Q: Is there a word for the opposite of being hollow? I'm looking for an adjective that describes an object that, rather than is just the outside, is instead just the inside.
Searching for antonyms of hollow, I have only found the word solid, which suggests both the inside and outside of an object.
I really need a way to describe something as being just the inside of something.
I can only think of words like core or heart to describe what I'm looking for, but those are nouns.

When a tree has only its exterior, the tree is hollow. It is a hollow tree.
When a tree has only its interior, the tree is _____. It is a _____ tree.


Perhaps tree was a bad example, it was the only object I could think of at the time that made sense to be a hollow object. Any object where that description could apply is a reasonable substitute, it does not have to be specifically about a tree.

A: In the case of a tree, I might use the term denuded.

denuded adj
   without the natural or usual covering; 

TFD
By the way, this is not really an antonym of hollow, because what you're looking for is not really an antonym, per se. It's more like a mirrored state, or a complementary state. The antonym of hollow would be filled or full or any of several others. The state you describe is something else again.

A: You can say that you have the core or heart of something.

A: I nominate skeleton.

The supporting framework, basic structure, or essential part of
  something:
the concrete skeleton of an unfinished building (No exterior)
the skeleton of a report (No exterior)

[From Oxford]

A: Exfoliated

  
*
  
*To remove (a layer of bark or skin, for example) in flakes or scales; peel.
  
*To cast off in scales, flakes, or splinters.
  

...if the exterior has been removed by some process.
FreeDictionary

A: I guess the word you are looking for is marrow. However, I don't know if it suits the example you gave. I'd rather say naked or bare.

A: I'd probably go with skinned, the past participle of "to skin", which is defined by oxforddictionaries.com as to "remove the skin from (an animal or a fruit or vegetable)".  Similar words might be "stripped", or, less pleasantly, "flayed".
