external agent的用法和樣例：
例句
- A structure or substance not normally present but produced by an external agent or action, such as a structure seen in a microscopic specimen after fixation that is not present in the living tissue.
矯作物非正常出現而是由外界因素或作用所產(chǎn)生的一種結構或物質(zhì)，如經(jīng)固定后，在顯微鏡下才能看到的標本中的一種活生命組織中不會(huì )出現的結構
