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