Complex sentence: Of these the Montgenèvre and the Brenner were the most frequented, while it will be noticed that in the Central Alps only two passes (the Splügen and the Septimer) were certainly known to the Romans.
Split into simpler sentences: Of these the Montgenèvre and the Brenner were the most frequented. In the Central Alps only two passes (the Splügen and the Septimer) were certainly known to the Romans.