Pressure inside the tympanic cavity of middle ear is kept equal to that outside the ear drum by …………… .

Explanation

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks how the pressure inside the tympanic cavity of the middle ear is kept equal to the pressure outside the eardrum.

2. **Identifying the Structure Involved**: The structure responsible for equalizing the pressure is the Eustachian tube.

3. **Function of the Eustachian Tube**: The Eustachian tube is a canal that connects the middle ear to the nasopharynx (the upper part of the throat behind the nose). Its primary function is to regulate air pressure in the middle ear.

...