The Structure of the Violin
How sound is produced

Vibrations from the strings are transmitted to the top plate and bottom plate through the bridge, and this reverberates within the hollow body, producing the rich, brilliant tone characteristic of the violin.
A bowed string vibrates and moves in a circular motion that produces the fundamental tone, while the vibration produces overtones like a rippling wave. This complex movement of the string is transmitted to the body by the bridge. The bridge transmits this vibration to the top plate of the violin through two fundamental movements; one in which it pushes down on the top plate alternately one foot at a time, and the other in which both feet push down on the top plate simultaneously.

There is another small yet important part: the sound post. The sound post is a post sandwiched between the top plate and the bottom plate underneath the bridge, and it transmits the vibrations from the top plate to the bottom plate. It also serves to preserve the shape of the body.
If you look inside a violin, you can see the bass bar running up the left hand side as you face the instrument. The piece sitting under the bridge on the right-hand side is the sound post.