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   125780篇
  免费   3419篇
  国内免费   1181篇
  2023年   153篇
  2022年   437篇
  2021年   745篇
  2020年   527篇
  2019年   656篇
  2018年   12464篇
  2017年   11144篇
  2016年   8342篇
  2015年   2233篇
  2014年   2147篇
  2013年   2427篇
  2012年   6776篇
  2011年   15066篇
  2010年   13429篇
  2009年   9470篇
  2008年   11572篇
  2007年   12983篇
  2006年   1810篇
  2005年   1927篇
  2004年   2390篇
  2003年   2213篇
  2002年   1836篇
  2001年   995篇
  2000年   813篇
  1999年   597篇
  1998年   320篇
  1997年   290篇
  1996年   227篇
  1995年   234篇
  1994年   230篇
  1993年   236篇
  1992年   367篇
  1991年   371篇
  1990年   313篇
  1989年   320篇
  1988年   293篇
  1987年   266篇
  1986年   245篇
  1985年   236篇
  1984年   185篇
  1983年   173篇
  1982年   145篇
  1980年   140篇
  1979年   172篇
  1978年   160篇
  1976年   136篇
  1975年   145篇
  1974年   142篇
  1972年   347篇
  1971年   344篇
排序方式: 共有10000条查询结果,搜索用时 187 毫秒
91.
92.

Background  

Evolution of parasite traits is inextricably linked to their hosts. For instance one common definition of parasite virulence is the reduction in host fitness due to infection. Thus, traits of infection must be viewed in both protagonists and may be under shared genetic and physiological control. We investigated these questions on the oomycete Hyaloperonospora arabidopsis (= parasitica), a natural pathogen of the Brassicaceae Arabidopsis thaliana.  相似文献   
93.
Sexual selection has driven the evolution and elaboration of a wide variety of displays and ornaments in male nonhuman primates, including capes, cheek flanges, and sexual coloration. Among the most sexually dimorphic of all primates is the drill (Mandrillus leucophaeus), the males of which can be 3 times the mass of females, possess large canines, and exhibit extremely bright sexual skin coloration. However, the function of male coloration in this species has never been examined. Here, we present data on male color (measured objectively using digital photography), dominance rank, measures of male-female association, and key sexual behaviors, of adult male drills (n = 17) living in four semi free-ranging enclosures at the Drill Rehabilitation and Breeding Center in Nigeria. We test the hypothesis that male coloration is a badge of status, indicating dominance rank, and the hypothesis that male coloration attracts females. We found that male coloration did indicate rank, and that high ranking, strongly colored males were more likely to associate with adult females, and more specifically with fully tumescent females. These males also engaged in more sexual activity. However, measures of male-female association and sexual behaviors were not related to male color once rank had been taken into consideration; i.e., for males of a given rank, females did not prefer those that were more colorful. We discuss the results in light of what is known about the wild drill social system, in which unfamiliar individuals may come together in “supergroups,” and in which quality indicators of competitive ability may be particularly important.  相似文献   
94.
95.
96.

Backgound  

It has been reported that Toll-like receptor 4 (TLR4) deficiency reduces infarct size after myocardial ischemia/reperfusion (MI/R). However, measurement of MI/R injury was limited and did not include cardiac function. In a chronic closed-chest model we assessed whether cardiac function is preserved in TLR4-deficient mice (C3H/HeJ) following MI/R, and whether myocardial and systemic cytokine expression differed compared to wild type (WT).  相似文献   
97.
98.
99.
ADHD is a neurodevelopmental syndrome that often persists into adulthood. It is possible that different criteria are necessary for older adults than younger adults: the manifestations of ADHD could change with age; other conditions with onset in later life share presenting symptoms with ADHD; different contextual challenges and patterns of compensatory support may exist. For these reasons, we reviewed evidence for the validity of DSM ADHD criteria in adulthood for individuals over the age of 50. Specifically, we evaluated evidence that the DSM criteria for ADHD identify a valid syndrome in older adults based on clinical presentation, laboratory or testing findings, absence of alternate diagnosis to explain symptoms, course of the syndrome, or familial presence of the condition. We found evidence that various ADHD criteria identify subjects with clinical presentations similar to that seen in younger adults, but only 92 well-described cases have been reported in the literature. ADHD traits also may be less common in the general population of older adults than in younger adults, suggesting that the threshold for an atypical burden of ADHD traits may be lower in older populations. Future research can establish a richer basis for validity of diagnostic criteria for ADHD in older adults.  相似文献   
100.
The need for selection of the optimal material for the manufacturing of cardio-patches can be resolved by the use of cryostored autologous pericardial tissue. This short communication is a concise fragment of a large-scale research and demonstrates only the efficiency of cell culturing before and after pericardial preservation in the low temperature conditions.  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