全文获取类型
收费全文 | 2743篇 |
免费 | 236篇 |
出版年
2022年 | 18篇 |
2021年 | 36篇 |
2020年 | 18篇 |
2019年 | 22篇 |
2018年 | 30篇 |
2017年 | 23篇 |
2016年 | 48篇 |
2015年 | 101篇 |
2014年 | 97篇 |
2013年 | 148篇 |
2012年 | 159篇 |
2011年 | 146篇 |
2010年 | 93篇 |
2009年 | 77篇 |
2008年 | 122篇 |
2007年 | 114篇 |
2006年 | 106篇 |
2005年 | 112篇 |
2004年 | 105篇 |
2003年 | 90篇 |
2002年 | 101篇 |
2001年 | 86篇 |
2000年 | 78篇 |
1999年 | 82篇 |
1998年 | 45篇 |
1997年 | 36篇 |
1996年 | 28篇 |
1995年 | 31篇 |
1994年 | 26篇 |
1993年 | 25篇 |
1992年 | 59篇 |
1991年 | 55篇 |
1990年 | 54篇 |
1989年 | 52篇 |
1988年 | 57篇 |
1987年 | 35篇 |
1986年 | 33篇 |
1985年 | 48篇 |
1984年 | 24篇 |
1983年 | 41篇 |
1982年 | 27篇 |
1981年 | 29篇 |
1980年 | 18篇 |
1979年 | 22篇 |
1978年 | 21篇 |
1977年 | 23篇 |
1975年 | 16篇 |
1974年 | 19篇 |
1973年 | 19篇 |
1972年 | 19篇 |
排序方式: 共有2979条查询结果,搜索用时 15 毫秒
141.
Cecilia U. D. Stenfors Linda Magnusson Hanson Gabriel Oxenstierna T?res Theorell Lars-G?ran Nilsson 《PloS one》2013,8(4)
Background
Cognitive complaints involving problems with concentration, memory, decision-making and thinking are relatively common in the work force. The sensitivity of both subjective and objective cognitive functioning to common psychiatric conditions, stress levels and to cognitive load makes it plausible that psychosocial working conditions play a role in cognitive complaints. Thus, this study aimed to test the associations between psychosocial work factors and cognitive complaints in nationally representative samples of the Swedish work force. Cross-sectional (n = 9751) and prospective (n = 3644; two time points two years apart) sequential multiple regression analyses were run, adjusting for general confounders, depressive- and sleeping problems. Additional prospective analyses were run adjusting for baseline cognitive complaints.Cross-sectional results
High quantitative demands, information and communication technology (ICT) demands, underqualification and conflicts were positively associated with cognitive complaints, while social support, good resources at work and overqualification were negatively associated with cognitive complaints in all models. Skill discretion and decision authority were weakly associated with cognitive complaints. Conflicts were more strongly associated with cognitive complaints in women than in men, after adjustment for general confounders.Prospective results
Quantitative job demands, ICT demands and underqualification were positively associated with future cognitive complaints in all models, including when adjusted for baseline cognitive complaints. Decision authority was weakly positively associated with future cognitive complaints, only after adjustment for depressive- and sleeping problems respectively. Social support was negatively associated with future cognitive complaints after adjustment for general confounders and baseline cognitive complaints. Skill discretion and resources were negatively associated with future cognitive complaints after adjustment for general confounders. The associations between quantitative demands and future cognitive complaints were stronger in women.Discussion/Conclusions
The findings indicate that psychosocial working conditions should be taken into account when considering cognitive complaints among employees. 相似文献142.
143.
