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   680314篇
  免费   83084篇
  国内免费   450篇
  763848篇
  2016年   7655篇
  2015年   11436篇
  2014年   13046篇
  2013年   18452篇
  2012年   20985篇
  2011年   21138篇
  2010年   14131篇
  2009年   13164篇
  2008年   18668篇
  2007年   19332篇
  2006年   17792篇
  2005年   17558篇
  2004年   17135篇
  2003年   16746篇
  2002年   16023篇
  2001年   29243篇
  2000年   29539篇
  1999年   23844篇
  1998年   8947篇
  1997年   9372篇
  1996年   9084篇
  1995年   8297篇
  1994年   8381篇
  1993年   8244篇
  1992年   19737篇
  1991年   18914篇
  1990年   18420篇
  1989年   18223篇
  1988年   16636篇
  1987年   16341篇
  1986年   15060篇
  1985年   14956篇
  1984年   12649篇
  1983年   11165篇
  1982年   8829篇
  1981年   8230篇
  1980年   7561篇
  1979年   12413篇
  1978年   9675篇
  1977年   9092篇
  1976年   8597篇
  1975年   9226篇
  1974年   9929篇
  1973年   9803篇
  1972年   8944篇
  1971年   8223篇
  1970年   7055篇
  1969年   6938篇
  1968年   6164篇
  1967年   5461篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
The effect of the chromosomal ends of Tetrahymena thermophila on the stability of linear transforming molecules in the filamentous fungus Podospora anserina was tested. A derivative of an integrative vector for this fungus has been constructed, so that after linearization, the ends of the plasmid are the telomeric sequences of T. thermophila. After transformation, this linear molecule was maintained as an extrachromosomal plasmid with no integrated copies in about 50% of the transformants. Under selective conditions, there was approximately one linear molecule per 5 to 10 nuclei, and these extrachromosomal molecules were rapidly lost under nonselective conditions. The circular plasmid carrying an inverted repeat of T. thermophila telomeres could be linearized and processed in vivo.  相似文献   
72.
73.
74.
OBJECTIVE--To test the hypothesis that farmers are at high risk of hip osteoarthritis and to investigate possible causes for such a hazard. DESIGN--Cross sectional survey. SETTING--Five rural general practices. SUBJECTS--167 male farmers aged 60-76 and 83 controls from mainly sedentary jobs. All those without previous hip replacement underwent radiography of the hip. MAIN OUTCOME MEASURES--Hip replacement for osteoarthritis or radiological evidence of hip osteoarthritis. RESULTS--Prevalence of hip osteoarthritis was higher in farmers than controls and especially in those who had farmed for over 10 years (odds ratio 9.3, 95% confidence interval 1.9 to 44.5). The excess could not be attributed to any one type of farming, and heavy lifting seems the likely explanation. CONCLUSIONS--Manual handling in agriculture should be limited where possible. Consideration should be given to making hip osteoarthritis a prescribed industrial disease in farmers. There may be wider implications for the prevention of hip osteoarthritis in the general population.  相似文献   
75.
76.
The metabolism of the cholestatic triterpene acid reduced lantadene A has been studied in susceptible and resistant rats and in sheep which are susceptible to intoxication. Sheep and susceptible female rats produced a similar major metabolite and rats produced a second metabolite which was a glucuronide. These metabolites were also observed in extracts of bile canalicular membranes prepared from intoxicated rats. Resistant male and female rats produced a similar major metabolite which was different to those synthesized by susceptible animals. It is concluded that in rats and sheep there is a correlation between the type of metabolites produced in the liver and the susceptibility to intoxication by reduced lantadene A.  相似文献   
77.
78.
79.
An earlier report (1) of an adverse effect of high doses of oxytocin on human memory included results of studies on women receiving oxytocin as part of the treatment to induce 2nd trimester therapeutic abortion. These women served as their own controls. We have now been able to study a group of women who have been treated in all ways like the original group, with the exception that they did not receive oxytocin. The results from this external control corroborate the finding that oxytocin affected memory.  相似文献   
80.
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