A canal with nerves and blood vessels.
I know this because my just-barely-sixteen-MONTH-old fell, shattered her tooth, and exposed the pulp (canal with nerve and blood vessels).
So, it's just a tooth but it was sooooooo sad. My sweet little 16 month old was doing her daily laps around the living room, dining room, and kitchen when she suddenly fell. She didn't trip over anything, like a toy or her feet. One minute she was up, and the next she was down.
When I picked her up she had little white pieces of tooth on her lip. Of course I didn't know that's what it was at first, until I brushed it away and felt it's 'chalkiness'.
"Oh my goodness! That was her tooth."
There was no blood and she only cried for a short time. I was pacing and making phone calls trying to decide if this was an emergency - almost half her tooth was gone - or something to be taken care of at home. How do you mend a broken tooth?
Suddenly I am brought out of my panic when I hear a huge crash in the kitchen. Before running to my daughter's side I had been mixing bread dough in my mixer. It causes my mixer to shake a little so I usually stand by it to keep it from knocking things off the counter.
Well, it fell and pulled some drying dishes off the counter with it. THANKFULLY, my oldest was far enough away that it didn't hurt her. And, thankfully again, she was standing right by the door and could easily exit the room.
I ran around to see if she was okay. She told me her tummy hurt. I yanked up her shirt and was relieved to see no poking glass or cuts. She was just scared, not hurt!
We made it to the dentist and he referred us to a specialist. We saw her the next day and she pulled the tooth. My poor little baby will have no tooth until she's at least 7 years old. We can have a false tooth put in when she has more teeth in her mouth, but for now she's our little toothless girl.

So, it's just a tooth. It's not a leg or an eye or a finger. Just a tooth. BUT, it was still so sad and tragic to deal with as a parent. I was beside myself. Outwardly I think I handled things pretty well - not fantastic, but well. But inside I was really panicky. I kept wondering why I am so weak when it comes to these situations. Then I realized I still did everything I needed to. I wondered if everyone felt panicky on the inside while handling similar situations?
It's been one week today since it happened, and my husband and I still look at each other and sigh for the premature loss of a baby tooth.