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   804606篇
  免费   94663篇
  国内免费   967篇
  900236篇
  2018年   7090篇
  2017年   6761篇
  2016年   9973篇
  2015年   14644篇
  2014年   16740篇
  2013年   23219篇
  2012年   27048篇
  2011年   27510篇
  2010年   18359篇
  2009年   16822篇
  2008年   24177篇
  2007年   25046篇
  2006年   23157篇
  2005年   22435篇
  2004年   22270篇
  2003年   20955篇
  2002年   20308篇
  2001年   34892篇
  2000年   35015篇
  1999年   28050篇
  1998年   10555篇
  1997年   10774篇
  1996年   10337篇
  1995年   9637篇
  1994年   9468篇
  1993年   9351篇
  1992年   23053篇
  1991年   22373篇
  1990年   21997篇
  1989年   21443篇
  1988年   19521篇
  1987年   18746篇
  1986年   17520篇
  1985年   17202篇
  1984年   14522篇
  1983年   12578篇
  1982年   9729篇
  1981年   8793篇
  1980年   8192篇
  1979年   13677篇
  1978年   10849篇
  1977年   9834篇
  1976年   9166篇
  1975年   10175篇
  1974年   11114篇
  1973年   10891篇
  1972年   9787篇
  1971年   9042篇
  1970年   7600篇
  1969年   7454篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
51.
Formation of rings from Drosophila DNA fragments   总被引:1,自引:0,他引:1  
  相似文献   
52.
Four myeloid cell lines (M1, WEHI-3B D+, FDC-P1, and 32D) were screened for the presence of J11d antigen. One of these cell lines, the myeloid leukemia M1, was found to express a high level of J11d antigen on the cell surface. Recombinant mouse leukemic inhibitory factor (rm-LIF), recombinant human LIF (rh-LIF), and steroids (hydrocortisone and dexamethasone) could induce M1 cells to undergo monocytic differentiation. The level of J11d antigen was greatly reduced after treatment of the cells with LIF or steroids. Western blotting revealed that the apparent molecular weight of the J11d antigen on M1 cells was 45-48 kDa. Furthermore, the level of J11d mRNA was also reduced during LIF-induced differentiation of M1 cells.  相似文献   
53.
54.
The way a protein responds to mutation provides key insights into its architecture and energetics. Mutations are improving the understanding both of protein folding and stability, and of the adaptability of the hydrophobic core. The importance of intermolecular effects in crystal structures is being emphasized and new insights into the correspondence between crystal and solution structures are being developed.  相似文献   
55.
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   
56.
We propose a mathematical model for pendelluft flow in a single airway bifurcation. The model is motivated by an apparatus used in an experimental study of the pendelluft by Ultman et al. (1988). We derive differential equations governing the fluid flow, which directly connect physiological parameters to the variables determining the pendelluft; this approach allows us to include nonlinearity in the model. If nonlinearity is neglected, our model is identical to the R-I-C circuits used by previous investigators. If nonlinearity is retained, we show that pendelluft can occur even in perfectly symmetric airway bifurcations. For the specific apparatus used in the experiments of High et al. (1991), we demonstrate that two qualitatively different pendelluft flows can occur in the system.  相似文献   
57.
58.
59.
60.
Besides vobtusine and vobtusine-lactone, deoxyvobtusine was isolated from the leaves of Voacanga grandifolia (Miq. Rolfe. Spectral and chemical evi  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