
The Doctor Who Wiki describes her thus:
Her life was extended through a series of seven hundred and eight plastic surgery operations, until she was nothing but a piece of skin stretched onto a frame with eyes and a mouth, connected to a brain in a jar below.
She's translucent, and we can see her blood vessels easily enough. Her eyes can be seen thru her skin, as can her mouth. One thing that looks like an object on her back.. is actually a hole to the other side. If you look at the attached pictures, you can see that even from the front view, she has a hole in the back of her mouth.
Not much else to see. Her brain is in the green/blue fluid in the jar below her (see the last picture.)