extradosn.拱背(拱洞的外弧面；外拱线)extraadj.额外的n.额外之物；临时演员adv.特别地extra-foliaceous叶外的extra hollow organ外腑extragalactic nova河外新星extravascular mass血管外量, 血管外物质extra-long tunnel特长隧道extracellular space细胞外间隙extra-wide bridge超宽桥面extracellular water细胞外液, 细胞外水分extra high tenacity超高强度extramarital affair外遇; 婚外恋