全文获取类型
收费全文 | 783897篇 |
免费 | 85016篇 |
国内免费 | 3437篇 |
专业分类
872350篇 |
出版年
2021年 | 7328篇 |
2018年 | 8506篇 |
2017年 | 7733篇 |
2016年 | 10808篇 |
2015年 | 14264篇 |
2014年 | 16837篇 |
2013年 | 23766篇 |
2012年 | 26275篇 |
2011年 | 26396篇 |
2010年 | 17720篇 |
2009年 | 16085篇 |
2008年 | 22648篇 |
2007年 | 23029篇 |
2006年 | 21681篇 |
2005年 | 20617篇 |
2004年 | 20190篇 |
2003年 | 19234篇 |
2002年 | 18473篇 |
2001年 | 36584篇 |
2000年 | 36513篇 |
1999年 | 28810篇 |
1998年 | 9642篇 |
1997年 | 10395篇 |
1996年 | 9700篇 |
1995年 | 8897篇 |
1994年 | 8656篇 |
1993年 | 8631篇 |
1992年 | 23112篇 |
1991年 | 22352篇 |
1990年 | 21502篇 |
1989年 | 20981篇 |
1988年 | 19250篇 |
1987年 | 18040篇 |
1986年 | 16819篇 |
1985年 | 16789篇 |
1984年 | 13738篇 |
1983年 | 11612篇 |
1982年 | 8788篇 |
1981年 | 7964篇 |
1980年 | 7532篇 |
1979年 | 13014篇 |
1978年 | 9962篇 |
1977年 | 9367篇 |
1976年 | 8504篇 |
1975年 | 9404篇 |
1974年 | 10264篇 |
1973年 | 10004篇 |
1972年 | 9114篇 |
1971年 | 8384篇 |
1970年 | 7314篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
All plant cells are provided with the necessary rigidity to withstand the turgor by an exterior cell wall. This wall is composed
of long crystalline cellulose microfibrils embedded in a matrix of other polysaccharides. The cellulose microfibrils are deposited
by mobile membrane bound protein complexes in remarkably ordered lamellar textures. The mechanism by which these ordered textures
arise, however, is still under debate. The geometrical model for cell wall deposition proposed by Emons and Mulder (Proc.
Natl. Acad. Sci. 95, 7215–7219, 1998) provides a detailed approach to the case of cell wall deposition in non-growing cells, where there is no evidence for the
direct influence of other cellular components such as microtubules. The model successfully reproduces even the so-called helicoidal
wall; the most intricate texture observed. However, a number of simplifying assumptions were made in the original calculations.
The present work addresses the issue of the robustness of the model to relaxation of these assumptions, by considering whether
the helicoidal solutions survive when three aspects of the model are varied. These are: (i) the shape of the insertion domain,
(ii) the distribution of lifetimes of individual CSCs, and (iii) fluctuations and overcrowding. Although details of the solutions
do change, we find that in all cases the overall character of the helicoidal solutions is preserved. 相似文献
52.
Formation of rings from Drosophila DNA fragments 总被引:1,自引:0,他引:1
53.
54.
Transcriptive expression during sea urchin embryogenesis 总被引:2,自引:0,他引:2
55.
56.
57.
T. B. Lapirova V. R. Mikryakova A. S. Mavrin G. A. Yinogradova 《Journal of Evolutionary Biochemistry and Physiology》2000,36(1):47-51
The paper presents data on a change in lysozyme content in tissues of spleen, liver and heart in fry of the Lena River sturgeon
exposed to the presence of sublethal concentrations of Hg2+, Cd2+, and Cu2+ under conditions of chronic experiment. It has been shown that the lysozyme content in fish tissues varies and has a phasic
character. The amplitude of fluctuations of this parameter depends on the moment of sampling, nature of the toxicant, and
structural-functional organization of the studied organs. 相似文献
58.
Subsequent to observations that pulmonary responses to antigen challenge are of different magnitudes in sensitized rats that are anesthetized with different drugs, we conducted studies to test whether the alterations in responses were due to changes in airway responsiveness to cholinergic or serotonergic challenge, opioid-receptor mediated events, or changes in mast cell mediator release. Immunoglobulin E-sensitized rats anesthetized with ketamine/urethan had larger changes in lung resistance and plasma histamine after pulmonary antigen challenge compared with rats anesthetized with fentanyl-droperidol. Blockade of opioid receptors with naloxone did not affect the responses. In unsensitized rats, airway responses to aerosolized methacholine were similar for the two anesthetics, indicating unchanged smooth muscle responsiveness; however, airway responses to intravenous serotonin were enhanced by ketamine and ablated by droperidol. We conclude that ketamine- and droperidol-induced alterations of pulmonary allergic responses are due to changes in sensitivity to serotonin and in mast cell mediator release. We speculate that mast cell mediator release may be modulated by a serotonin receptor-linked mechanism. 相似文献
59.
60.
The goal of this study was to develop a new implantable transducer for measuring anterior cruciate ligament (ACL) graft tension postoperatively in patients who have undergone ACL reconstructive surgery. A unique approach was taken of integrating the transducer into a femoral fixation device. To devise a practical in vivo calibration protocol for the fixation device transducer (FDT), several hypotheses were investigated: (1) The use of a cable versus the actual graft as the means for applying load to the FDT during calibration has no significant effect on the accuracy of the FDT tension measurements; (2) the number of flexion angles at which the device is calibrated has no significant effect on the accuracy of the FDT measurements; (3) the friction between the graft and femoral tunnel has no significant effect on measurement accuracy. To provide data for testing these hypotheses, the FDT was first calibrated with both a cable and a graft over the full range of flexion. Then graft tension was measured simultaneously with both the FDT on the femoral side and load cells, which were connected to the graft on the tibial side, as five cadaver knees were loaded externally. Measurements were made with both standard and overdrilled tunnels. The error in the FDT tension measurements was the difference between the graft tension measured by the FDT and the load cells. Results of the statistical analyses showed that neither the means of applying the calibration load, the number of flexion angles used for calibration, nor the tunnel size had a significant effect on the accuracy of the FDT. Thus a cable may be used instead of the graft to transmit loads to the FDT during calibration, thus simplifying the procedure. Accurate calibration requires data from just three flexion angles of 0, 45, and 90 deg and a curve fit to obtain a calibration curve over a continuous range of flexion within the limits of this angle group. Since friction did not adversely affect the measurement accuracy of the FDT, the femoral tunnel can be drilled to match the diameter of the graft and does not need to be overdrilled. Following these procedures, the error in measuring graft tension with the FDT averages less than 10 percent relative to a full-scale load of 257 N. 相似文献