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   727250篇
  免费   84947篇
  国内免费   438篇
  2018年   6215篇
  2016年   8684篇
  2015年   12491篇
  2014年   14352篇
  2013年   20170篇
  2012年   23097篇
  2011年   23227篇
  2010年   15601篇
  2009年   14271篇
  2008年   20446篇
  2007年   21334篇
  2006年   19444篇
  2005年   19022篇
  2004年   18445篇
  2003年   17688篇
  2002年   17358篇
  2001年   35453篇
  2000年   35868篇
  1999年   28240篇
  1998年   9616篇
  1997年   10062篇
  1996年   9675篇
  1995年   8839篇
  1994年   8900篇
  1993年   8620篇
  1992年   22931篇
  1991年   22063篇
  1990年   21353篇
  1989年   20892篇
  1988年   19032篇
  1987年   18101篇
  1986年   16643篇
  1985年   16386篇
  1984年   13591篇
  1983年   11819篇
  1982年   9042篇
  1981年   8129篇
  1980年   7532篇
  1979年   12970篇
  1978年   9882篇
  1977年   9024篇
  1976年   8363篇
  1975年   9128篇
  1974年   9743篇
  1973年   9561篇
  1972年   8711篇
  1971年   7812篇
  1970年   6775篇
  1969年   6524篇
  1968年   5811篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
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   
92.
93.
94.
95.
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   
96.
97.
98.
β-Xylosidases are hydrolytic enzymes which play an important role in xylan degradation, hydrolyzing xylobiose and xylooligosaccharides to xylose from the non-reducing end. Filamentous fungi are particularly interesting producers of this enzyme from an industrial point of view, due to the fact that they secrete β-xylosidases into the medium. Besides, fungal β-xylosidases are highly advantageous for their elevated activity levels and specificity. Interest in xylanolytic enzymes has been increasing, for their possible application in many biotechnological processes. This fact has driven the isolation, purification and characterization of several β-xylosidases. In this review, the mechanisms of action, substrate specificities, physicochemical characteristics, regulation at molecular level, molecular cloning and classification of filamentous fungal β-xylosidases are described. The potential industrial applications of fungal β-xylosidases will also be presented.  相似文献   
99.
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   
100.
The changes in transport activity of tonoplast proton pumps under the influence of exogenous NO donator and modulation of Ca2+ concentration jointly and separately were investigated at different stages of ontogenesis and under hyperosmotic stress. The results suggest that both exogenous NO donator and Ca2+ ions can influence the activity of transport processes related to tonoplast and this influence is especially evident in the period of growth and accumulation of metabolites. Under hyperosmotic stress, H+-pyrophosphatase plays a more important role than H+-ATPase: the activity of the former increases 2.3-fold compared to the control osmotic conditions, whereas the activity of H+-ATPase is practically unchanged. H+-pyrophosphatase was more responsive to the presence of exogenous NO donator and to variations in Ca2+ concentration. The effects of exogenous NO donator on tonoplast proton pumps depended on the concentration of Ca2+, which apparently can mediate NO action.  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