Sights are the big difference. The other is the barrel contour. The Model 65 will either have a 3" or 4" heavy barrel, where the Model 66 has a 2 1/2", 4" or 6" barrel with the lighter contour of the Model 19.
The autonomic nervous system provides for involuntary muscle function - the work of breathing, digestion, and so forth. On some folks, that's a pure waste of ingenuity.