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   300806篇
  免费   27848篇
  国内免费   274篇
  2021年   2760篇
  2018年   3583篇
  2017年   3395篇
  2016年   4812篇
  2015年   5962篇
  2014年   7127篇
  2013年   9517篇
  2012年   11154篇
  2011年   11701篇
  2010年   7835篇
  2009年   7014篇
  2008年   10072篇
  2007年   10324篇
  2006年   9812篇
  2005年   9347篇
  2004年   9197篇
  2003年   8775篇
  2002年   8587篇
  2001年   12795篇
  2000年   12614篇
  1999年   9903篇
  1998年   3499篇
  1997年   3300篇
  1996年   3180篇
  1995年   2926篇
  1994年   2897篇
  1993年   2780篇
  1992年   7755篇
  1991年   7439篇
  1990年   7514篇
  1989年   7296篇
  1988年   6773篇
  1987年   6384篇
  1986年   5729篇
  1985年   6013篇
  1984年   4872篇
  1983年   4236篇
  1982年   3066篇
  1981年   2851篇
  1980年   2677篇
  1979年   4399篇
  1978年   3415篇
  1977年   3128篇
  1976年   3071篇
  1975年   3462篇
  1974年   3716篇
  1973年   3722篇
  1972年   3202篇
  1971年   2982篇
  1970年   2673篇
排序方式: 共有10000条查询结果,搜索用时 437 毫秒
991.
The leaves of Strychnos wallichiana Steud. ex. DC. from Bangladesh contain icajine and novacine as their major alkaloids. Smaller amounts of strychnine, brucine, pseudostrychnine, pseudobrucine, N-methyl-sec.-pseudo-β-colubrine, 14-hydroxyicajine, strychnine N-oxide, and brucine N-oxide are also present. The new bases 14 hydroxynovacine and icajine N-oxide have been isolated.  相似文献   
992.
Pseudomonas aeruginosa and P. denitrificans accumulate more protoheme and considerably more protoporphyrin during anaerobic growth under denitrifying conditions than during aerobic growth. In Escherichia coli, the small accumulation of protoporphyrin and protoheme which occurs during anaerobic growth is slightly stimulated by nitrate and markedly stimulated by oxygen.  相似文献   
993.
994.
Receptors for estradiol, progesterone and dihydrotestosterone have been demonstrated in normal growing breasts of young females below 25 years.  相似文献   
995.
996.
In a previous study, we demonstrated trophic effects of vitamin A and its active metabolite, retinoic acid (RA), on perinatal rat spinal cord neurons and astrocytes in vitro. We now report that RA increases the survival of cholinergic neurons without affecting that of GABAergic neurons. These results were supported by measured levels of acetylcholinesterase (AChE), choline acetyltransferase (ChAT), and glutamic acid decarboxylase (GAD) activities, key enzymes of acetylcholine and gamma-aminobutyric acid metabolism, respectively, which showed RA-induced increases in AChE and ChAT levels but no elevations of GAD activity. In contrast to these phenotype-specific effects, most neurons showed RA-induced increases in neuritic outgrowth, density, and silver impregnation. Taken together, these results demonstrate neurotransmitter-specific and generalized effects of RA on developing CNS neurons.  相似文献   
997.
998.
This paper proposes a methodology for establishing a set of sustainability indicators as a baseline to measure the state of a city, and after appropriate actions are taken, to assess performance, to determine if changes point to improvements regarding goals. The methodology starts with choosing a procedure for identification, scope, evaluation, and level of indicators to build the initial dataset. As a second step, produces a reduced set of indicators, complying with all criteria imposed, and selected under the condition of extracting the maximum amount of information from the initial dataset.  相似文献   
999.
1000.
Recently, in another study, we observed that indomethacin, a prostaglandin synthetase inhibitor, significantly increased NK activity in both normal and rheumatoid arthritis (RA) peripheral blood (PB) but not in RA synovial fluid (SF). Because macrophages are a major source of prostaglandins, we examined the effect of macrophage-enriched adherent cells (AC) on NK activity as measured by a 3-hr Cr-release assay with K 562 cells. The removal of AC resulted in increased (p less than 0.01) NK activity in both normal and RA PB. In contrast, the removal of AC from RA SF resulted in a significant decrease (p less than 0.001) of NK activity. By using only nonadherent cells (NAC), NK activity in RA SF and synovial tissue (ST) was significantly reduced when compared to autologous RA PB (p less than 0.001). Enhancement of NK activity of SF NAC by both poly I:C and IL 2 was not dependent on AC. Mixing experiments demonstrated that the addition of synovial AC for 16 hr increased NK activity of synovial NAC to a level similar to that of unseparated mononuclear cells, whereas autologous PB AC suppressed NK activity of PB NAC. PB AC, when added to SF NAC, also increased NK activity. Supernatants from synovial mononuclear cells were stimulatory of synovial NAC NK activity, whereas normal PB mononuclear supernatants were suppressive. These observations document 1) a significant reduction of NAC-mediated NK activity in the rheumatoid joint as compared to PB from the same patient, and 2) that AC modulate NK activity differently in the rheumatoid joint as compared to RA or normal PB.  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