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   748190篇
  免费   82426篇
  国内免费   561篇
  831177篇
  2018年   6863篇
  2017年   6625篇
  2016年   9248篇
  2015年   12012篇
  2014年   14150篇
  2013年   20852篇
  2012年   22921篇
  2011年   23486篇
  2010年   15887篇
  2009年   14587篇
  2008年   20858篇
  2007年   21465篇
  2006年   20253篇
  2005年   19458篇
  2004年   19247篇
  2003年   18427篇
  2002年   17819篇
  2001年   36159篇
  2000年   36172篇
  1999年   28346篇
  1998年   9339篇
  1997年   10080篇
  1996年   9402篇
  1995年   8610篇
  1994年   8355篇
  1993年   8391篇
  1992年   22849篇
  1991年   22161篇
  1990年   21323篇
  1989年   20826篇
  1988年   19149篇
  1987年   17957篇
  1986年   16740篇
  1985年   16716篇
  1984年   13685篇
  1983年   11562篇
  1982年   8757篇
  1981年   7956篇
  1980年   7523篇
  1979年   13003篇
  1978年   9964篇
  1977年   9367篇
  1976年   8506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
  1969年   6971篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
51.
52.
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   
53.
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   
54.
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.
Trigger finger is a relatively common clinical entity, most frequently caused by stenosing tenosynovitis. Several other conditions not related to tenosynovitis also have been described as a cause of triggering, and these have been reviewed. We present a rare anomaly of the fourth lumbrical muscle insertion as a cause of triggering of the right little finger. This was completely relieved following excision of the anomalous muscle. This rare anatomic variant should be added to the list of potential causes of trigger finger.  相似文献   
60.
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