illustration - If I reference a medical textbook or website for hand drawing bones is that considered copyright infringement?

I can't quite figure this one out. I am not copying a custom piece of work, I am copying something that is available in nature, bones. I am also not tracing it, I am drawing by hand using them as a reference for accuracy. So, is that considered copyright infringement? Does it depend on how similar my drawings are to the ones I visually copied?

So if I look at a medical website or book and see a knee bone, then I draw one on paper or in a graphics editing tool by looking, with my eyes, at the referenced image, is that going to get me in trouble somehow?

I mean, surely I don't have to go find my own skeleton to draw from... right??