What are sinuses ?

Explanation

**Step-by-Step Text Solution:**

1. **Definition of Sinuses**: Sinuses are a connected system of hollow cavities located within the skull. These cavities are lined with mucous membranes and are interconnected.

2. **Types of Sinuses**: There are several types of sinuses in the human skull, including:
- **Frontal Sinus**: Located in the forehead region.
- **Ethmoid Sinus**: Found behind the eyes.
- **Sphenoid Sinus**: Located behind the nasal cavity.
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