全文获取类型
收费全文 | 1521篇 |
免费 | 101篇 |
出版年
2023年 | 9篇 |
2022年 | 17篇 |
2021年 | 23篇 |
2020年 | 16篇 |
2019年 | 33篇 |
2018年 | 35篇 |
2017年 | 36篇 |
2016年 | 45篇 |
2015年 | 53篇 |
2014年 | 62篇 |
2013年 | 87篇 |
2012年 | 109篇 |
2011年 | 104篇 |
2010年 | 78篇 |
2009年 | 42篇 |
2008年 | 71篇 |
2007年 | 65篇 |
2006年 | 74篇 |
2005年 | 65篇 |
2004年 | 54篇 |
2003年 | 57篇 |
2002年 | 67篇 |
2001年 | 23篇 |
2000年 | 9篇 |
1999年 | 14篇 |
1998年 | 18篇 |
1997年 | 14篇 |
1996年 | 22篇 |
1995年 | 9篇 |
1994年 | 15篇 |
1993年 | 14篇 |
1992年 | 11篇 |
1991年 | 13篇 |
1990年 | 16篇 |
1989年 | 14篇 |
1988年 | 11篇 |
1987年 | 12篇 |
1984年 | 12篇 |
1983年 | 9篇 |
1982年 | 23篇 |
1981年 | 14篇 |
1980年 | 7篇 |
1979年 | 8篇 |
1978年 | 8篇 |
1977年 | 7篇 |
1976年 | 8篇 |
1975年 | 11篇 |
1974年 | 12篇 |
1973年 | 15篇 |
1971年 | 7篇 |
排序方式: 共有1622条查询结果,搜索用时 31 毫秒
131.
132.
We studied the response of the barn owl annual productivity to the common vole population numbers and variability to test the effects of environmental stochasticity on their life histories. Current theory predicts that temporal environmental variability can affect long-term nonlinear responses (e.g., production of young) both positively and negatively, depending on the shape of the relationship between the response and environmental variables. At the level of the Czech Republic, we examined the shape of the relationship between the annual sum of fledglings (annual productivity) and vole numbers in both non-detrended and detrended data. At the districts’ level, we explored whether the degree of synchrony (measured by the correlation coefficient) and the strength of the productivity response increase (measured by the regression coefficient) in areas with higher vole population variability measured by the s-index. We found that the owls’ annual productivity increased linearly with vole numbers in the Czech Republic. Furthermore, based on district data, we also found that synchrony between dynamics in owls’ reproductive output and vole numbers increased with vole population variability. However, the strength of the response was not affected by the vole population variability. Additionally, we have shown that detrending remarkably increases the Taylor’s exponent b relating variance to mean in vole time series, thereby reversing the relationship between the coefficient of variation and the mean. This shift was not responsible for the increased synchrony with vole population variability. Instead, we suggest that higher synchrony could result from high food specialization of owls on the common vole in areas with highly fluctuating vole populations. 相似文献
133.
Julia C. McKay Michael Corbacio Karel Tyml Frank S. Prato Alex W. Thomas 《Bioelectromagnetics》2010,31(1):64-76
A 225 µT, extremely low frequency, pulsed electromagnetic field (PEMF) that was designed for the induction of antinociception, was tested for its effectiveness to influence blood flow within the skeletal microvasculature of a male Sprague–Dawley rat model (n = 103). Acetylcholine (0.1, 1.0, or 10 mM) was used to perturb normal blood flow and to delineate differential effects of the PEMF, based on degree of vessel dilation. After both 30 and 60 min of PEMF exposure, we report no effects on peak perfusion response to acetylcholine (with only 0.2% of the group difference attributed to exposure). Spectral analysis of blood flow data was generated to obtain information related to myogenic activity (0.15–0.40 Hz), respiratory rate (0.4–2.0 Hz), and heart rate (2.0–7.0 Hz), including the peak frequency within each of the three frequency regions identified above, peak power, full width at half maximum (FWHM), and mean within band. No significant effects due to exposure were observed on myogenic activity of examined blood vessels, or on heart rate parameters. Anesthesia‐induced respiratory depression was, however, significantly reduced following PEMF exposure compared to shams (although exposure only accounted for 9.4% of the group difference). This set of data suggest that there are no significant acute physiological effects of 225 µT PEMF after 30 and 60 min of exposure on peak blood flow, heart rate, and myogenic activity, but perhaps a small attenuation effect on anesthetic‐induced respiratory depression. Bioelectromagnetics 31:64–76, 2010. © 2009 Wiley‐Liss, Inc. 相似文献
134.
Josef Patzak Vladimír NesvadbaAlena Henychová Karel Krofta 《Biochemical Systematics and Ecology》2010
The goal of this study was to characterize European wild hops (Humulus lupulus L.) by chemical and molecular genetic analyses in comparison to cultivated hops and North American wild hops. The contents of alpha and beta and acids varied from 0.45% to 5.55% and from 1.22% to 5.73% in European wild hops, respectively. Low bitter acid contents, alpha/beta acid ratios of lower than 1.0 and cohumulone content not exceeding 30% were typical as well as for traditional European cultivars. The lower myrcene content, the presence of farnesene and high selinene content were typical for European wild hops. We evaluated molecular genetic diversity in European wild hops by microsatellite and gene-specific markers and found that this variability did not correlate with the chemical characteristics. Our phylogenetic analysis confirmed overlapping variability and close genetic relationships in Europe, the separation of wild hops from the Caucasus region and the high diversity of North American wild hops. 相似文献
135.
136.
