全文获取类型
收费全文 | 72966篇 |
免费 | 373篇 |
国内免费 | 746篇 |
专业分类
系统科学 | 1447篇 |
丛书文集 | 860篇 |
教育与普及 | 320篇 |
理论与方法论 | 546篇 |
现状及发展 | 35986篇 |
研究方法 | 1583篇 |
综合类 | 31153篇 |
自然研究 | 2190篇 |
出版年
2013年 | 1026篇 |
2012年 | 1072篇 |
2011年 | 3257篇 |
2010年 | 784篇 |
2009年 | 895篇 |
2008年 | 1303篇 |
2007年 | 1577篇 |
2006年 | 1590篇 |
2005年 | 1707篇 |
2004年 | 2735篇 |
2003年 | 2218篇 |
2002年 | 1946篇 |
2001年 | 1956篇 |
2000年 | 1533篇 |
1999年 | 1333篇 |
1998年 | 674篇 |
1997年 | 796篇 |
1994年 | 704篇 |
1993年 | 699篇 |
1992年 | 1202篇 |
1991年 | 995篇 |
1990年 | 1082篇 |
1989年 | 883篇 |
1988年 | 845篇 |
1987年 | 874篇 |
1986年 | 915篇 |
1985年 | 1118篇 |
1984年 | 941篇 |
1983年 | 804篇 |
1982年 | 863篇 |
1981年 | 913篇 |
1980年 | 1037篇 |
1979年 | 1812篇 |
1978年 | 1565篇 |
1977年 | 1548篇 |
1976年 | 1316篇 |
1975年 | 1329篇 |
1974年 | 1454篇 |
1973年 | 1532篇 |
1972年 | 1645篇 |
1971年 | 1723篇 |
1970年 | 2018篇 |
1969年 | 1738篇 |
1968年 | 1635篇 |
1967年 | 1584篇 |
1966年 | 1385篇 |
1965年 | 1071篇 |
1959年 | 611篇 |
1958年 | 942篇 |
1957年 | 724篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
131.
Experiments were carried out to elucidate the role of central catecholamines in regulating catecholamine metabolism in the vas deferens and adrenal gland of the rat. Rats were injected intracerebroventricularly (i.c.v.) with either vehicle or 6-hydroxydopamine (6-OHDA). Groups of animals pretreated with vehicle or 6-OHDA (i.c.v.) were injected intraperitoneally (i.p.) with alpha-methyl-para-tyrosine (AMT), a tyrosine hydroxylase inhibitor. Catecholamine turnover rates were estimated by determining norepinephrine or epinephrine content after administrating AMT.Central norepinephrine and dopamine contents decreased significantly (p<0.05) after treatment with 6-OHDA and AMT. The norepinephrine content of the vas deferens of rats pretreated with 6-OHDA was markedly reduced (p<0.001) after administration of AMT, whereas that of the vehicle-treated rats remained unchanged. Administration of 6-OHDA had no effect on the norepinephrine or epinephrine content of the adrenal gland.The present results indicate that central monoaminergic neurons have an inhibitory effect on the adrenergic neurons of the vas deferens. In contrast, this inhibitory regulation does not appear to be exerted on the adrenal glands. 相似文献
132.
A topoisomerase II inhibitor, novobiocin, and a deacetylase inhibitor, butyrate, synergistically transformed human liver cells into fibroblast-like cells. This morphological change was associated with an increased production of procollagen type III peptide and a simultaneous assembly of actin, tubulin, vimentin and cytokeratin. Novobiocin and butyrate had no marked effect on the phosphorylation state of cytokeratin proteins, but synergistically enhanced [3H]acetate uptake. From these results, it can be speculated that protein acetylation plays an important role in inducing the assembly of cytoskeletal proteins and the morphological transformation of human liver cells. 相似文献
133.
A diffusion chamber technique based on time-lag analysis for the estimation of effective diffusion coefficients of radiolabelled macromolecules of varying molecular weights through native mucus gel is reported. For all solutes studied, a reduction in effective diffusion coefficients was observed with a retardation of solute flux in both aqueous and mucus layers. Over the molecular weight range of solutes investigated (126-186,000 Daltons), a consistent effect of molecular weight was evident with regard to the retarding effect of mucus. No apparent or absolute molecular weight cut-off for macromolecular transfer was exhibited. However, at high molecular weights (greater than 30,000 Daltons) the retardation was greatly enhanced. The results confirm that mucus can be regarded as a gel with finite pores, but that it does not constitute an absolute barrier to even high molecular weight solutes. 相似文献
134.
从中国南海西沙群岛永胜岛附近海域采集到的一种海绵(种属未定)分离到两种新的三羟基长链酰胺类化合物S—Ⅱ(C_(49)H_(99)NO_4,M=765,m.p.138-140℃)和S—Ⅲ(C_(45)H_(93)NO_4,M=723,m.p.92—93℃),通过对它们的IR,MS,~1HNMR,~(13)CNMR及~(13)CNMR DEPT等波谱数据分析,S—Ⅱ和S—Ⅲ的结构分别确定为N-(1-乙基-3,22-二羟基)二十二烷基-2-甲基-4-羟基二十四脂肪酸酰胺和N-(1-羟甲基-2-乙基-4-甲基-3,18-二羟基)十九烷基—二十三脂肪酸酰胺。 相似文献
135.
本文提出了精测安培表内阻的新方法,并将其与其它测量方法进行了比较。 相似文献
136.
本文介绍了在水电工程规划设计时,如何利用三次自然样条插值函数在有限个方案的基础上自动生成无限多个方案,然后采用多目标综合决策模型在其中选择最优方案的一种方法.文中还给出了程序流程,并以某水电站正常蓄水位选择作为实例,对方法进行了验证,结果与专家决策完全吻合. 相似文献
137.
The gas-liquid-liquid reactive extraction system for preparing hydrogen peroxide via anthraquinone was investigated. The oxidation reaction of hydrogenated working solution was combined with the extraction of hydrogen peroxide from working solution in a sieve plate column. The reaction of 2-ethylanthrahydroquionone with oxygen and the liquid-liquid extraction of hydrogen peroxide take place simultaneously. The oxygen was introduced with hydrogenated working solution through a nozzle in the bottom of the column, which worked as agitated air as well as oxidation reagent. The results showed the oxidation and extraction do not hamper each other, on the contrary, the presence of oxidation gas in the column can promote the transfer of hydrogen peroxide from organic phase to aqueous phase, thus the reaction efficiency and extraction efficiency increased with increasing gas superficial velocity. Furthermore, the oxidation efficiency is almost 100% and the extraction efficiency is higher than 90% in this process. 相似文献
138.
F. Acerbi 《Archive for History of Exact Sciences》2003,57(6):465-502
139.
就近几年图书馆界关于新世纪馆员意识转换及社会角色转换等方面的研究进行了综述,指出新时期的馆员要实现自我社会角色的转换,首先是要明确转换的必要性,然后采取有效的策略逐步地、系统地推进转换,才能取得较好的效果。 相似文献
140.
提出了一种基于径向网络的增量学习算法 ,在对新样本进行学习时 ,通过恢复受干扰样本的方式来尽可能保持已有的知识不变 .模拟实验证明了本算法的正确性和有效性 . 相似文献