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   7707篇
  免费   84篇
  国内免费   53篇
系统科学   41篇
丛书文集   90篇
教育与普及   13篇
理论与方法论   17篇
现状及发展   3367篇
研究方法   410篇
综合类   3823篇
自然研究   83篇
  2016年   48篇
  2012年   107篇
  2011年   208篇
  2010年   56篇
  2009年   53篇
  2008年   153篇
  2007年   177篇
  2006年   134篇
  2005年   140篇
  2004年   240篇
  2003年   126篇
  2002年   133篇
  2001年   296篇
  2000年   294篇
  1999年   216篇
  1992年   122篇
  1991年   125篇
  1990年   107篇
  1989年   100篇
  1988年   112篇
  1987年   121篇
  1986年   114篇
  1985年   160篇
  1984年   122篇
  1983年   99篇
  1982年   86篇
  1981年   116篇
  1980年   110篇
  1979年   251篇
  1978年   198篇
  1977年   183篇
  1976年   140篇
  1975年   159篇
  1974年   212篇
  1973年   173篇
  1972年   193篇
  1971年   212篇
  1970年   273篇
  1969年   211篇
  1968年   188篇
  1967年   210篇
  1966年   181篇
  1965年   144篇
  1959年   53篇
  1958年   104篇
  1957年   76篇
  1956年   73篇
  1955年   52篇
  1954年   74篇
  1948年   49篇
排序方式: 共有7844条查询结果,搜索用时 531 毫秒
81.
Field stimulation of isolated, spirally-cut cervix from day-22 pregnant rats produced contractions which could be inhibited by tetrodotoxin or hyoscine and potentiated by propranolol. The rat cervix would appear to receive both cholinergic and noradrenergic innervations whose transmitters activate muscarinic cholinoceptors and beta-adrenoceptors respectively.  相似文献   
82.
N Aoki  L J Degroot 《Experientia》1979,35(11):1515-1517
Suppressor cell induction can be demonstrated during antigen specific blastogenesis by using the same methods which have shown induction of suppressor cells by Con A. Since suppressor cells are rapidly generated during antigen specific blastogenesis, they must regulate the final level of blastogenesis induced during the seven day in vitro incubation.  相似文献   
83.
Summary Viable cerebellar perikarya of mixed cell type obtained from 7–9-day-old rats were maintained in monolayer culture for up to 12 days. During this time extensive neurite formation and outgrowth occurred. The large majority of the cells developing in culture were tentatively identified as granule neurons. This identification was based on the large number isolated from the starting tissue, and the cells general morphological features in culture such as perikaryal and nuclear size, the bipolar nature of neurite extension, and their migratory behaviour.D.N. Currie was supported by a grant from Hoechst Pharmaceuticals, Ld, U.K.J.C. was supported by an S.R.C. Grant to S.P.R. RoseThe authors would like to thank Kathleen Tear for excellent technical assistance and members of the Brain Research Group for helpful discussions.  相似文献   
84.
85.
Zusammenfassung Die i.v. Verabreichung von LSD an Ratten steigert den Blutkreislauf im Hirnstamm sowie im frontalen und parietalen Cortex selektiv. Es wurden durch 6-Nor-LSD, 2-Methyl-LSD und 8, 9-LSD weder wahrnehmbare Verhaltensänderungen noch Änderungen des regionalen Blutkreislaufs verursacht.  相似文献   
86.
The DNP fibres gamma-irradiated under aerobic condition showed a reduction of their diameter, while no remarkable changes were observed in the DNP fibres irradiated under hypoxic condition by scanning electron microscopy.  相似文献   
87.
Summary An ecdysone 20-hydroxylase enzyme system that converts -ecdysone to 20-hydroxyecdysone was prepared from the midgut of the tobacco hornworm prepupa. This partially purified enzyme is NADPH dependent and is localized in the mitochondrial fraction of the midgut tissue.  相似文献   
88.
系统地阐述了铁氧体可变特征阻抗传输线的设计理论,结合德国重离子物理研究所交叉指强流直线加速器的束载对高频功率馈送的输入阻抗的影响,设计了一个可以在强的加速柬载下实现高频功率源和加速腔高频输入阻抗良好匹配的铁氧体可变阻抗适配器。  相似文献   
89.
To ensure reliable land vehicle positioning, the Global Positioning System (GPS) is one of the best techniques commonly used in present-day positioning systems. However, GPS signals are not available if line of sight with the satellites is lost. In this respect, the addition of the Dead Reckoning (DR) method to complement the GPS unit would enhance the reliability of land vehicle positioning system. For implementing the DR method, the magnetic sensor and accelerometer are used for obtaining heading and velocity or distance information. Calibration of two sensors' dynamic model and DR algorithm are also introduced in this paper. The test result utilizing the GPS and DR methods is analyzed in this paper. It utilize spositioning information from GPS receiver when GPSsignal is available, otherwise DR is started to compensate GPS outage with the GPS output as the original point. The results showed that the error in east and north produced by the DR algorithm for a short time interval can be limitedwithin an acceptable range.  相似文献   
90.
The marine spongeTheonella swinhoei (lithistid Family Theonellidae, Order Astrophorida) has yielded many important, bioactive natural products, most of which share structural features with bacterial natural products. The presence of microbial symbionts inT. swinhoei has been reported, and it was originally suggested that the cytotoxic macrolide swinholide A and many of the bioactive cyclic peptides fromT. swinhoei were all produced by symbiotic cyanobacteria. By transmission electron microscopy, we found four distinct cell populations to be consistently present inT. swinhoei: eukaryotic sponge cells, unicellular heterotrophic bacteria, unicellular cyanobacteria and filamentous heterotrophic bacteria. Purification and chemical analyses of each cell type showed the macrolide swinholide A to be limited to the mixed population of unicellular heterotrophic bacteria, and an anti-fungal cyclic peptide occurred only in the filamentous heterotrophic bacteria. Contrary to prior speculation, no major metabolites were located in the cyanobacteria or sponge cells.  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