A 3D reconstruction of the paranasal sinuses in a human (from Koppe et al. 1999). You also have paratympanic sinuses that pneumatize the mastoid process of the temporal bone (feel for an inferiorly-directed, thumb-size protuberance right behind each ear). An x-ray of a pig skull, from here. Can you see the outline of the brain-shaped endocranial cavity? How about in this x-ray of a rhino skull? Image courtesy of Kent Sanders.