ischium的复数形式怎么写？

ischium的音标：ischium的英式发音音标为：['ɪskɪəm]ischium的美式发音音标为：['ɪskɪrm]...

The sick cow frequently attempts urination and pulsates the urethra over the ischium arch.病牛频频企图排尿,搏动坐骨弓上的尿道....

n.坐骨...