All sphincters muscles are connected. For instance, if your mouth is open (jaw relaxed), your cervix is open. It may be hard to consciously relax one's cervix to decrease pain in birth, but anyone can relax their jaw (and by connection the cervix lets go of tension). Another sphincter muscle is the pupil in the eye. If it is allowed to dilate, it likewise helps the cervix dilate--thus the advantage of birthing in low light. Sphincters are shy muscles. They close up if they do not have privacy. For instance, imagine going #2 and having people barge in on you. Not going to happen (that sphincter muscle would close up). Same with birth--privacy is a huge need in order for a women's body to be able to open.
Melanie Ellison, Durango CO
Childbirth fascinates me, and I always love learning more tidbits about it and sharing them with you. I also post photo galleries here from my photoshoots. Sign up for the RSS feed below so you don't miss a post!