Structural anatomy of the human head
Successive frontal slices of adult male, brain removed.
As can be seen in these images, the human skull is a rather complex maze of hollow cavities, thin walls, and hidden structures.
There are four sets of paranasal sinuses in the front of the head: the maxillary sinuses (right below the eyes), frontal sinuses (above the eyes, in the hard part of the forehead), ethmoid sinuses (between/behind the nose and eyes), and the sphenoid sinuses (in the sphenoid bone, under the pituitary gland, in the center of the skull - can be seen in the bottom-most plates).
In addition to those sinuses, you can also see the Eustachian tubes, which connect the ear to the nasopharynx and regulate pressure in the middle ear; the curled nasal concha, which regulate the air flow through our nose, keeping it a relatively constant humidity and temperature; and the falx cerebri, a sickle-shaped sheet of dura mater that divides the right and left hemispheres of the brain.
Studies in the Anatomy and Surgery of the Nose and Ear. Adam E. Smith, 1918.