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?
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