全文获取类型
收费全文 | 8608篇 |
免费 | 880篇 |
国内免费 | 4篇 |
专业分类
9492篇 |
出版年
2023年 | 60篇 |
2022年 | 150篇 |
2021年 | 281篇 |
2020年 | 138篇 |
2019年 | 183篇 |
2018年 | 220篇 |
2017年 | 206篇 |
2016年 | 356篇 |
2015年 | 540篇 |
2014年 | 560篇 |
2013年 | 581篇 |
2012年 | 743篇 |
2011年 | 760篇 |
2010年 | 489篇 |
2009年 | 384篇 |
2008年 | 545篇 |
2007年 | 535篇 |
2006年 | 407篇 |
2005年 | 403篇 |
2004年 | 402篇 |
2003年 | 350篇 |
2002年 | 322篇 |
2001年 | 85篇 |
2000年 | 61篇 |
1999年 | 63篇 |
1998年 | 77篇 |
1997年 | 35篇 |
1996年 | 49篇 |
1995年 | 27篇 |
1994年 | 28篇 |
1993年 | 23篇 |
1992年 | 28篇 |
1991年 | 30篇 |
1990年 | 25篇 |
1989年 | 13篇 |
1988年 | 14篇 |
1987年 | 12篇 |
1986年 | 16篇 |
1985年 | 26篇 |
1984年 | 26篇 |
1983年 | 15篇 |
1982年 | 12篇 |
1981年 | 31篇 |
1980年 | 22篇 |
1979年 | 11篇 |
1978年 | 17篇 |
1976年 | 8篇 |
1972年 | 9篇 |
1971年 | 9篇 |
1966年 | 8篇 |
排序方式: 共有9492条查询结果,搜索用时 15 毫秒
21.
22.
This study examines the effect of various plasma proteins from man on the spontaneous contractile activity of the rat portal vein. Albumin, gamma-globulin, alpha-globulin, beta-globulin (the major plasma proteins), and immunoglobulin IgG (the major immunoglobulin present in the gamma-globulin fraction) were obtained commercially. Mesenteric portal vein strips were prepared from rats and placed in a physiological salt solution in muscle baths for the measurement of longitudinal mechanical response. Portal veins exposed to albumin or gamma-globulin showed a dose-dependent increase in the spontaneous activity, whereas those exposed to alpha-globulin or alpha- and beta-globulin together showed a dose-dependent inhibition of spontaneous activity. Immunoglobulin IgG produced a dose-dependent increase in the spontaneous activity similar to that of gamma-globulin. The increased spontaneous activity produced by albumin was not prevented by ouabain but was inhibited by phentolamine. Spontaneous contractile activity was stimulated by albumin in the chemically (6-hydroxydopamine) denervated portal vein. These findings indicate that albumin acts in a manner similar to noradrenaline. The increased spontaneous activity caused by gamma-globulin (IgG) was inhibited by ouabain or verapamil. The effect of IgG was not dependent on alpha-adrenergic, cholinergic, histaminergic, serotoninergic, or renin angiotensin systems nor was it affected by removal of the endothelium. These observations may have implications in the pathophysiology of essential hypertension. 相似文献
23.
24.
Jonathan A. Williams Carmen Huesa Mikael J. Turunen James A. Oo Oskars Radzins Wilf Gardner James F.C. Windmill Hanna Isaksson K. Elizabeth Tanner John S. Riddell Sylvie Coupaud 《Journal of musculoskeletal & neuronal interactions》2022,22(2):212
Objective:Characterise the spatiotemporal trabecular and cortical bone responses to complete spinal cord injury (SCI) in young rats.Methods:8-week-old male Wistar rats received T9-transection SCI and were euthanised 2-, 6-, 10- or 16-weeks post-surgery. Outcome measures were assessed using micro-computed tomography, mechanical testing, serum markers and Fourier-transform infrared spectroscopy.Results:The trabecular and cortical bone responses to SCI are site-specific. Metaphyseal trabecular BV/TV was 59% lower, characterised by fewer and thinner trabeculae at 2-weeks post-SCI, while epiphyseal BV/TV was 23% lower with maintained connectivity. At later-time points, metaphyseal BV/TV remained unchanged, while epiphyseal BV/TV increased. The total area of metaphyseal and mid-diaphyseal cortical bone were lower from 2-weeks and between 6- and 10-weeks post-SCI, respectively. This suggested that SCI-induced bone changes observed in the rat model were not solely attributable to bone loss, but also to suppressed bone growth. No tissue mineral density differences were observed at any time-point, suggesting that decreased whole-bone mechanical properties were primarily the result of changes to the spatial distribution of bone.Conclusion:Young SCI rat trabecular bone changes resemble those observed clinically in adult and paediatric SCI, while cortical bone changes resemble paediatric SCI only. 相似文献
25.
