In OS X.2 (can't confirm on other versions), when you hold the mouse button down while pointing to a scroll arrow UI widget, it scrolls the window as expected. But then when you move the mouse (still holding the button) over the scroll arrow of the opposite direction, it also activates and scrolls the other way.
In OS 9, each arrow needed a new click-and-hold. Holding the mouse down and dragging off the original button caused nothing to happen. So switching directions meant taking your finger off the mouse button and re-engaging the opposite scroll arrow.
All of the arrows (I prefer a double-arrow at each end) seem to act as one button while you hold the mouse down. However, the horizontal arrows are independent from the vertical ones. This is a very small thing, but it may be useful to you if you like to fine tune your view using the arrows!
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040127145327135