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   448215篇
  免费   51844篇
  国内免费   742篇
  2018年   4203篇
  2017年   4043篇
  2016年   5769篇
  2015年   8346篇
  2014年   9698篇
  2013年   13071篇
  2012年   15497篇
  2011年   15948篇
  2010年   10562篇
  2009年   9586篇
  2008年   13988篇
  2007年   14431篇
  2006年   13551篇
  2005年   12952篇
  2004年   12978篇
  2003年   12111篇
  2002年   11779篇
  2001年   18998篇
  2000年   18936篇
  1999年   15129篇
  1998年   5657篇
  1997年   5668篇
  1996年   5393篇
  1995年   5177篇
  1994年   4884篇
  1993年   4890篇
  1992年   12517篇
  1991年   12474篇
  1990年   12235篇
  1989年   11701篇
  1988年   10871篇
  1987年   10240篇
  1986年   9776篇
  1985年   9535篇
  1984年   7924篇
  1983年   6858篇
  1982年   5177篇
  1981年   4664篇
  1980年   4340篇
  1979年   7382篇
  1978年   5990篇
  1977年   5306篇
  1976年   4991篇
  1975年   5804篇
  1974年   6325篇
  1973年   6157篇
  1972年   5485篇
  1971年   5100篇
  1970年   4293篇
  1969年   4219篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
In this paper the recent population changes of the Wild Boar in different European countries is analysed through the study of hunting statistics. A simultaneous increase in numbers is observed throughout the whole area during the period 1965–1975. From 1975 onwards the population stabilizes itself apart from in peripheral areas like Finland. Potentially favourable factors which play a part in this process are discussed and certain reproductive and dispersive characteristics which favour its invasive behaviour are discussed.  相似文献   
122.
123.
Administration of purified pertussis toxin to rats induced persistent tachycardia, (observed in conscious rats but not after pithing); as little as 0.05 microgram/100 g produced a significant effect. Pertussis toxin-treatment did not affected the pressor response produced in the pithed rats by the alpha 2-adrenergic agonist methoxamine but markedly diminished the pressor effect of the alpha 2-adrenergic agonists clonidine and azepexole. A role of adenylate cyclase inhibition in the action of postsynaptic vascular alpha 2-adrenergic receptors is suggested.  相似文献   
124.
Recombinant human insulin.   总被引:1,自引:0,他引:1  
Insulin is a well-characterized peptide that can be produced by recombinant DNA technology for human therapeutic use. A brief overview of insulin production from both traditional mammalian pancreatic extraction and recombinant bacterial and yeast systems is presented, and detection techniques, including electrophoresis, are reviewed. Analytical systems for insulin separation are principally based on reversed-phase chromatography, which resolves the deamidation product(s) (desamido insulin) of insulin, proinsulin, and insulin. Process-scale separation is a multistep process and includes ion exchange, reversed-phase, and size exclusion chromatography. Advantages and/or disadvantages of various separation approaches, as described by the numerous literature references on insulin purification, are presented.  相似文献   
125.
126.
127.
Chronic reduction of gravitational load in the rear limbs of rats to simulate the influence of near-zero gravity in skeletal muscles has been shown previously to elicit atrophy in the soleus muscle. Use of this model by the present investigation indicates that soleus atrophy was characterized by a decline in the number of fibers in groups that contained the slow isoenzyme of myosin and which were classified as type I from intensity of staining to myofibrillar actomyosin adenosinetriphosphatase (ATPase) and to NADH tetrazolium reductase. Furthermore total fiber number was not changed, whereas fibers containing the intermediate isoenzyme and those classified as type IIa increased. There results could be explained by either a change in the composition within existing fibers or a simultaneous loss of slow fibers and de novo synthesis of intermediate and fast fibers. Evidence for transformation included an absence of embryonic or neonatal myosin in muscles from suspended rats and the constant fiber number that was unchanged by 4 wk of suspension. Furthermore although fiber areas of both groups of type I and IIa fibers declined during suspension, variability of the fiber areas within each group did not increase.  相似文献   
128.
129.
In an attempt to relate the translational characteristics of alfalfa mosaic virus (A1MV) RNAs to their structure [Ravelonandro et al. (1983) Nucleic Acids Res. 11, 2815-2826; Gehrke et al. (1983) Biochemistry 22, 5157-5164] we measured the relative affinities (discrimination ratios) of these RNAs for the initiation complex, in the wheat germ extract and in the nuclease-treated reticulocyte lysate, using a competition method designed by Brendler et al. [(1981) J. Biol. Chem. 256, 11747-11754]. As a prerequisite of this study we ascertained that the molecular mass distribution of the translation products was independent of RNA concentration in both translation systems. In the wheat germ extract the discrimination ratios are very similar for two strains of A1MV (S and B) which differ mainly by the presence (strain S) or absence (strain B) of a stable 5'-proximal hairpin. Hence this structure has no bearing on discrimination. Taking the affinity of RNA 3 as reference, the following orders of magnitude are found for the affinities of the different RNAs in the wheat germ: RNA 3, 1.0; RNA 1, 10; RNA 2, 60; RNA 4, 150. In the reticulocyte lysate the discrimination ratios are not significantly different from the wheat germ. Thus it seems that the mechanism of discrimination is essentially the same in the two translation systems, despite a difference in rate-limitation.  相似文献   
130.
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