A voltage stimulus applied to {TAIL ~ the drain} forces current to flow through the channel coupling the drain to {HEAD ~ the source} 
A voltage stimulus applied to { TAIL ~ the drain } forces current to flow through the channel coupling the drain to { HEAD ~ the source }
OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD