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   159篇
  免费   2篇
系统科学   2篇
理论与方法论   2篇
现状及发展   53篇
研究方法   34篇
综合类   61篇
自然研究   9篇
  2021年   1篇
  2020年   2篇
  2019年   2篇
  2018年   5篇
  2017年   2篇
  2016年   8篇
  2015年   4篇
  2014年   4篇
  2013年   15篇
  2012年   10篇
  2011年   20篇
  2010年   5篇
  2008年   11篇
  2007年   13篇
  2006年   7篇
  2005年   5篇
  2004年   6篇
  2003年   3篇
  2002年   10篇
  2001年   3篇
  2000年   4篇
  1999年   2篇
  1998年   1篇
  1997年   3篇
  1995年   1篇
  1994年   1篇
  1993年   1篇
  1990年   1篇
  1982年   2篇
  1980年   1篇
  1978年   2篇
  1973年   1篇
  1971年   3篇
  1970年   1篇
  1964年   1篇
排序方式: 共有161条查询结果,搜索用时 250 毫秒
51.
Navarrete A  van Schaik CP  Isler K 《Nature》2011,480(7375):91-93
The human brain stands out among mammals by being unusually large. The expensive-tissue hypothesis explains its evolution by proposing a trade-off between the size of the brain and that of the digestive tract, which is smaller than expected for a primate of our body size. Although this hypothesis is widely accepted, empirical support so far has been equivocal. Here we test it in a sample of 100 mammalian species, including 23 primates, by analysing brain size and organ mass data. We found that, controlling for fat-free body mass, brain size is not negatively correlated with the mass of the digestive tract or any other expensive organ, thus refuting the expensive-tissue hypothesis. Nonetheless, consistent with the existence of energy trade-offs with brain size, we find that the size of brains and adipose depots are negatively correlated in mammals, indicating that encephalization and fat storage are compensatory strategies to buffer against starvation. However, these two strategies can be combined if fat storage does not unduly hamper locomotor efficiency. We propose that human encephalization was made possible by a combination of stabilization of energy inputs and a redirection of energy from locomotion, growth and reproduction.  相似文献   
52.
53.
Newly generated olfactory receptor axons grow from the peripheral to the central nervous system aided by olfactory ensheathing cells (OECs). Thus, OEC transplantation has emerged as a promising therapy for spinal cord injuries and for other neural diseases. However, these cells do not present a uniform population, but instead a functionally heterogeneous population that exhibits a variety of responses including adhesion, repulsion, and crossover during cell–cell and cell–matrix interactions. Some studies report that the migratory properties of OECs are compromised by inhibitory molecules and potentiated by chemical gradients. Here, we demonstrated that rodent OECs express all the components of the Nogo receptor complex and that their migration is blocked by myelin. Next, we used cell tracking and traction force microscopy to analyze OEC migration and its mechanical properties over myelin. Our data relate the decrease of traction force of OEC with lower migratory capacity over myelin, which correlates with changes in the F-actin cytoskeleton and focal adhesion distribution. Lastly, OEC traction force and migratory capacity is enhanced after cell incubation with the Nogo receptor inhibitor NEP1-40.  相似文献   
54.
55.
In discussions on intestinal protection, the protective capacity of mucus has not been very much considered. The progress in the last years in understanding the molecular nature of mucins, the main building blocks of mucus, has, however, changed this. The intestinal enterocytes have their apical surfaces covered by transmembrane mucins and the whole intestinal surface is further covered by mucus, built around the gel-forming mucin MUC2. The mucus of the small intestine has only one layer, whereas the large intestine has a two-layered mucus where the inner, attached layer has a protective function for the intestine, as it is impermeable to the luminal bacteria.  相似文献   
56.
This study offers a detailed analysis of an episode of the popularization of astronomy which took place in Portugal, a peripheral country of Europe, and occurring in the early twentieth century. The episode was driven by the 28 May 1900 total solar eclipse which was seen on the Iberian Peninsula (Portugal and Spain). Instead of focusing on one of the ends of the popularization process, we analyze the circulation of knowledge among scientists and the public, contrast the aims of the various expeditions, professional and amateur, which took place on Portuguese soil, analyze their repercussions in the Portuguese astronomical landscape, and the different ways used by the Portuguese political elite and astronomical community to successfully appropriate this astronomical event to serve their varied agendas, political, social and scientific. In this episode of public enthusiasm for science, a central figure emerged in the network of the official commission, professional and amateur communities and the ‘general public’: Frederico Tomás Oom (1864–1930), an astronomer of the Lisbon Astronomical Observatory. This paper aims to illustrate the different layers of the circulation process, and at proving that the popularization of science was not a unidirectional process from scientists to lay people nor did it serve only a particular agenda, be it political, social or scientific.  相似文献   
57.
58.
利用二苯基苦基肼分析法发现,等离子处理后碳纤维表面上将产生2~6个/nm ̄2游离基.它们会在30h内转化成其它基团,再复转为酚羟基后慢慢消失.等离子处理的作用-方面可导致游离基或介稳基团的产生,另一方面又不断消除它们.提高处理功率或延长处理时间在一定程度上有利于基团的形成,但超过一定限度后反会适得其反.利用等离子处理产生的游离基可使碳纤维在参与乙烯基单体聚合时,产生接枝聚合分子链.  相似文献   
59.
The lakes of Titan   总被引:1,自引:0,他引:1  
The surface of Saturn's haze-shrouded moon Titan has long been proposed to have oceans or lakes, on the basis of the stability of liquid methane at the surface. Initial visible and radar imaging failed to find any evidence of an ocean, although abundant evidence was found that flowing liquids have existed on the surface. Here we provide definitive evidence for the presence of lakes on the surface of Titan, obtained during the Cassini Radar flyby of Titan on 22 July 2006 (T16). The radar imaging polewards of 70 degrees north shows more than 75 circular to irregular radar-dark patches, in a region where liquid methane and ethane are expected to be abundant and stable on the surface. The radar-dark patches are interpreted as lakes on the basis of their very low radar reflectivity and morphological similarities to lakes, including associated channels and location in topographic depressions. Some of the lakes do not completely fill the depressions in which they lie, and apparently dry depressions are present. We interpret this to indicate that lakes are present in a number of states, including partly dry and liquid-filled. These northern-hemisphere lakes constitute the strongest evidence yet that a condensable-liquid hydrological cycle is active in Titan's surface and atmosphere, in which the lakes are filled through rainfall and/or intersection with the subsurface 'liquid methane' table.  相似文献   
60.
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