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   1432752篇
  免费   137917篇
  国内免费   1387篇
  2021年   16376篇
  2019年   16234篇
  2018年   17902篇
  2017年   16619篇
  2016年   28379篇
  2015年   42815篇
  2014年   50888篇
  2013年   77207篇
  2012年   39634篇
  2011年   28379篇
  2010年   44229篇
  2009年   45547篇
  2008年   26804篇
  2007年   25144篇
  2006年   29934篇
  2005年   30876篇
  2004年   30075篇
  2003年   27618篇
  2002年   25474篇
  2001年   38338篇
  2000年   36071篇
  1999年   34345篇
  1998年   24714篇
  1997年   24689篇
  1996年   24084篇
  1995年   23622篇
  1994年   23461篇
  1993年   22659篇
  1992年   30810篇
  1991年   29488篇
  1990年   28100篇
  1989年   28517篇
  1988年   26435篇
  1987年   25180篇
  1986年   24019篇
  1985年   25875篇
  1984年   25010篇
  1983年   22124篇
  1982年   21923篇
  1981年   21170篇
  1980年   19700篇
  1979年   21696篇
  1978年   19753篇
  1977年   18823篇
  1976年   17926篇
  1975年   17801篇
  1974年   18231篇
  1973年   18518篇
  1972年   16083篇
  1971年   14481篇
排序方式: 共有10000条查询结果,搜索用时 250 毫秒
991.
992.
993.
994.
The NHS reforms have come to mean all things to all men (and women). Identifying a market oriented purchaser-provider split as the conceptual heart of the reforms is still, however, useful. There are important perverse incentives in and around the NHS that are associated with the reforms; furthermore, many reactions to the resulting problems are paradoxical and often counterproductive. Hitherto most criticism of the reforms from the health policy and management community (as opposed to the professions and the public) has been tactical rather than fundamental. There are serious problems for the NHS associated both with the NHS market and with current, often tacit, strategies for the future of the service.  相似文献   
995.
996.
Movements of radio-tagged Common Buzzards Buteo buteo in their first year   总被引:1,自引:1,他引:0  
S. S. WALLS  R. E. KENWARD 《Ibis》1995,137(2):177-182
The movements of 96 radio-tagged juvenile Common Buzzards Buteo buteo were studied during 3 years in Dorset, southern England. Contrary to previous studies which implied that Common Buzzards are territorial, we found that 39% foraged within 1 km of the nest during their first winter, presumably within their parents' home range. Most (72%) of those that did not disperse were recorded making brief excursions during August and September, before opting for a "stay-at-home" strategy. There was no significant difference between the sexes in the tendency to make excursions or disperse, but females dispersed further than males. Buzzards which dispersed early tended to settle significantly farther from their nests than did late dispersers. Long-term radio-tracking has shown that 72% of the dispersing birds returned towards their natal area in the following breeding season. Such philopatry may be an important hindrance to avian recolonization following local extinctions.  相似文献   
997.
998.
A new failure model is introduced in the form of a four-parameter nonlinear differential equation, with failure probability as the dependent variable and failure time as the independent variable. The first parameter characterizes the location, the second the scale, and the other two the shape of the model. The type of the accompanying hazard function is immediately read off the shape parameters. The new model approximates the classical failure models with rather high precision, but also models cases where the failure density is skewed to the left. It can be used to analyze survival data objectively, based on the shape of the failure distribution. The computation of quantiles and moments is easy and fast. Nonlinear regression methods are used to estimate parameter values.  相似文献   
999.
Our goal is to examine the synthesis and deposition of corneal glycosaminoglycans (GAGs) in response to a wound created by the insertion of porous discs into stromal interlamellar pockets. The disc and the surrounding stromal tissue were assayed and compared to contralateral control stroma and to sham operated corneas at 14,42, and 84 days. The tissue and/or discs were removed and labeled with 35S-sulfate for 18 h; GAGs were extracted with 4 M guanidine–HCl. Extracts were chromatographed on Q-Sepharose columns, bound proteoglycans were eluted with a linear salt gradient, and radioactive fractions were analyzed. Total GAG content was determined colorimetrically, using dimethylmethylene blue. Specific GAGs were determined using enzymatic digestion with selective polysaccharide lyases and protein cores were examined using SDS–PAGE. The nonbound fractions from the chromatography were assayed for TGF-β using Western blot analysis and for hyaluronic acid using an 125I-radiometric assay. Specific GAGs were localized 42 days after the disc had been implanted in the stroma. The placement of the discs into the stroma resulted in a decrease in the total amount of GAG. However, the ratio of dermatan-chondroitin sulfate and heparan sulfate to keratan sulfate increased in the surrounding tissue and disc. Hyaluronic acid was elevated at day 14 in the surrounding tissue, and not until day 84 in the disc. Western blot analysis of surrounding tissue extracts revealed forms of TGF-β that migrated with an apparent molecular mass of 63 and 43 kDa. The results indicate that the insertion of discs into interlamellar pockets causes changes in the sulfation and proportion of the glycosaminoglycans in the surrounding tissue and the disc. These changes are coincident with the appearance of TGF-β. After 84 days, the population of glycosaminoglycans in the disc begins to resemble the surrounding stroma. This model will allow us to examine further the synthesis and deposition of proteins following an extensive wound in which cells must migrate to the wound site and then undergo extensive remodeling. © 1995 Wiley-Liss, Inc.  相似文献   
1000.
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