You are certainly allowed, but keep in mind that each of these lessons looks at the same problem - understanding how the things we're constructing exist in 3D space, how they break into simpler forms and how those forms relate to one another. The only difference between them are the topics we use as lenses through which to look at the same problem.

We're not actually studying anatomy/skeletal structure/etc. - all we're really paying attention to are the major forms that are externally visible. That said, if you feel looking up the anatomy helps you better understand what you're looking at, then that's fine. Just be sure not to build up that skeletal structure in your drawing, and instead approach the construction as shown in the lesson and its demos.