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   190515篇
  免费   6509篇
  国内免费   822篇
  2023年   599篇
  2022年   715篇
  2021年   1537篇
  2020年   1275篇
  2019年   1434篇
  2018年   13798篇
  2017年   12392篇
  2016年   10385篇
  2015年   4984篇
  2014年   4919篇
  2013年   6463篇
  2012年   10993篇
  2011年   18447篇
  2010年   15519篇
  2009年   11005篇
  2008年   14155篇
  2007年   15466篇
  2006年   4863篇
  2005年   4341篇
  2004年   4810篇
  2003年   4487篇
  2002年   3951篇
  2001年   2709篇
  2000年   2462篇
  1999年   1839篇
  1998年   905篇
  1997年   665篇
  1996年   691篇
  1995年   628篇
  1994年   542篇
  1992年   1126篇
  1991年   1031篇
  1990年   974篇
  1989年   1030篇
  1988年   855篇
  1987年   887篇
  1986年   797篇
  1985年   836篇
  1984年   738篇
  1983年   630篇
  1979年   728篇
  1978年   538篇
  1975年   629篇
  1974年   692篇
  1973年   642篇
  1972年   826篇
  1971年   804篇
  1970年   576篇
  1969年   649篇
  1968年   603篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
Analogs of deamino-oxytocin and deamino-oxypressin containing a CH2-NH group instead of an amide bond between positions 8 and 9 were synthesized. All tested compounds exhibit significantly lowered biological activities.  相似文献   
102.

Background  

Radial chromosome positioning in interphase nuclei is nonrandom and can alter according to developmental, differentiation, proliferation, or disease status. However, it is not yet clear when and how chromosome repositioning is elicited.  相似文献   
103.

Background  

In heterogeneous environments, sex-biased dispersal could lead to environmental adaptive parental effects, with offspring selected to perform in the same way as the parent dispersing least, because this parent is more likely to be locally adapted. We investigate this hypothesis by simulating varying levels of sex-biased dispersal in a patchy environment. The relative advantage of a strategy involving pure maternal (or paternal) inheritance is then compared with a strategy involving classical biparental inheritance in plants and in animals.  相似文献   
104.
105.
TNF inhibitors are currently considered both effective and cost-effective in patients with active rheumatoid arthritis (RA), particularly in patients who have not responded fully to methotrexate. There is substantial doubt about the cost-effectiveness of TNF inhibitors as initial treatment for active RA. New data from the National Data Bank for Rheumatic Diseases now question the current consensus in methotrexate failures. The data suggest that in routine clinical practice TNF inhibitors provide only modest incremental benefits over best conventional therapy. If confirmed, these observational studies suggest that the economic argument underpinning the widespread use of TNF inhibitors in established RA is unsustainable.  相似文献   
106.

Background  

DNA Microarrays have become the standard method for large scale analyses of gene expression and epigenomics. The increasing complexity and inherent noisiness of the generated data makes visual data exploration ever more important. Fast deployment of new methods as well as a combination of predefined, easy to apply methods with programmer's access to the data are important requirements for any analysis framework. Mayday is an open source platform with emphasis on visual data exploration and analysis. Many built-in methods for clustering, machine learning and classification are provided for dissecting complex datasets. Plugins can easily be written to extend Mayday's functionality in a large number of ways. As Java program, Mayday is platform-independent and can be used as Java WebStart application without any installation. Mayday can import data from several file formats, database connectivity is included for efficient data organization. Numerous interactive visualization tools, including box plots, profile plots, principal component plots and a heatmap are available, can be enhanced with metadata and exported as publication quality vector files.  相似文献   
107.
108.
109.
The in vitro determination of the effects of some pharmacological agents on the outer diameter and axial tension of human Fallopian tube ampullar segments showed that some drugs may act on the circular and longitudinal smooth muscle layers relatively separately. PGF2 alpha elicited an excitatory response in both muscle layers while norepinephrine and isoproterenol decreased the activity. Oxytocin and indomethacin had relatively limited effects on the spontaneous motility of the ampullar segments. The calcium antagonist verapamil inhibited the spontaneous periodic activity.  相似文献   
110.
Copper is an essential trace element that is extremely toxic to organisms and organs at high doses. We have investigated the histological and biochemical effects of a toxic dose of copper sulfate on the liver of term Ross broiler chicks. Fertilized eggs were divided into three groups: experimental, injected with 50 mcg/0.1 ml copper sulfate in the air chambers on day 1; sham, injected with 0.1 ml saline; and control, no injection. Term chicks were killed and their livers investigated histologically, with hematoxylin–eosin-stained sections examined under light microscopy, and biochemically, for malondialdehyde and glutathione levels. Histological examinations showed copper-treated samples with granular degeneration and necrosis of hepatocytes and impairment to the cell lining of the remark cords. The samples had a congestive appearance, with blood in the vena centralis and sinusoids, slight connective tissue increase, and lymphocyte infiltration. Control and sham group sections had normal appearances. As oxidative damage parameters, in the copper-treated group, malondialdehyde levels were increased and glutathione levels decreased. In the sham and control groups, there were no significant differences. At this toxic dose, copper sulfate shows oxidative damage according to the histology of term chick liver that are confirmed biochemically by the changes in malondialdehyde and glutathione levels.  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