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   991061篇
  免费   111493篇
  国内免费   664篇
  2018年   8407篇
  2016年   11930篇
  2015年   16714篇
  2014年   19648篇
  2013年   27447篇
  2012年   31107篇
  2011年   31640篇
  2010年   21541篇
  2009年   20100篇
  2008年   28519篇
  2007年   29237篇
  2006年   27610篇
  2005年   26418篇
  2004年   26180篇
  2003年   25284篇
  2002年   24579篇
  2001年   42522篇
  2000年   42754篇
  1999年   34280篇
  1998年   12816篇
  1997年   13358篇
  1996年   12812篇
  1995年   12101篇
  1994年   11897篇
  1993年   11693篇
  1992年   28694篇
  1991年   27903篇
  1990年   27586篇
  1989年   26874篇
  1988年   24823篇
  1987年   24251篇
  1986年   22179篇
  1985年   22510篇
  1984年   18609篇
  1983年   16213篇
  1982年   12646篇
  1981年   11329篇
  1980年   10782篇
  1979年   17762篇
  1978年   14161篇
  1977年   12885篇
  1976年   12309篇
  1975年   13441篇
  1974年   14481篇
  1973年   14305篇
  1972年   12740篇
  1971年   11849篇
  1970年   10230篇
  1969年   9594篇
  1968年   8769篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
3.
4.
A patient with chronic anemia is presented who radiologically showed prominent rugae of the stomach. Angiography demonstrated an arteriovenous malformation with a large feeding artery and prominent draining veins.  相似文献   
5.
6.
A conceptual model is proposed, describing potential Zostera marina habitats in the Wadden Sea, based on reported data from laboratory, mesocosm and field studies. Controlling factors in the model are dynamics, degree of desiccation, turbidity, nutrients and salinity. A distinction has been made between a higher and a lower zone of potential habitats, each suitable for different morphotypes of Z. marina. The model relates the decline of Z. marina in the Wadden Sea to increased sediment and water dynamics, turbidity, drainage of sediments (resulting in increased degree of desiccation) and total nutrient loads during the twentieth century. The upper and lower delineation of both the higher and the lower zone of potential Z. marina habitats appear to be determined by one or a combination of several of these factors. Environmental changes in one of these factors will therefore influence the borderlines of the zones. The lower zone of Z. marina will be mainly affected by increased turbidity, sediment dynamics, degree of desiccation during low tide and nutrient load. The higher zone will be affected by increases in water and sediment dynamics, desiccation rates and nutrient loads. Potential Z. marina habitats are located above approx. –0.80 m mean sea level (when turbidity remains at the same level as in the early 1990s) in sheltered, undisturbed locations, and preferably where some freshwater influence is present. At locations with a high, near-marine, salinity, the nutrient load has to be low to allow the growth of Z. marina. The sediment should retain enough water during low tide to keep the plants moist. Our results suggest that the return of Z. marina beds within a reasonable time-scale will require not only suitable habitat conditions, but also revegetation measures, as the changes in the environment resulting from the disappearance of Z. marina may impede its recovery, and the natural import of propagules will be unlikely. Furthermore, the lower zone of Z. marina may require a genotype that is no longer found in the Wadden Sea. Received: 26 April 1999 / Received in revised form: 15 October 1999 / Accepted: 16 October 1999  相似文献   
7.
8.
9.
Comparison of bone marrow extracellular matrices.   总被引:1,自引:0,他引:1  
We have compared the structure and composition of adult and fetal bovine bone marrow extracellular matrices. In contrast to fetal bone marrow, adult bone marrow has more oval fenestration and accumulation of adipocytes as well as lower protein content. These differences could be due to remodeling of bone marrow tissue as it develops. Zymogram analysis of matrix metalloproteinase (MMP) and tissue inhibitor of MMP (TIMP) activities showed that fetal, but not adult bone marrow extract contained a 96-kDa MMP and TIMP-1 and -2. These activities may contribute to the structural differences between adult and fetal bone marrow tissues.  相似文献   
10.
N Bains  J Hoey 《CMAJ》1998,159(2):163
  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