There is a distinction between learning "in" art and learning "through" art. Both approaches are necessary when integrating art curriculum. "In" art you might learn about Picasso's painting, but through art you could learn about the Spanish Civil War and Picasso's painting, Guernica. The difference might be subtle but it has a huge effect on what are students are retaining in the art room. This links back to Enduring Ideas, those over arching themes that we want our students to grasp through lessons and units.
Enduring ideas contain content that go beyond a single schools subject for the students to get a larger understanding of an idea. This doesn't mean however that we should divide it up into sections. Here's a little history about the Spanish Civil War, here's a painting about it, now here's a book about it.... That doesn't make the same connection in the students mind as it would if you blend all these ideas. Refer back to images, throw in literature, all the while giving information that applies to exactly what you are talking about.
Teaching doesn't have to be linear, it can be a bit messy. Our students aren't going to remember a string of facts that you spouted out at them about the Spanish Civil War but if you take the time to integrate different disciplines into a lesson or unit, without distinction of subject, they're going to take something away from that. Show pictures, give facts, tell stories, read relevant literature; but don't do it in order! MIX IT UP!