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   39592篇
  免费   115篇
  国内免费   185篇
系统科学   203篇
丛书文集   587篇
教育与普及   68篇
理论与方法论   148篇
现状及发展   17467篇
研究方法   1592篇
综合类   19158篇
自然研究   669篇
  2013年   364篇
  2012年   535篇
  2011年   1210篇
  2010年   235篇
  2008年   657篇
  2007年   769篇
  2006年   785篇
  2005年   718篇
  2004年   734篇
  2003年   675篇
  2002年   695篇
  2001年   1344篇
  2000年   1254篇
  1999年   807篇
  1992年   773篇
  1991年   571篇
  1990年   670篇
  1989年   684篇
  1988年   634篇
  1987年   679篇
  1986年   660篇
  1985年   792篇
  1984年   621篇
  1983年   530篇
  1982年   479篇
  1981年   490篇
  1980年   590篇
  1979年   1239篇
  1978年   1029篇
  1977年   985篇
  1976年   819篇
  1975年   869篇
  1974年   1202篇
  1973年   1017篇
  1972年   1053篇
  1971年   1211篇
  1970年   1522篇
  1969年   1116篇
  1968年   1142篇
  1967年   1068篇
  1966年   965篇
  1965年   674篇
  1964年   184篇
  1959年   366篇
  1958年   625篇
  1957年   424篇
  1956年   349篇
  1955年   338篇
  1954年   328篇
  1948年   234篇
排序方式: 共有10000条查询结果,搜索用时 671 毫秒
231.
Matthews HE  Irvine WM  Friberg P  Brown RD  Godfrey PD 《Nature》1984,310(5973):125-126
The cold dark interstellar Taurus Molecular Cloud One (TMC-1) is a rich source of acetylenic and polyacetylenic molecular species. As well as linear closed-shell molecules (H(C triple bond C)nCN) and symmetric rotors (CH3C triple bond CH, CH3C triple bond CCN), several radicals (C triple bond CH, C triple bond CCN, (C triple bond C2H) have also been identified, many of which had not been studied previously in the laboratory. Whether the observed abundances can be understood in terms of purely gas-phase ion-molecule chemical schemes, which produce reasonable agreement for the simplest polyatomic species, is unclear; alternative models involving the particulate interstellar grains as catalysts or sources have also been suggested. We now report the detection in TMC-1 of a new molecule, tricarbon monoxide (C3O), whose pure rotational spectrum has only very recently been studied in the laboratory. As C3O is the first known interstellar carbon chain molecule to contain oxygen, its existence places an important new constraint on chemical schemes for cold interstellar clouds. In fact, the observed abundance of tricarbon monoxide fits quite well into our model of galactochemistry.  相似文献   
232.
Summary High-affinity, Na+-independent binding of -alanine to a synaptosomal fraction of rat brain was potently inhibited by glycine and by some other -amino acids, but not by taurine or GABA. This binding mechanism, which was also sensitive to both bicuculline and strychnine, might involve synaptic receptors for both -alanine and glycine.This study was supported in part by Centro Ramón y Cajal and Fundacion Juan March.  相似文献   
233.
Summary Treating VY/WfL-A vy /a mice with 5-androstan-17-one, a mammalian glucose-6-phosphate dehydrogenase inhibitor, prevented the mice from becoming obese. The weight difference between treated and controlA vy /a mice was mainly due to a decreased accumulation of triacylglycerol. The compound did not suppress appetite, had no detectable toxicity and did not affect the lipogenesis rates in the liver and carcass. The weight-controlling effect of 5-androstan-17-one inA vy /a mice was reversible upon withdrawal of treatment.The authors wish to thank Mr W.R. Gibson and Drs C.G. Culbertson and P.N. Harris for performing the pathological examinations.  相似文献   
234.
Summary The secondary disease in PF1 strain combination depends upon the immune status of both partners. It is most clearly expressed if both donors and recipients are immunologically crippled by thymectomy and irradiation. Conspicuous reduction in the immune potential of parental parabionts induced by thymectomy and irradiation displayed their marked incidence of parabiotic death.  相似文献   
235.
Summary Specific genotypes of the xiphophorine fish develop neoplasms following treatment with N-methyl-N-nitrosourea or X-rays. Several of these neoplasms can be related to the presence of specific chromosomes. The implications of these findings are discussed.Supported by Deutsche Forschungsgemeinschaft through Sonderforschungsbereich 103 Zellenergetik und Zelldifferenzierung, Marburg (projects C 9 and C 10), and by Justus-Liebig-Universität Giessen. We are indebted to Prof. K. Frese, Veterinär-Pathologisches Institut, Giessen and Dr H. D. Mennel, Pathologisches Institut, Freiburg, for their help in the classification of neoplasms. We furthermore thank K. Klinke for breeding the fish. Dedicated to Prof. C. Kosswig on the occasion of his 75th birthday.The paper contains parts of the dissertations of S. Abdo, J. Haas and G. Kollinger.supported by the Egypt ministry of education.  相似文献   
236.
Summary -(4-chlorophenyl)-GABA, a GABA mimetic compound, acts as an inhibitor of GABA metabolism in both synaptosomal and extrasynaptosomal compartments. It has no significant action on GABA or Glu uptake by synaptosomes.  相似文献   
237.
Summary Pentobarbital concentrations of 10–100 M selectively inhibited the noradrenaline release evoked by activation of the nicotinic receptors on the terminals sympathetic nerves of the rabbit heart. Higher concentrations also decreased the noradrenaline release induced by KCl or by electrical stimulation of the nerve axons.This paper is dedicated to Professor Dr. G. Malorny on the occasion of his 65th birthday.Acknowledgment: We thank Mrs G. Thielecke for technical assistance.  相似文献   
238.
Summary Phenylketonuric squirrels have shown marked inhibition of alkaline phosphatatase in the olfactory lobes and cerebral hemispheres, whereas the Na+-K+-ATPase remained less altered. In the pathogenesis of phenylketonuria inhibition of alkaline phosphatase at the level of Blood-Brain Barrier (BBB), leads transport system to impaired functioning.This work was done in the Department of Zoology, University of Udaipur, Udaipur-313001, India.  相似文献   
239.
Summary The effects of L-glutamic acid and of some glutamate analogues have been studied on the central nervous system of the snailHeobania vermiculata, using conventional electrophysiological techniques. The glutamate H-response had the mean equilibrium value of-(57±4) mV and was associated with a Cl conductance change. The D-response to glutamate application showed an involvement of sodium ions. Aspartate was agonist of glutamate action and displayed similar equilibrium value of the H-response, whereas quisqualate H-response was non-invertible.The authors wish to express their thanks to Prof. A. Curatolo for his continual interest and advice, and to Dr J. Stinnakre for the criticism of the text.  相似文献   
240.
Summary Cyclophosphamide given to rats 2 or 5 days after an injection of Yoshida ascites sarcoma cured approximately the same proportion of animals, but the resistance to a subsequent tumor challenge was found only in rats treated with the drug 5 days after tumor injection.  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