William J. Sutherland Rosalind Aveling Thomas M. Brooks Mick Clout Lynn V. Dicks Liz Fellman Erica Fleishman David W. Gibbons Brandon Keim Fiona Lickorish Kathryn A. Monk Diana Mortimer Lloyd S. Peck Jules Pretty Johan Rockström Jon Paul Rodríguez Rebecca K. Smith Mark D. Spalding Femke H. Tonneijck Andrew R. Watkinson 《Trends in ecology & evolution》2014
26.
27.
Daniel Paredes Jay A. Rosenheim Rebecca Chaplin‐Kramer Silvia Winter Daniel S. Karp 《Ecology letters》2021,24(1):73-83
Diversifying agricultural landscapes may mitigate biodiversity declines and improve pest management. Yet landscapes are rarely managed to suppress pests, in part because researchers seldom measure key variables related to pest outbreaks and insecticides that drive management decisions. We used a 13‐year government database to analyse landscape effects on European grapevine moth (Lobesia botrana) outbreaks and insecticides across c. 400 Spanish vineyards. At harvest, we found pest outbreaks increased four‐fold in simplified, vineyard‐dominated landscapes compared to complex landscapes in which vineyards are surrounded by semi‐natural habitats. Similarly, insecticide applications doubled in vineyard‐dominated landscapes but declined in vineyards surrounded by shrubland. Importantly, pest population stochasticity would have masked these large effects if numbers of study sites and years were reduced to typical levels in landscape pest‐control studies. Our results suggest increasing landscape complexity may mitigate pest populations and insecticide applications. Habitat conservation represents an economically and environmentally sound approach for achieving sustainable grape production. 相似文献
28.
Anaerobic digestion of animal waste: effect of mixing 总被引:2,自引:0,他引:2
Karim K Thomas Klasson K Hoffmann R Drescher SR Depaoli DW Al-Dahhan MH 《Bioresource technology》2005,96(14):1607-1612
Six laboratory scale biogas mixed anaerobic digesters were operated to study the effect of biogas recycling rates and draft tube height on their performance. The digesters produced methane at 0.40-0.45 L per liter of digester volume per day. A higher methane production rate was observed in unmixed digesters, while increased biogas circulation rate reduced methane production. However, different draft tube heights caused no difference in the methane production rate. Air infiltration (up to 15% oxygen in the biogas) was observed in the digesters mixed by biogas recirculation. Slight air permeability of tubing or leakage on the vacuum side of the air pump may have caused the observed air infiltration. The similar performance of the mixed and unmixed digesters might be the result of the low solids concentration (50 g dry solids per liter of slurry) in the fed animal slurry, which could be sufficiently mixed by the naturally produced biogas. 相似文献
29.
30.
Habitat fragmentation has often been implicated in the decline of many species. For habitat specialists and/or sedentary species, loss of habitat can result in population isolation and lead to negative genetic effects. However, factors other than fragmentation can often be important and also need to be considered when assessing the genetic structure of a species. We genotyped individuals from 13 populations of the cooperatively breeding Brown‐headed Nuthatch Sitta pusilla in Florida to test three alternative hypotheses regarding the effects that habitat fragmentation might have on genetic structure. A map of potential habitat developed from recent satellite imagery suggested that Brown‐headed Nuthatch populations in southern Florida occupied smaller and more isolated habitat patches (i.e. were more fragmented) than populations in northern Florida. We also genotyped individuals from a small, isolated Brown‐headed Nuthatch population on Grand Bahama Island. We found that populations associated with more fragmented habitat in southern Florida had lower allelic richness than populations in northern Florida (P = 0.02), although there were no differences in heterozygosity. Although pairwise estimates of FST were low overall, values among southern populations were generally higher than northern populations. Population assignment tests identified K = 3 clusters corresponding to a northern cluster, a southern cluster and a unique population in southeast Florida; using sampling localities as prior information revealed K = 7 clusters, with greater structure only among southern Florida populations. The Bahamas population showed moderate to high differentiation compared with Florida populations. Overall, our results suggest that fragmentation could affect gene flow in Brown‐headed Nuthatch populations and is likely to become more pronounced over time. 相似文献