Pipe

A pipe is a nob. The picture shows an input line and two output lines connected by a dendrite, which consists of four pipes.
 * Pipes are sections of dendrites.
 * Pipes are visualized by straight lines
 * Pipes are directed. They have a startpoint and an endpoint. When the model runs, impulses flow from the startpoint to the endpoint. The direction of a pipe can be visualized by an arrows. Use the Arrows on/off command to control the display of arrows.
 * At the startpoint of a pipe is either an input line, a neuron or a knot
 * At the endpoint of a pipe is either an output line, a neuron or a knot