The dorsal ridge housed avionics and HF antennas. From this document:
The 6R featured reverse thrust. However, to receive FAA certification the Caravelle 6 Rs for United had to be changed to comply with US regulations. The most noticeable difference are the larger cockpit windows and the removal of the dorsal fin (the long fin on top of the fuselage), that housed avionics. The 6R's not built for United did have the dorsal fin, though. United introduced the Caravelle 6R on July 1961 between New York and Chicago. It was present only on the first models.
The non metallic radome is more visible here:
The HF antenna was not required when flying only over land and using only VHF aids, but was required for HF navigation (Loran) and HF communications.
The Caravelle was the realization of the motto: "A medium haul aircraft doesn't fly, it takes off and lands". To increase regularity the aircraft was fitted with numerous electronics systems. This would soon become known as "avionics".
In 1962, the Caravelle was the first aircraft able to land automatically with a Sud-Lear 102 autopilot. Orly was specifically equipped for such landing and FAA was associated to the tests. In 1969 Air Inter did the first commercial "Phase IIIA" landing at Orly in very low visibility.