Andrea Staršíchová Eva Lincová Zuzana Pernicová Alois Kozubík Karel Souček 《Cellular signalling》2010,22(11):1734-1744
Chronic inflammation plays an important role in the initiation and progression of various human diseases including benign prostatic hyperplasia or prostate cancer. Here we show that the proinflammatory cytokine interleukin-6 (IL-6) has prosurvival effects and chronically activates the Jak2/STAT3 signalling pathway in a model of benign prostatic hyperplasia (BPH-1). We demonstrate that the antiinflammatory cytokine transforming growth factor-β1 (TGF-β1), which also permanently activates its canonical signalling pathway through SMAD proteins in BPH-1 cells, modifies the effects of IL-6 on cell proliferation. Importantly, TGF-β1 inhibits IL-6 signal transduction by decreasing the phosphorylation levels of STAT3. This effect is associated with decreased expression of Jak2 at both mRNA and protein levels. Moreover, we showed that TGF-β1 inhibits IL-6-induced expression of the cancer-associated gene MUC1. These observations demonstrated a novel interaction between TGF-β1 and IL-6 signalling and suggested another mechanism of how defects in TGF-β signalling, frequently associated with prostate pathologies, can contribute to the disruption of tissue homeostasis. 相似文献
137.
It has been proposed that some specific locations of bird's nests have higher intrinsic chances of being depredated than other locations. This predicts that fates of consecutive nesting attempts at the same site should be repeatable. We used 20 pairs of old thrush nests to simulate repeated nesting attempts at the same sites, both within and between breeding seasons (n=40 sites×2 trials×2 years=160). Each nest was monitored by a camera to record multiple predation events and to identify predators. Predation by all predator species was repeatable during a 15-day trial. Predation by principal predators (jay Garrulus glandarius , marten Martes martes / foina ) and total predation (all species combined) was not correlated within pairs of simultaneously exposed nests or within samples of nests from particular study plot, and not repeatable for individual nests between-trials or between-years. These findings suggest short-term effect of predator memory causing revisitation of previously depredated nests during a current nesting trial (all predators); do not support an effect of nest site features on multiple nest discoveries and/or an effect of nest location on repeated random encounters with the same nest (principal predators). Long-term repeatability and correlation within pairs of simultaneously exposed nests was detectable only in occasional predators (great spotted woodpecker Dendrocopos major , possibly also squirrel Sciurus vulgaris ), which suggests effect of nest location combined with site fidelity and individual foraging specialization of these predators. We conclude that repeatability of nest predation depends on the time scale considered and the local predator community. We caution against spurious findings of repeatable nest predation resulting simply from statistical properties of correlation in binary data (nest fates). 相似文献
138.
139.
Melek Güler-Yüksel Naomi B Klarenbeek Yvonne PM Goekoop-Ruiterman Jeska K de Vries-Bouwstra Sjoerd M van der Kooij Andreas H Gerards H Karel Ronday Tom WJ Huizinga Ben AC Dijkmans Cornelia F Allaart Willem F Lems 《Arthritis research & therapy》2010,12(3):R96
Introduction
To investigate whether accelerated hand bone mineral density (BMD) loss is associated with progressive joint damage in hands and feet in the first year of rheumatoid arthritis (RA) and whether it is an independent predictor of subsequent progressive total joint damage after 4 years.Methods
In 256 recent-onset RA patients, baseline and 1-year hand BMD was measured in metacarpals 2-4 by digital X-ray radiogrammetry. Joint damage in hands and feet were scored in random order according to the Sharp-van der Heijde method at baseline and yearly up to 4 years.Results
68% of the patients had accelerated hand BMD loss (>-0.003 g/cm2) in the first year of RA. Hand BMD loss was associated with progressive joint damage after 1 year both in hands and feet with odds ratios (OR) (95% confidence intervals [CI]) of 5.3 (1.3-20.9) and 3.1 (1.0-9.7). In univariate analysis, hand BMD loss in the first year was a predictor of subsequent progressive total joint damage after 4 years with an OR (95% CI) of 3.1 (1.3-7.6). Multivariate analysis showed that only progressive joint damage in the first year and anti-citrullinated protein antibody positivity were independent predictors of long-term progressive joint damage.Conclusions
In the first year of RA, accelerated hand BMD loss is associated with progressive joint damage in both hands and feet. Hand BMD loss in the first year of recent-onset RA predicts subsequent progressive total joint damage, however not independent of progressive joint damage in the first year. 相似文献140.
Karel Královec Zbyněk Roček Pavla Žáková Vladimíra Mužáková 《Journal of morphology》2010,271(9):1078-1093
We use histological techniques and computer‐aided three‐dimensional reconstructions made from serial histological sections to describe the ontogeny of the ethmoidal endocranium of discoglossid frog Discoglossus pictus. We identify a pattern of development for the suprarostral cartilage that differs from previous findings and probably represents the ancestral anuran pattern. The nasal cartilages, including the inferior prenasal cartilage, are de novo adult structures. The only larva‐derived structures of the adult nasal capsules are the posterior aspects of the solum nasi and septum nasi. We also identify patterns of development for the ethmoid plate and postnasal wall that occur during early in ontogenesis. These patterns are associated with development events during metamorphic climax. The pattern of timing of chondrification of the anterior nasal cartilages more closely coincides with that of the neobatrachian species than that recorded for the pelobatid frog Spea. In addition, this study supports a sister taxon relationship between Discoglossus and Alytes. J. Morphol. 271:1078‐1093, 2010. © 2010 Wiley‐Liss, Inc. 相似文献