ScrollBarArrow 

Every ScrollBar has two ScrollBarArrows. A ScrollBarArrow is actually a Motif
ArrowButton. Click on a ScrollBarArrow to scroll the window's contents in the 
direction that the arrow points. A click on a ScrollBarArrow sinks the arrow 
into the ScrollBar similar to the way a PushButton sinks when you click on it.
Use the first (and often the middle) mouse button on a ScrollBarArrow. The 
window scrolls a single line at a time when you click on a ScrollBarArrow. If 
you mouse down and hold, the window will scroll continuously until you release 
the button or the window's contents cannot scroll anymore.

See also: ScrollBar, ScrollBarSlider, ScrollBarTrough, ScrolledWindow
