用法：①ADJ. Periodic events or situations happen occasionally， at fairly regular intervals.
例句：They especially emphasized the need for natural， serene settings where hurried urban dwellers could periodically escape from the city. (2002-01)
例句：This periodic extinction might be due to intersection of the Earth’ s orbit with a cloud of comets， but this theory is purely speculative. (2002-08)
例句：Periodically， molten material wells out of the Earth’s interior to invade the surface layers or to flow onto the surface itself. This material cools into a wide variety of igneous rocks. (1999-01)
例句：The woolly mammoth， along- haired rhinoceros， and other mammals have been periodically exposed in the tundra of Siberia， the hair and red flesh still frozen in cold storage.(1997-12)
例句：Periodic exhibitions of the members’ paintings also included works by noted English artists of the day， borrowed from embryonic private collections in the city. (1999-10)
用法：① ADJ. A peripheral activity or issue is one which is not very important compared with other activities or issues.
例句：Over 100 designated areas exist around the periphery of the Carbbean Sea.(2000-01)
②ADJ. Peripheral areas of land are ones which are on the edge of a larger area.
例句：These are the central nervous system (the nerve cord mentions above)， consisting of the brain and spinal cord， and the peripheral nervous system， consisting of the cranial， spinal， and peripheral nerves， together with their motor and sensory endings.