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   777524篇
  免费   92452篇
  国内免费   899篇
  2018年   6942篇
  2016年   9374篇
  2015年   13393篇
  2014年   15608篇
  2013年   21837篇
  2012年   24982篇
  2011年   25734篇
  2010年   17195篇
  2009年   15863篇
  2008年   22674篇
  2007年   23346篇
  2006年   21865篇
  2005年   21015篇
  2004年   20775篇
  2003年   19936篇
  2002年   19321篇
  2001年   33629篇
  2000年   33542篇
  1999年   26758篇
  1998年   10017篇
  1997年   10360篇
  1996年   9867篇
  1995年   9307篇
  1994年   9007篇
  1993年   9172篇
  1992年   22600篇
  1991年   22448篇
  1990年   21751篇
  1989年   21213篇
  1988年   19524篇
  1987年   18757篇
  1986年   17635篇
  1985年   17624篇
  1984年   14717篇
  1983年   12747篇
  1982年   9740篇
  1981年   8805篇
  1980年   8328篇
  1979年   13883篇
  1978年   11247篇
  1977年   10096篇
  1976年   9546篇
  1975年   10741篇
  1974年   11491篇
  1973年   11294篇
  1972年   10136篇
  1971年   9367篇
  1970年   7853篇
  1969年   7660篇
  1968年   6951篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
Maleylated bovine serum albumin (maleyl-BSA) and other polyanionic polymers that are recognized by cell surface receptors on macrophages have been shown to induce chemotaxis, protease secretion, and tumoricidal function in this cell type. In this paper the effect of maleyl-BSA on Ia antigen expression has been evaluated. In a fashion similar to LPS, maleyl-BSA suppressed IFN-gamma-induced expression of Ia in a time- and dose-dependent manner. Also like LPS, maleyl-BSA stimulated the production and secretion of substantial amounts of PGE2 over a 24-hr period. This did not, however, appear to be the primary mechanism by which expression of Ia was suppressed, because co-treatment of the cells with indomethacin, which totally inhibited the production of PGE2, only minimally affected the suppressive activity. Surprisingly, the suppressive activity of both maleyl-BSA and LPS could be largely abrogated by co-treatment of the cells with cyclohexamide during the time period when Ia expression was sensitive to suppression. This effect was selective in that PGE2- or dibutyryl cyclic AMP-induced suppression of Ia expression was not affected by cyclohexamide treatment. The data support the concept that there are multiple molecular mechanisms involved in the negative regulation of IFN-gamma-induced Ia expression in macrophages. Such mechanisms may include, in addition to the synthesis of PGE2 and consequent elevation in intracellular levels of cyclic AMP, one or more proteins made early after treatment with either maleyl-BSA or LPS. Thus the function of some of these early gene products may be to regulate expression of functional genes such as that encoding Ia antigen.  相似文献   
112.
Experiments were made on rabbit fed an atherogenic diet (0.5 g/kg cholesterol) singly for 15 and 24 h and repeatedly for 3, 9 and 30 days. At early stages of lipid metabolism distress the interrelationship was established between blood rheological disorders and microcirculatory abnormalities. The dependence of the initial reaction of some rheological characteristics on their initial level was marked.  相似文献   
113.
114.
With DNA sequencing now getting cheaper more quickly than data storage or computation, the time may have come for genome informatics to migrate to the cloud.  相似文献   
115.
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   
116.
117.
Synthesis of prenylquinones in chloroplasts   总被引:3,自引:0,他引:3  
  相似文献   
118.
119.
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   
120.
Puerto Rican populations of two species of sea anemones (Bunodosoma cavernata and B. granulifera) which had previously been considered one were assayed electrophoretically for enzymes encoded by 12 loci. The two species shared no common allozymes at 6 of the 12 loci. Genetic distance and identity values based on these allozymes were computed for the Puerto Rican populations and for B. cavernata from Florida and B. granulifera from Panama. The Puerto Rican populations of both species had much higher genetic identities for their geographically distant conspecifics than for each other. These results indicate that the two species are reproductively isolated and should be considered as separate valid species. Average heterozygosities are presented which are the first published for coelenterate species.  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