首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   304970篇
  免费   37344篇
  国内免费   206篇
  2016年   3267篇
  2015年   4984篇
  2014年   5730篇
  2013年   7865篇
  2012年   9021篇
  2011年   9049篇
  2010年   6150篇
  2009年   5718篇
  2008年   7909篇
  2007年   8143篇
  2006年   7722篇
  2005年   7512篇
  2004年   7390篇
  2003年   7380篇
  2002年   7057篇
  2001年   11207篇
  2000年   11277篇
  1999年   9319篇
  1998年   3712篇
  1997年   3823篇
  1996年   3793篇
  1995年   3540篇
  1994年   3538篇
  1993年   3523篇
  1992年   8263篇
  1991年   8033篇
  1990年   7822篇
  1989年   7736篇
  1988年   7296篇
  1987年   7245篇
  1986年   6701篇
  1985年   6862篇
  1984年   5763篇
  1983年   5165篇
  1982年   4133篇
  1981年   3975篇
  1980年   3616篇
  1979年   5950篇
  1978年   4677篇
  1977年   4459篇
  1976年   4262篇
  1975年   4601篇
  1974年   5061篇
  1973年   4951篇
  1972年   4608篇
  1971年   4143篇
  1970年   3651篇
  1969年   3675篇
  1968年   3278篇
  1967年   2828篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
191.
192.
193.
194.
195.
Five cases of coincident salmonellosis and ulcerative colitis are described. In three the diagnostic combination was recognized late. If corticosteroid therapy is given for the colitis once the combination is recognized, then simultaneous systemic antibiotic cover is advisable. One of the five patients died during corticosteroid therapy alone with salmonella septicaemia due to a usually non-invasive organism.  相似文献   
196.
Seven patients with “coeliac axis compression syndrome” are reported. Five were treated surgically, but only two did well. A survey of 200 healthy adults showed epigastric bruits in 6·5%; only one of these had dyspepsia, though dyspepsia was present in 12·5% overall.Caution is urged in attributing a causal relationship between coeliac axis compression and pain and in proceeding to arteriography when compression is suspected on clinical grounds.  相似文献   
197.
Genetic programming for development in Drosophila   总被引:2,自引:0,他引:2  
  相似文献   
198.
At Birmingham University since 1966 the major medical examinations began to be replaced by a process of continuous assessment. This is based on fairly frequent tests, usually at the end of each course, including short essay questions, multiple-choice questions, and tests of ability in wards and laboratory. At the end of the first final-year course 96 students underwent assessment. Of these 77 passed; 19 were referred for re-examination, of whom a further 12 passed subsequently. The scheme was found suitable by both teachers and external examiners.  相似文献   
199.
200.
Hydrodynamic forces can affect survival as well as limit the movement of motile benthic animals. An animal's danger of dislodgement depends on the hydrodynamic forces it experiences in its microhabitat relative to the force required to dislodge it (tenacity) from the substratum. We measured water flow and substratum characteristics in two different habitats of the shore crab Pachygrapsus crassipes: a wave-swept rocky shore and an intertidal mudflat. The maximum water velocities and accelerations in the microhabitats of the crabs at the wave-swept site were three times and two times greater, respectively, than at the mudflat site. In the laboratory, we measured the tenacity of crabs of various sizes on different substrata, and also measured their drag, lift and added-mass coefficients. Using these data, we calculated the flow conditions under which crabs would be overturned or sheared off the substratum in their two habitats. The net horizontal force (drag plus acceleration reaction) required to dislodge a crab on a rugose rock substratum was an order of magnitude greater than on smooth rock and two orders of magnitude greater than on mud. Our calculations indicate that, under non-storm conditions, crabs will not be dislodged from the substratum in either the mudflat or the wave-swept habitat when grasping the substratum with maximum tenacity. Moving crabs have lower tenacity and our calculations predict that hydrodynamic forces will restrict the mobility of large crabs more than that of small ones on smooth, but not on rugose rock.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号