The second input signal is applied to {HEAD ~ a base} or gate of {TAIL ~ the third transistor} with regard to the reference point 
of
The second input signal is applied to { HEAD ~ a base } or gate of { TAIL ~ the third transistor } with regard to the reference point
OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH