Final answer:
The 'lower sinus' on either side of the nose is actually the maxillary sinus. It is one of the paranasal sinuses located in the maxillary bones below the orbits. The paranasal sinuses serve various functions, including reducing bone mass and adding resonance to the voice.
Step-by-step explanation:
The sinus on either side of the nose that is referred to as the 'lower sinus' is actually the maxillary sinus. It is one of the paranasal sinuses, which are hollow, air-filled spaces located within certain bones of the skull. The maxillary sinuses are the largest sinuses and are located in the right and left maxillary bones below the orbits.
The paranasal sinuses serve to reduce bone mass, lighten the skull, and add resonance to the voice. However, they can also get congested and filled with fluid, causing pain and discomfort.