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   433159篇
  免费   48459篇
  国内免费   164篇
  2018年   4101篇
  2016年   5577篇
  2015年   7363篇
  2014年   8679篇
  2013年   11879篇
  2012年   13528篇
  2011年   14008篇
  2010年   9579篇
  2009年   8820篇
  2008年   12625篇
  2007年   13127篇
  2006年   12276篇
  2005年   11756篇
  2004年   11743篇
  2003年   11008篇
  2002年   10778篇
  2001年   17496篇
  2000年   17491篇
  1999年   13998篇
  1998年   5109篇
  1997年   5331篇
  1996年   4965篇
  1995年   4669篇
  1994年   4532篇
  1993年   4578篇
  1992年   11639篇
  1991年   11562篇
  1990年   11321篇
  1989年   10923篇
  1988年   10505篇
  1987年   10090篇
  1986年   9379篇
  1985年   9245篇
  1984年   7759篇
  1983年   6722篇
  1982年   5179篇
  1981年   4630篇
  1980年   4469篇
  1979年   7431篇
  1978年   5873篇
  1977年   5407篇
  1976年   5210篇
  1975年   5598篇
  1974年   6304篇
  1973年   6171篇
  1972年   5769篇
  1971年   5227篇
  1970年   4636篇
  1969年   4576篇
  1968年   4422篇
排序方式: 共有10000条查询结果,搜索用时 468 毫秒
41.
Censuses of buffalo and wildebeest in the Serengeti-Mara region have been carried out by various workers at intervals since 1958. The methods of these censuses, which normally employ a total count, are described and the errors inherent in such techniques are analysed in detail. After the appropriate corrections had been made the results showed that both populations have increased considerably over the years 1961–1971.  相似文献   
42.
43.
Summary Gramineae pollination from a pollen monitoring station located in the eastern suburb of Perugia and meteorological correlations are reported. The data refers to the year 1989. Grass pollen peak pollination was from May to July; in this period the influence of relative humidity and of temperature on pollen concentration was very high. Phenological observations, to identify the time of maximum stamen extension in the most common genera in the area, are also reported. During the period of investigation the counts of pollen grains over four-hour periods showed a regular diurnal rhythm with peaks of concentration in the four-hour period 16.00–20.00. Aerosporological data and meteorological data related to four-hour periods were correlated following different criteria.  相似文献   
44.
45.
Zooplankton data collected during September 1995 in the NorthWest Atlantic at 4139'N, 4958'W (the location of the siteof the ‘Titanic’ wreck) were analysed. The regioninvestigated was characterized by a very sharp frontal zonebetween the Gulf Stream and the main stream of the LabradorCurrent. The total plankton biomass in the water column wasvery high. The macroplankton biomass values below the 600 mlayer were significantly higher as compared with the similarvalues measured before in other productive boreal regions ofthe Atlantic and Pacific oceans. A lot of dead mesoplanktonanimals occurred in the deep layers. The reason was that thecold-water mesoplankton advected by the Labrador Current diedoff intensively within the deep layers of the frontal zone andwere used as a food resource by the macroplankton carnivoresand scavengers that were very abundant there.  相似文献   
46.
Reconstruction of bony structures of the face is always a problem as big as the defect and the function that must be replaced. Everything from simple grafts to sophisticated distant bone flaps has been used. Based on the studies of Cutting et al., Psillakis et al., and Casanova et al., we have developed the full-thickness galeoparietal bone flap, initially for mandibular reconstruction, but of great use for all maxillofacial reconstructions. From July of 1987 to December of 1988, 14 patients have been operated on. The experience with this flap is shown in four patients as follows: primary reconstruction of a mandible as a result of ameloblastoma, secondary reconstruction of a mandible with associated old fractures and malalignment of segments, bilateral malar reconstruction in a patient with Treacher Collins syndrome, and severe sequelae of an already treated Romberg case. Small variations could be made to best accommodate the technique used to the defect we were treating. Some technical details, the experience, the results, and possible sequelae or complications are also discussed.  相似文献   
47.
48.
49.
50.
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