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   449436篇
  免费   55658篇
  国内免费   283篇
  505377篇
  2018年   3836篇
  2017年   3520篇
  2016年   5483篇
  2015年   8182篇
  2014年   9177篇
  2013年   12899篇
  2012年   14868篇
  2011年   14982篇
  2010年   9886篇
  2009年   9142篇
  2008年   13069篇
  2007年   13561篇
  2006年   12255篇
  2005年   12071篇
  2004年   11826篇
  2003年   11265篇
  2002年   10765篇
  2001年   21637篇
  2000年   21776篇
  1999年   17395篇
  1998年   6221篇
  1997年   6471篇
  1996年   6298篇
  1995年   5700篇
  1994年   5749篇
  1993年   5569篇
  1992年   13649篇
  1991年   12966篇
  1990年   12648篇
  1989年   12501篇
  1988年   11160篇
  1987年   10785篇
  1986年   9870篇
  1985年   9651篇
  1984年   8183篇
  1983年   7072篇
  1982年   5522篇
  1981年   5003篇
  1980年   4650篇
  1979年   7682篇
  1978年   5919篇
  1977年   5425篇
  1976年   5062篇
  1975年   5363篇
  1974年   5807篇
  1973年   5658篇
  1972年   5100篇
  1971年   4727篇
  1970年   3920篇
  1969年   3850篇
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
41.
42.
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   
43.
44.
45.
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   
46.
47.
48.
49.
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   
50.
EEG activity was recorded in rats submitted to osmotic opening of the BBB by intracarotid mannitol infusion.This procedure produced an immediate short-lasting depression of the EEG and a tardive paroxysmal EEG activity. Both these phenomena were more relevant on the ipsilateral hemisphere. In some instances a tonico-clonic seizure was recorded.Pre-treatment with diazepam abolished the occurrence of the tardive EEG and behavioral modifications.In accord with previous findings, focal seizure activity is likely to be responsible for the metabolic abnormalities associated with osmotic opening of the BBB. This preparation therefore produces in the brain unphysiological states in respect to local metabolism and electrical function.  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