The set of all such vectors engenders one outgoing and one ingoing null congruence. Outgoing transport is provided by kinesin, and ingoing return traffic is provided by dynein. The ventilation-system mostly consists of two tubes for ingoing and outgoing air. Outgoing and ingoing tracks use different sets of motor proteins. There is a pair of principal null congruences (one "ingoing" and one "outgoing"). It can be played by ingoing MIDI signals from software, sequencers or MIDI keyboards. Ingoing retrograde transport carries cell waste materials from the axon terminal to the cell body.