Christian Ehrnthaller Markus Huber-Lang Per Nilsson Ronny Bindl Simon Redeker Stefan Recknagel Anna Rapp Tom Mollnes Michael Amling Florian Gebhard Anita Ignatius 《PloS one》2013,8(11)
There is increasing evidence that complement may play a role in bone development. Our previous studies demonstrated that the key complement receptor C5aR was strongly expressed in the fracture callus not only by immune cells but also by bone cells and chondroblasts, indicating a function in bone repair. To further elucidate the role of complement in bone healing, this study investigated fracture healing in mice in the absence of the key complement molecules C3 and C5. C3-/- and C5-/- as well as the corresponding wildtype mice received a standardized femur osteotomy, which was stabilized using an external fixator. Fracture healing was investigated after 7 and 21 days using histological, micro-computed tomography and biomechanical measurements. In the early phase of fracture healing, reduced callus area (C3-/-: -25%, p=0.02; C5-/-: -20% p=0.052) and newly formed bone (C3-/-: -38%, p=0.01; C5-/-: -52%, p=0.009) was found in both C3- and C5-deficient mice. After 21 days, healing was successful in the absence of C3, whereas in C5-deficient mice fracture repair was significantly reduced, which was confirmed by a reduced bending stiffness (-45%; p=0.029) and a smaller callus volume (-17%; p=0.039). We further demonstrated that C5a was activated in C3-/- mice, suggesting cleavage via extrinsic pathways. Our results suggest that the activation of the terminal complement cascade in particular may be crucial for successful fracture healing. 相似文献
144.
145.
Abstract Single stranded RNA molecules can assume a wide range of tertiary structures beyond the canonical A-form double helix. Certain sequences, termed motifs, are more common than a random distribution would suggest. The existence of such motifs can be rationalized in structural terms. In this study, we have investigated the intrinsic structural stability of RNA terminal loop motifs using multiple MD simulations in explicit water. Representative loops were chosen from the major tetraloop motifs, including also the U-turn motif. Not all loops retain their folded starting structure, but lowering the temperature to 277 K, or adding adjacent base pairs from the stem to which the motif is attached, helps stabilizing the folded loop structure. 相似文献
146.
147.
A fourth species of the Holarctic Ilybius angustior complex, Ilybius minakawai n. sp., is described from the Island of Sakhalin in the Russian far east. Male genitalia are diagnostic within this species complex, although from body size and shape the new species cannot be separated from I. churchillensis Wallis from north Alaska and the Canadian low arctic. The small and narrow body characteristic of these two species represents an extreme of the cold climate form of the widespread I. angustior (Gyllenhal). Low variation within the studied large ribosomal and cytochrome c oxidase subunit I mitochondrial genes suggest that speciation events within the I. angustior-complex are fairly recent. In combination with known geographical ranges, the low genetic variation within this species complex suggest speciation within Pleistocene refugia including Beringia. 相似文献
148.
Vlad Dincă Mårten Runquist Mårten Nilsson Roger Vila 《Biological journal of the Linnean Society. Linnean Society of London》2013,109(4):817-829
Polyommatus ripartii is a biogeographically and taxonomically poorly understood species of butterfly with a scattered distribution in Europe. Recently, it has been shown that this species includes several European endemic and localized taxa (galloi, exuberans, agenjoi) that were previously considered species and even protected, a result that poses further questions about the processes that led to its current distribution. We analysed mitochondrial DNA and the morphology of P. ripartii specimens to study the phylogeography of European populations. Three genetically differentiated but apparently synmorphic lineages occur in Europe that could be considered evolutionarily significant units for conservation. Their strongly fragmented and counterintuitive distribution seems to be the result of multiple range expansions and contractions along Pleistocene climatic oscillations. Remarkably, based on the 79 specimens studied, these genetic lineages do not seem to extensively coexist in the distributional mosaic, a phenomenon most evident in the Iberian Peninsula. One of the important gaps in the European distribution of P. ripartii is reduced by the discovery of new Croatian populations, which also facilitate a better understanding of the biogeography of the species. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 109 , 817‐829. 相似文献
149.
150.
Records of air-borne pollen and spores were made during a six month period in the Stockholm area in 1972. This paper presents phenological data and volumetric measurements obtained by means of a Burkard trap. Attention was paid to both arboreal pollen and non-arboreal pollen and spores, identified to family, genus or species level. Spores of certain common Pteridophytes were also included in the investigation, but not Bryophytes or fungal spores. 相似文献