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   392247篇
  免费   41478篇
  国内免费   146篇
  433871篇
  2018年   4740篇
  2017年   4394篇
  2016年   6128篇
  2015年   8240篇
  2014年   9209篇
  2013年   12931篇
  2012年   14563篇
  2011年   14124篇
  2010年   9470篇
  2009年   8050篇
  2008年   12045篇
  2007年   12281篇
  2006年   11672篇
  2005年   10831篇
  2004年   10913篇
  2003年   10357篇
  2002年   10035篇
  2001年   16028篇
  2000年   15950篇
  1999年   12721篇
  1998年   4598篇
  1997年   4540篇
  1996年   4396篇
  1995年   4205篇
  1994年   4189篇
  1993年   4023篇
  1992年   10615篇
  1991年   10120篇
  1990年   9925篇
  1989年   9742篇
  1988年   9011篇
  1987年   8660篇
  1986年   7816篇
  1985年   7971篇
  1984年   6611篇
  1983年   5784篇
  1982年   4578篇
  1981年   4070篇
  1980年   3871篇
  1979年   6684篇
  1978年   4979篇
  1977年   4709篇
  1976年   4364篇
  1975年   4835篇
  1974年   5230篇
  1973年   5251篇
  1972年   4842篇
  1971年   4498篇
  1970年   3916篇
  1969年   3880篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The effects of intraventricular administration of lysine-vasopressin on pain sensitivity in the rat were determined in the tail-flick test. Vasopressin (16–100 μg) was found to induce potent and dose-dependent antinociceptive actions, lasting up to one hour. An additional experiment demonstrated that analgesia induced by vasopressin was not blocked by naloxone, suggesting that this analgesia is independent of opiate receptor systems. Vasopressin was also found to be equally effective in elevating tail-flick latency after systemic administration. These results, together with others, suggest a possible role of vasopressin systems in the regulation of pain sensitivity.  相似文献   
992.
993.
Enumeration of potentially pathogenic bacteria from sewage sludges.   总被引:9,自引:8,他引:1       下载免费PDF全文
To ascertain the health risks that may be posed by the land application of sewage sludges, a scheme was devised to determine the types and numbers of pathogenic and potentially pathogenic bacteria present in sludges. A processing treatment was adapted to sludge to give a homogenate which yielded the greatest numbers of viable bacteria. Conventional methods were successful in enumerating Klebsiella, Staphylococcus, gram-negative enteric bacteria, and commonly used indicator organisms. Modifications of conventional methods improved the enumeration of Salmonella, Mycobacterium sp., fluorescent Pseudomonas sp., and Clostridium perfringens. However, Shigella methodology yielded only one isolate. Utilizing the proposed scheme, the population densities of these organisms were estimated in three domestic wastewater sludges. In light of these results, the potential impact of land application of sewage sludges is discussed.  相似文献   
994.
995.
We analyzed the pattern of correlations among fitness components, herbivory, and resin characteristics in a natural all-aged stand of ponderosa pine, to infer the strength and mechanism of natural selection on plant chemistry. Male and female cone production were monitored yearly for 15 years, and levels of herbivory for 9 years in 165 trees. Resin flow rate and monoterpene composition were determined for these same trees. Multiple regression of fitness components on resin characteristics showed significant associations consistent with directional selection for increased resin flow rates and increased proportions of α- and β-pinene, myrcene and terpinolene. However, negative correlations among monoterpene fractions of the resin constrained the overall selection. Selective herbivory by aphids approached statistical significance and monoterpenes showed some (non-significant) effect as deterrents against deer browse. Resin characteristics were not correlated with attack by cone insects or porcupines. However, the association between resin characteristics and fitness is significantly different from that predicted by the path coefficients involving herbivores. Therefore the hypothesis that these herbivores mediate selection on the resin is not supported by the observed pattern of correlations among resin characteristics, herbivory, growth and fecundity. In this population, most of the association between resin characteristics and fitness appears to be mediated by some other factor independent of attack by herbivore species present. Received: 18 March 1996/Accepted: 18 July 1996  相似文献   
996.
A double-blind controlled trial in 24 lepromatous leprosy patients in reaction showed that clofazimine (Lamprene) controlled symptoms of erythema nodosum leprosum reaction in lepromatous leprosy better than prednisolone. Clofazimine also appeared to be significantly superior in preventing recurrence once the reaction had been controlled. There was a statistically significant rise in serum albumin among inpatients on clofazimine as compared with patients on prednisolone, but no difference in terms of neurological status, bacterial index, morphological index, and renal functions. Red/black hyperpigmentation was seen among practically all patients on clofazimine. No other side-effects or deleterious systemic effects were observed.  相似文献   
997.
998.
999.
A number of recombinant cell lines which produce human tissue plasminogen activator (tPA) was obtained using different hosts--mouse, rat, hamster, simian and human cell lines. All types of recombinant lines secreted active r-tPA into conditioned medium. A slight difference between molecular weights of secreted variants of r-tPA was mediated by the different mechanisms of protein modification. Treatment of some recombinant cell lines with different substances resulted in increased levels of r-tPA production.  相似文献   
1000.
Interactions of luciferases isolated from Vibrio fischeri 6 and Escherichia coli JM109(pF3) (bearing cloned V. fischeri luxAB genes) with FMN reductase isolated from E. coli JM109 were studied. FMN reductase formed a stable complex with luciferase, suggesting similar properties of the FMN reductases in the taxonomically close families Vibrionaceae and Enterobacteriaceae.  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