I will get back to my dog. It is well known a dog have one of greatest sensors of smell. I take it the sense of smell is a gas chromatograph , were it identifies a variety of compounds and the organism identifies pleasant and unpleasant. My Puch, liked, ham, cold cuts chicken . hot dogs . ( everything is identified by smell ). come time , hot dog no good any more, the followed by cold cuts, than ham, and ended with ham and then nothing any more . Then if you force ( mildly )he will vomit. So this becomes a signal some thing in the system is wrong. Since the system is based on circulating fluids, some compound is introduced with time into the system in larger quantity until reaches a threshold.: then some part in the brain will signal the involuntary reflex to initiate the reverse peristaltic action. So the question is what is the growing compound with time and were is it produced.