Every spring, John Marshall Law School professor Susann M. MacLachlan begins her animal law class with a simple student directive. Drop this course. If you ever want to eat a Big Mac again, she tells them — or really, any meat — do yourself a favor and get out now. Nobody does. It’s is a course, MacLachlan said, that can be “very depressing.” “There’s the criminal law aspect — animal torture, animal hoarding — or factory farming and the conditions of the …