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   1354270篇
  免费   127705篇
  国内免费   1418篇
  2021年   16742篇
  2019年   16185篇
  2018年   17792篇
  2017年   16503篇
  2016年   28124篇
  2015年   42445篇
  2014年   50494篇
  2013年   76771篇
  2012年   36954篇
  2011年   25578篇
  2010年   43617篇
  2009年   45055篇
  2008年   24247篇
  2007年   22156篇
  2006年   27378篇
  2005年   28428篇
  2004年   27380篇
  2003年   25241篇
  2002年   23283篇
  2001年   36013篇
  2000年   33226篇
  1999年   32088篇
  1998年   24428篇
  1997年   24406篇
  1996年   23775篇
  1995年   23325篇
  1994年   23106篇
  1993年   22353篇
  1992年   28703篇
  1991年   27303篇
  1990年   25544篇
  1989年   26290篇
  1988年   24110篇
  1987年   23005篇
  1986年   21857篇
  1985年   23725篇
  1984年   23301篇
  1983年   20638篇
  1982年   21082篇
  1981年   20087篇
  1980年   18843篇
  1979年   19451篇
  1978年   18280篇
  1977年   17485篇
  1976年   16623篇
  1975年   16091篇
  1974年   16480篇
  1973年   16759篇
  1972年   14190篇
  1971年   13050篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
993.
994.
995.
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   
996.
997.
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   
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