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   78380篇
  免费   6650篇
  国内免费   39篇
  2022年   501篇
  2021年   979篇
  2020年   583篇
  2019年   745篇
  2018年   954篇
  2017年   882篇
  2016年   1483篇
  2015年   2474篇
  2014年   2811篇
  2013年   3866篇
  2012年   4743篇
  2011年   4895篇
  2010年   3206篇
  2009年   2890篇
  2008年   4292篇
  2007年   4372篇
  2006年   4198篇
  2005年   4181篇
  2004年   4247篇
  2003年   3831篇
  2002年   3830篇
  2001年   891篇
  2000年   631篇
  1999年   915篇
  1998年   1204篇
  1997年   861篇
  1996年   794篇
  1995年   765篇
  1994年   748篇
  1993年   692篇
  1992年   636篇
  1991年   617篇
  1990年   589篇
  1989年   626篇
  1988年   539篇
  1987年   517篇
  1986年   466篇
  1985年   596篇
  1984年   749篇
  1983年   642篇
  1982年   761篇
  1981年   795篇
  1980年   723篇
  1979年   502篇
  1978年   565篇
  1977年   529篇
  1976年   530篇
  1975年   404篇
  1974年   500篇
  1973年   459篇
排序方式: 共有10000条查询结果,搜索用时 171 毫秒
41.
42.
43.
A recent literature review of commentaries and ‘state of the art’ articles from researchers in psychiatric genetics (PMG) offers a consensus about progress in the science of genetics, disappointments in the discovery of new and effective treatments, and a general optimism about the future of the field. I argue that optimism for the field of psychiatric molecular genetics (PMG) is overwrought, and consider progress in the field in reference to a sample estimate of US National Institute of Mental Health funding for this paradigm for the years 2008 and 2009. I conclude that the amounts of financial investment in PMG is questionable from an ethical perspective, given other research and clinical needs in the USA.  相似文献   
44.
45.
46.
47.
Practically all animals must find food while avoiding predators.An individual's perception of predation risk may depend on manyfactors, such as distance to refuge and group size, but it isunclear whether individuals respond to different factors ina similar manner. We tested whether flocks of foraging starlingsresponded in the same way to an increased perception of predationrisk by assessing three factors: (1) neighbor distances, (2)habitat obstruction, and (3) recent exposure to a predator.We found that in all three scenarios of increased risk, starlingsreduced their interscan intervals (food-searching bouts), whichincreased the frequency of their vigilance periods. We thenexamined how one of these factors, habitat obstruction, affectedescape speed by simulating an attack with a model predator.Starlings were slower to respond in visually obstructed habitats(long grass swards) and slower when they had their head downin obstructed habitats than when they had their head down inopen habitats. In addition, reaction times were quicker whenstarlings could employ their peripheral fields of vision. Ourresults demonstrate that different sources of increased riskcan generate similar behavioral responses within a species.The degree of visibility in the physical and social environmentaffects both the actual and perceived risk of predation.  相似文献   
48.
49.
John R. Polito 《CMAJ》2008,179(10):1037-1038
  相似文献   
50.
We previously reported that aged mice lacking complement factor H (CFH) exhibit visual defects and structural changes in the retina. However, it is not known whether this phenotype is age-related or is the consequence of disturbed development. To address this question we investigated the effect of Cfh gene deletion on the retinal phenotype of young and mid-age mice. Cfh −/− mouse eyes exhibited thickening of the retina and reduced nuclear density, but relatively normal scotopic and photopic electroretinograms. At 12 months there was evidence of subtle astroglial activation in the Cfh −/− eyes, and significant elevation of the complement regulator, decay-accelerating factor (DAF) in Müller cells. In the retinal pigment epithelium (RPE) of young control and Cfh −/− animals mitochondria and melanosomes were oriented basally and apically respectively, whereas the apical positioning of melanosomes was significantly perturbed in the mid-age Cfh −/− RPE. We conclude that deletion of Cfh in the mouse leads to defects in the retina that precede any marked loss of visual function, but which become progressively more marked as the animals age. These observations are consistent with a lifelong role for CFH in retinal homeostasis.  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