If Vc rises too high {HEAD ~ more current} would flow through Q2 to the input terminal 10 to bring Vc down to {TAIL ~ the correct level} 
If Vc rises too high { HEAD ~ more current } would flow through Q2 to the input terminal 10 to bring Vc down to { TAIL ~ the correct level }
OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL