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   40101篇
  免费   4148篇
  国内免费   12篇
  2022年   288篇
  2021年   676篇
  2020年   487篇
  2019年   568篇
  2018年   704篇
  2017年   636篇
  2016年   1050篇
  2015年   1730篇
  2014年   1886篇
  2013年   2335篇
  2012年   2869篇
  2011年   2795篇
  2010年   1857篇
  2009年   1615篇
  2008年   2260篇
  2007年   2255篇
  2006年   2135篇
  2005年   1906篇
  2004年   1898篇
  2003年   1697篇
  2002年   1655篇
  2001年   658篇
  2000年   611篇
  1999年   624篇
  1998年   436篇
  1997年   353篇
  1996年   343篇
  1995年   351篇
  1994年   309篇
  1993年   306篇
  1992年   415篇
  1991年   356篇
  1990年   351篇
  1989年   368篇
  1988年   367篇
  1987年   339篇
  1986年   269篇
  1985年   298篇
  1984年   305篇
  1983年   273篇
  1982年   258篇
  1981年   232篇
  1980年   210篇
  1979年   217篇
  1978年   182篇
  1977年   186篇
  1976年   183篇
  1975年   199篇
  1974年   164篇
  1973年   169篇
排序方式: 共有10000条查询结果,搜索用时 140 毫秒
51.
52.
Since its commencement in 2001, a program to facilitate fish passage on a major stretch of Australia's longest river has installed eight structures, testing and modifying their design as they go. What are the results so far and what are the implications for future directions?  相似文献   
53.
54.
The subcellular localization of the ω-hydroxylase of Saccharomycopsis lipolytica was assessed by the analytical fractionation technique, originally described by de Duve C., Pressman, B.C., Gianetto, R., Wattiaux, R. and Appelmans, F., and hitherto little, if at all, applied to yeast. Protoplasts were separated in six fractions by differential centrifugation. Some of these fractions were further fractioned by density gradient centrifugation. The distribution of ω-hydroxylase and 15 other constituents chosen as possible markers of its subcellular membranes has been established. ω-Hydroxylase resulted in being bound to a membrane that containes also cytochrome P-450 and NADPH-cytochrome c reductase. This membrane clearly differs from five other subcellular entities. (1) Mitochondria were characterized by particulate malate dehydrogenase, particulate Antimycin A-insensitive NADH-cytochrome c reductase, oligomycin-sensitive and K+-stimulated ATPase pH 9. (2) Most if not all of the catalase and urate oxidase is peroxisomal. (3) Free ribosomes account for most RNA. (4) Nucleoside diphosphatase is for the first time reported in a yeast and appears to belong to an homogeneous population of small membranes. (5) The soluble compartment contains magnesium pyrophosphatase, alkaline phosphatase, 5′-nucleotidase and part of the NADH-cytochrome c reductase. Latent arylesterase and ATPase pH7 have an unspecific distribution. Alkaline phosphodiesterase I has not been detected.  相似文献   
55.
56.
The clinical and laboratory findings in seven children with Kawasaki disease are reviewed. Four of the patients had the more complicated course that has characterized the cases diagnosed in North America. This suggests that the benign forms are often mistaken for other febrile illnesses. The patients were two girls and five boys ranging in age from 4 months to 7 years; six were Caucasian and one was a North American Indian. Fever, redness of the oral mucosa, an erythematous or scarlatiniform rash and cervical adenopathy were seen in all; six patients had the characteristic fingertip desquamation and nonexudative conjunctivitis. Cardiac involvement occurred in four patients, two of whom had coronary artery aneurysm or thrombosis. Arthritis or arthralgia was seen in six patients, and aseptic meningitis occurred in four. Of the three patients with jaundice two underwent laparotomy and excision of a hydropic gallbladder; one of them died from Klebsiella pneumoniae sepsis and disseminated intravascular coagulopathy.  相似文献   
57.
58.
The camel tick,Hyalomma dromedarii, exhibited positive scototaxis in an arena, e.g. it oriented towards a black or grey target in front of a white background. The degree of the scototactic response varied with the size and the elevation of the target, with its luminance contrast, with its shape and with the speed by which the target was moved: (1) the response to stationary and moving targets increased with increasing target size; (2) presentation of the targets at an elevation of 11o–15o induced the highest response; (3) the response decreased with decreasing luminance contrast of the target; (4) targets with the shape of a disk, a triangle standing on a vertex, a vertical bar or a silhouette of a dromedary caused high responses; a low response was observed when the target was a horizontal bar and there was no response to a striped pattern; (5) the smaller the size of a disk, the faster it had to be moved to elicit an optimum response.The smallest disk which elicited a significant response appeared under a visual angle of 4.8o for a thick at the starting point. The smallest dromedary-shaped silhouette which elicited a significant response corresponded to the silhouette of a real dromedary at a distance of 18 m.  相似文献   
59.
60.
Exposure to interparental violence is associated with negative outcomes, such as depression, post-traumatic stress disorder and reduced cognitive abilities. However, little is known about the potential effects of witnessing domestic violence during childhood on gray matter volume (GMV) or cortical thickness. High-resolution 3.0 T volumetric scans (Siemens Trio Scanner) were obtained on 52 subjects (18–25 years) including 22 (6 males/16 females) with a history of visually witnessing episodes of domestic violence, and 30 (8 males/22 females) unexposed control subjects, with neither a current nor past DSM-IV Axis I or II disorder. Potential confounding effects of age, gender, level of parental verbal aggression, parental education, financial stress, full scale IQ, and total GMV, or average thickness were modeled using voxel based morphometry and FreeSurfer. Witnessing domestic violence subjects had a 6.1% GMV reduction in the right lingual gyrus (BA18) (P = 0.029, False Discovery Rate corrected peak level). Thickness in this region was also reduced, as was thickness in V2 bilaterally and left occipital pole. Theses regions were maximally sensitive to exposure to witnessing domestic violence between 11–13 years of age. Regional reductions in GMV and thickness were observed in both susceptible and resilient witnessing domestic violence subjects. Results in subjects witnessing domestic violence were similar to previously reported results in subjects with childhood sexual abuse, as the primary region affected was visual cortex. Brain regions that process and convey the adverse sensory input of the abuse may be specifically modified by this experience, particularly in subjects exposed to a single type of maltreatment. Exposure to multiple types of maltreatment is more commonly associated with morphological alterations in corticolimbic regions. These findings fit with preclinical studies showing that visual cortex is a highly plastic structure.  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