An animal's fart can come via (通过) its anus (肌工门)， if it has one,
or its cloaca (泄殖腔) - a combined opening for urination (排尿) and defecation (排便). The gas
release makes the orifice (开口) vibrate (振动) against an organism's (生物体的) sphincter (括约肌) muscles, creating
a sound.
这里的against具体什么意思？倚靠吗？
