When drawing additional masses in this course we use a simple outward curve where the mass is exposed to fresh air. Outside this course you can draw as you please. There is some utility in this kind of thinking when drawing animals (or the human figure) as they are mostly composed of positive forms, which make outward curves. Often something that may at a glance look like an inward curve may be the result of two (or more) outward curves meeting. But that's not a general rule, simply something that may be helpful to think about.

We'll carve the eye sockets into the cranial ball as shown in the informal head demo and rhino head demo wherever possible. There are a few outliers where this isn't feasible, such as a hammerhead shark, where we'd need to mount a secondary form onto the cranial ball and inscribe eye sockets into that instead.