You know how you 2 leggeds find it funny watching us shake our legs uncontrollably when we get a good belly rub? Well, it is not really funny. It is a reflex you know. Like your knee jerk reaction.
[And yest that knee jerk thing IS funny!]
The "saddle region" on our body is made up of the back, sides and flanks.
Specific topography as to where etc, varies from dog to dog just like you 2 leggeds have individualized body zones. Rubbing or scratching a dog in the saddle region will cause the hind leg
to simulate a scratching motion. This is called the "scratch reflex." In 4leggeds the spinal nerves pass all the way to chest and abdomen, and stimulation of these nerves will cause a feeling of "itchiness" (similar to the irritation of a flea) causing the rear leg to involuntarily attempt to scratch it. Rubbing or scratching of the chest and abdomen will make the leg extends to different lengths or reaches to varying heights depending upon the area that is stimulated.
Vets use the scratch reflex or lack of, as a diagnostic tool,when they suspect spinal or neck injury, just like your doc taps your knees just below the knee cap to gauge nerve reaction. Or just like the Babinsky reflex. :) I love that word. Babinsky.... sounds exotic. Just means toe curling reflex.
BAaaaBiiinnnsky , love saying it.
And one more thing. A 4 legged knows when you are causing the itch with a bell scratch, we sort of like watching you get all worked up and frankly you act as if the leg moving is YOUR reward... for a good belly scratch. YOU ARE ODD.