Ventril Fins

They help with stability and directional control. They prevent the aircraft from side slipping at specific speed ranges. The F-111 did not have them and it would try to swap ends in a tight turn. It also wobbled a lot at low speeds. After they were installed, it solved their problems. The F-8 ventral fins were added in an effort for better slow speed handeling.

Here is a nice shot of those "ventral fins" you asked about

http://www.airliners.net/Airliners_net_image.file?filename=1/6/5/492561.jpg&ZyXtCe=MDczOTIz&id=492561&ViD=big

The ol' Dump-n-Burn!!!

Well I am a F-16 mechanic so I can say what purpose they serve on the Fighting Falcon.
As berny13 says they help with stability and directional control. On the Falcon it is at high angle of attack that they are most needed. When the Falcon has a high angle of attack the airflow around the vertical fin is interrupted by the fuselage and the wings and directional stabilty would be lost with out ventral fins. Hope this helps.
By the way jimz66 did you get the F-16 links you asked about in your F-16C topic

Yes I did thank you. I looked at them breifly. I have to go back to them. Thanks for your help guys. I think of little things like this sorry they are little tidbits.