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   44492篇
  免费   4726篇
  国内免费   12篇
  2021年   559篇
  2020年   383篇
  2019年   471篇
  2018年   594篇
  2017年   561篇
  2016年   970篇
  2015年   1745篇
  2014年   1711篇
  2013年   2263篇
  2012年   2571篇
  2011年   2267篇
  2010年   1577篇
  2009年   1464篇
  2008年   1977篇
  2007年   1979篇
  2006年   1762篇
  2005年   1708篇
  2004年   1622篇
  2003年   1410篇
  2002年   1413篇
  2001年   1372篇
  2000年   1348篇
  1999年   1207篇
  1998年   666篇
  1997年   622篇
  1996年   605篇
  1995年   584篇
  1994年   538篇
  1993年   573篇
  1992年   1142篇
  1991年   857篇
  1990年   895篇
  1989年   857篇
  1988年   746篇
  1987年   674篇
  1986年   697篇
  1985年   778篇
  1984年   605篇
  1983年   472篇
  1982年   391篇
  1981年   345篇
  1980年   292篇
  1979年   427篇
  1978年   373篇
  1977年   281篇
  1976年   259篇
  1975年   235篇
  1974年   322篇
  1973年   286篇
  1972年   221篇
排序方式: 共有10000条查询结果,搜索用时 93 毫秒
101.
When sending text messages on their mobile phone to friends, children often use a special type of register, which is called textese. This register allows the omission of words and the use of textisms: instances of non-standard written language such as 4ever (forever). Previous studies have shown that textese has a positive effect on children’s literacy abilities. In addition, it is possible that children’s grammar system is affected by textese as well, as grammar rules are often transgressed in this register. Therefore, the main aim of this study was to investigate whether the use of textese influences children’s grammar performance, and whether this effect is specific to grammar or language in general. Additionally, studies have not yet investigated the influence of textese on children’s cognitive abilities. Consequently, the secondary aim of this study was to find out whether textese affects children’s executive functions. To investigate this, 55 children between 10 and 13 years old were tested on a receptive vocabulary and grammar performance (sentence repetition) task and various tasks measuring executive functioning. In addition, text messages were elicited and the number of omissions and textisms in children’s messages were calculated. Regression analyses showed that omissions were a significant predictor of children’s grammar performance after various other variables were controlled for: the more words children omitted in their text messages, the better their performance on the grammar task. Although textisms correlated (marginally) significantly with vocabulary, grammar and selective attention scores and omissions marginally significantly with vocabulary scores, no other significant effects were obtained for measures of textese in the regression analyses: neither for the language outcomes, nor for the executive function tasks. Hence, our results show that textese is positively related to children’s grammar performance. On the other hand, use of textese does not affect—positively nor negatively—children’s executive functions.  相似文献   
102.
103.
Weakly electric fish produce electric signals with a specialised organ in their tail. In addition, they are electrosensitive and can perceive their self-generated signals (for electrolocation) and electric signals of other electric fishes (for electrocommunication). Mormyrids possess three types of peripheral electroreceptor organs, one used for electrocommunication and two types involved in electolocation. They are innervated by afferent fibres, which project to different zones in the electrosensory lateral line lobe (ELL) in the medulla. Brain circuits for electrolocation and electrocommunication are separated almost throughout the whole brain. Electrolocation pathways run from the ELL-cortex to the torus semicircularis of the midbrain and then via the valvula cerebelli towards the telencephalon. Pathways involved in electrocommunication run from the nucleus of the ELL to another part of the torus and from there through the isthmic granule nucleus to the valvula. In addition, a pathway via the preglomerular complex to the telencephalon might exist. In both the electrolocation and the electrocommunication circuits, prominent recurrent pathways are present.  相似文献   
104.
105.
106.
107.
Differential hybridization analysis revealed two cDNA clones, pBUS19 and pBUS30, to be overexpressed in progressionally advanced rat prostatic tumors. Northern blot analysis suggested the clones to be related although no homology in nucleotide sequence could be shown. Isolation and characterization of a pBUS19-related clone, pJG116, and computer-assisted database comparison showed that all three clones could be mapped within a rat-specific endogenous retrovirus. The importance of overexpression of retroviral sequences in advanced prostatic cancer remains unclear.  相似文献   
108.
The Lipomycetaceae (Endomycetales) are known from the generaDipodascopsis, Lipomyces andZygozyma with budding anamorphic states inMyxozyma. The family is easily recognized culturally and physiologically but is phenotypically and ecologically extremely diverse. This natural taxon is phylogenetically distinct from the Saccharomycetaceae, but probably related to the Dipodascaceae. The possible evolution of the lipomycetaceous anamorphs is discussed.  相似文献   
109.
Control of antigenic variation in African trypanosomes   总被引:8,自引:0,他引:8  
  相似文献   
110.
Macrophages are essential for normal wound repair and many of their effects on healing wounds are likely to be mediated by the secretion of cytokines. This study examines the appearance of messenger RNA (mRNA) for cachectin/tumor necrosis factor (TNF), IL 1, and macrophage inflammatory proteins 1 and 2 (MIP-1 and MIP-2), as well as the mature peptides, in a model of wound healing using wound chambers. RNA for all four cytokines can be detected in wound inflammatory cells by polymerase chain reaction amplification throughout the first 7 days. Cachectin/TNF and IL 1 protein levels peaked on the first day after wound chamber implantation, and MIP-1 and MIP-2 were detected only on day 3. The data suggest that these cytokines participate in the early inflammatory response to wounding.  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