As far as the spells are concerned an organ is an organ (lung, brain, liver, kidney, stomach, heart, etc. - though I guess a heart injury could require both muscle and organ law and brain would require nerve and organ). That is how jaranka and I worked the critical injuries to specific organs.
Which organ is injured would only be important in determining the penalties and the spells necessary to heal them.
A solid organ (liver, kidney, spleen, etc.) would bleed more and a hollow organ would more likely to be injured by a spell like vacuum (e.g., impact critical).
The possible law book could also encapsulate called shots (with appropriate critcals) as this is noticeably missing right now. I have been asked, "I would like to strike him in the leg." It would be nice to be able to pull out the book and simulate it.