全文获取类型
收费全文 | 100977篇 |
免费 | 3816篇 |
国内免费 | 2582篇 |
出版年
2022年 | 590篇 |
2021年 | 1003篇 |
2020年 | 857篇 |
2019年 | 949篇 |
2018年 | 1832篇 |
2017年 | 1763篇 |
2016年 | 3640篇 |
2015年 | 7410篇 |
2014年 | 7154篇 |
2013年 | 6870篇 |
2012年 | 6359篇 |
2011年 | 3446篇 |
2010年 | 3049篇 |
2009年 | 2886篇 |
2008年 | 1536篇 |
2007年 | 1358篇 |
2006年 | 1289篇 |
2005年 | 7243篇 |
2004年 | 5908篇 |
2003年 | 4077篇 |
2002年 | 1589篇 |
2001年 | 1402篇 |
2000年 | 568篇 |
1999年 | 1726篇 |
1998年 | 486篇 |
1992年 | 1983篇 |
1991年 | 2059篇 |
1990年 | 2120篇 |
1989年 | 2040篇 |
1988年 | 1982篇 |
1987年 | 1838篇 |
1986年 | 1641篇 |
1985年 | 1666篇 |
1984年 | 1102篇 |
1983年 | 848篇 |
1982年 | 482篇 |
1979年 | 1081篇 |
1978年 | 762篇 |
1977年 | 610篇 |
1976年 | 631篇 |
1975年 | 875篇 |
1974年 | 998篇 |
1973年 | 1007篇 |
1972年 | 956篇 |
1971年 | 928篇 |
1970年 | 820篇 |
1969年 | 830篇 |
1968年 | 734篇 |
1967年 | 752篇 |
1966年 | 585篇 |
排序方式: 共有10000条查询结果,搜索用时 343 毫秒
101.
Anna M. Kauppi Alicia Edin Ingrid Ziegler Paula M?lling Anders Sj?stedt ?sa Gylfe Kristoffer Str?lin Anders Johansson 《PloS one》2016,11(1)
A metabolomics approach for prediction of bacteremic sepsis in patients in the emergency room (ER) was investigated. In a prospective study, whole blood samples from 65 patients with bacteremic sepsis and 49 ER controls were compared. The blood samples were analyzed using gas chromatography coupled to time-of-flight mass spectrometry. Multivariate and logistic regression modeling using metabolites identified by chromatography or using conventional laboratory parameters and clinical scores of infection were employed. A predictive model of bacteremic sepsis with 107 metabolites was developed and validated. The number of metabolites was reduced stepwise until identifying a set of 6 predictive metabolites. A 6-metabolite predictive logistic regression model showed a sensitivity of 0.91(95% CI 0.69–0.99) and a specificity 0.84 (95% CI 0.58–0.94) with an AUC of 0.93 (95% CI 0.89–1.01). Myristic acid was the single most predictive metabolite, with a sensitivity of 1.00 (95% CI 0.85–1.00) and specificity of 0.95 (95% CI 0.74–0.99), and performed better than various combinations of conventional laboratory and clinical parameters. We found that a metabolomics approach for analysis of acute blood samples was useful for identification of patients with bacteremic sepsis. Metabolomics should be further evaluated as a new tool for infection diagnostics. 相似文献
102.
103.
104.
105.
106.
Valeria L. Martin-Albarracin Guillermo C. Amico Daniel Simberloff Martin A. Nu?ez 《PloS one》2015,10(11)
Introduction and naturalization of non-native species is one of the most important threats to global biodiversity. Birds have been widely introduced worldwide, but their impacts on populations, communities, and ecosystems have not received as much attention as those of other groups. This work is a global synthesis of the impact of nonnative birds on native ecosystems to determine (1) what groups, impacts, and locations have been best studied; (2) which taxonomic groups and which impacts have greatest effects on ecosystems, (3) how important are bird impacts at the community and ecosystem levels, and (4) what are the known benefits of nonnative birds to natural ecosystems. We conducted an extensive literature search that yielded 148 articles covering 39 species belonging to 18 families -18% of all known naturalized species. Studies were classified according to where they were conducted: Africa, Asia, Australasia, Europe, North America, South America, Islands of the Indian, of the Pacific, and of the Atlantic Ocean. Seven types of impact on native ecosystems were evaluated: competition, disease transmission, chemical, physical, or structural impact on ecosystem, grazing/ herbivory/ browsing, hybridization, predation, and interaction with other non-native species. Hybridization and disease transmission were the most important impacts, affecting the population and community levels. Ecosystem-level impacts, such as structural and chemical impacts were detected. Seven species were found to have positive impacts aside from negative ones. We provide suggestions for future studies focused on mechanisms of impact, regions, and understudied taxonomic groups. 相似文献
107.
Many animals exhibit different behaviors in different seasons. The photoperiod can have effects on migration, breeding, fur growth, and other processes. The cyclic growth of the fur and feathers of some species of mammals and birds, respectively, is stimulated by the photoperiod as a result of hormone-dependent regulation of the nervous system. To further examine this phenomenon, we evaluated the Arbas Cashmere goat (Capra hircus), a species that is often used in this type of research. The goats were exposed to an experimentally controlled short photoperiod to study the regulation of cyclic cashmere growth. Exposure to a short photoperiod extended the anagen phase of the Cashmere goat hair follicle to increase cashmere production. Assessments of tissue sections indicated that the short photoperiod significantly induced cashmere growth. This conclusion was supported by a comparison of the differences in gene expression between the short photoperiod and natural conditions using gene chip technology. Using the gene chip data, we identified genes that showed altered expression under the short photoperiod compared to natural conditions, and these genes were found to be involved in the biological processes of hair follicle growth, structural composition of the hair follicle, and the morphogenesis of the surrounding skin appendages. Knowledge about differences in the expression of these genes as well as their functions and periodic regulation patterns increases our understanding of Cashmere goat hair follicle growth. This study also provides preliminary data that may be useful for the development of an artificial method to improve cashmere production by controlling the light cycle, which has practical significance for livestock breeding. 相似文献
108.
109.
110.