humans have the ability to effectively swallow upside down, what makes this possible?

Question

explain clearly plz!!!

Answer 1

http://answers.yahoo.com/question/index?qid=20070802162159AA9cCWB

Answer 2

http://www.madsci.org/posts/archives/2005-03/1110582380.An.r.html

Answer 3

http://hes.ucfsd.org/gclaypo/digestive_system.html#Can we swallow if we are upside

http://www.pennmedicine.org/encyclopedia/em_DisplayAnimation.aspx?gcid=000097&ptid=17