As a result, it will cause the stretching of breast muscles. (intelligentmother.com)
Vacuum stretches, and then releases, your nipple stimulating a release of oxytocin, which signals the smooth breast muscle to contract, ejecting the milk. (newbabynewpaltz.com)
The birds have changed a great deal since those early days: Some commercially bred turkeys have breast muscles so large that the birds can't get close enough to mate. (sciencenews.org)