全文获取类型
收费全文 | 6032篇 |
免费 | 352篇 |
国内免费 | 1篇 |
专业分类
6385篇 |
出版年
2023年 | 62篇 |
2022年 | 85篇 |
2021年 | 163篇 |
2020年 | 112篇 |
2019年 | 140篇 |
2018年 | 187篇 |
2017年 | 147篇 |
2016年 | 228篇 |
2015年 | 325篇 |
2014年 | 375篇 |
2013年 | 412篇 |
2012年 | 519篇 |
2011年 | 522篇 |
2010年 | 317篇 |
2009年 | 236篇 |
2008年 | 326篇 |
2007年 | 354篇 |
2006年 | 344篇 |
2005年 | 258篇 |
2004年 | 253篇 |
2003年 | 214篇 |
2002年 | 211篇 |
2001年 | 52篇 |
2000年 | 36篇 |
1999年 | 33篇 |
1998年 | 50篇 |
1997年 | 39篇 |
1996年 | 44篇 |
1995年 | 32篇 |
1994年 | 28篇 |
1993年 | 24篇 |
1992年 | 28篇 |
1991年 | 18篇 |
1990年 | 16篇 |
1989年 | 20篇 |
1988年 | 17篇 |
1987年 | 8篇 |
1986年 | 11篇 |
1985年 | 14篇 |
1984年 | 7篇 |
1983年 | 11篇 |
1982年 | 12篇 |
1981年 | 7篇 |
1979年 | 12篇 |
1978年 | 10篇 |
1977年 | 9篇 |
1976年 | 6篇 |
1975年 | 9篇 |
1974年 | 8篇 |
1973年 | 8篇 |
排序方式: 共有6385条查询结果,搜索用时 15 毫秒
91.
Jane Elith Catherine H. Graham Robert P. Anderson Miroslav Dudík Simon Ferrier Antoine Guisan Robert J. Hijmans Falk Huettmann John R. Leathwick Anthony Lehmann Jin Li Lucia G. Lohmann Bette A. Loiselle Glenn Manion Craig Moritz Miguel Nakamura Yoshinori Nakazawa Jacob McC. M. Overton A. Townsend Peterson Steven J. Phillips Karen Richardson Ricardo Scachetti-Pereira Robert E. Schapire Jorge Soberón Stephen Williams Mary S. Wisz Niklaus E. Zimmermann 《Ecography》2006,29(2):129-151
Prediction of species' distributions is central to diverse applications in ecology, evolution and conservation science. There is increasing electronic access to vast sets of occurrence records in museums and herbaria, yet little effective guidance on how best to use this information in the context of numerous approaches for modelling distributions. To meet this need, we compared 16 modelling methods over 226 species from 6 regions of the world, creating the most comprehensive set of model comparisons to date. We used presence-only data to fit models, and independent presence-absence data to evaluate the predictions. Along with well-established modelling methods such as generalised additive models and GARP and BIOCLIM, we explored methods that either have been developed recently or have rarely been applied to modelling species' distributions. These include machine-learning methods and community models, both of which have features that may make them particularly well suited to noisy or sparse information, as is typical of species' occurrence data. Presence-only data were effective for modelling species' distributions for many species and regions. The novel methods consistently outperformed more established methods. The results of our analysis are promising for the use of data from museums and herbaria, especially as methods suited to the noise inherent in such data improve. 相似文献
92.
Background
Costs of tuberculosis diagnosis and treatment may represent a significant burden for the poor and for the health system in resource-poor countries.Objectives
The aim of this study was to analyze patients'' costs of tuberculosis care and to estimate the incremental cost-effectiveness ratio (ICER) of the directly observed treatment (DOT) strategy per completed treatment in Rio de Janeiro, Brazil.Methods
We interviewed 218 adult patients with bacteriologically confirmed pulmonary tuberculosis. Information on direct (out-of-pocket expenses) and indirect (hours lost) costs, loss in income and costs with extra help were gathered through a questionnaire. Healthcare system additional costs due to supervision of pill-intake were calculated considering staff salaries. Effectiveness was measured by treatment completion rate. The ICER of DOT compared to self-administered therapy (SAT) was calculated.Principal Findings
DOT increased costs during the treatment phase, while SAT increased costs in the pre-diagnostic phase, for both the patient and the health system. Treatment completion rates were 71% in SAT facilities and 79% in DOT facilities. Costs per completed treatment were US$ 194 for patients and U$ 189 for the health system in SAT facilities, compared to US$ 336 and US$ 726 in DOT facilities. The ICER was US$ 6,616 per completed DOT treatment compared to SAT.Conclusions
Costs incurred by TB patients are high in Rio de Janeiro, especially for those under DOT. The DOT strategy doubles patients'' costs and increases by fourfold the health system costs per completed treatment. The additional costs for DOT may be one of the contributing factors to the completion rates below the targeted 85% recommended by WHO. 相似文献93.
Sandoval A Arikkath J Monjaraz E Campbell KP Felix R 《Cellular and molecular neurobiology》2007,27(7):901-908
(1) Voltage-gated Ca2+ (CaV) channels are multi-subunit membrane complexes that allow depolarization-induced Ca2+ influx into cells. The skeletal muscle L-type CaV channels consist of an ion-conducting CaV1.1 subunit and auxiliary α2δ−1, β1 and γ1 subunits. This complex serves both as a CaV channel and as a voltage sensor for excitation–contraction coupling. (2) Though much is known about the mechanisms by which
the α2δ−1 and β1 subunits regulate CaV channel function, there is far less information on the γ1 subunit. Previously, we characterized the interaction of γ1 with the other components of the skeletal CaV channel complex, and showed that heterologous expression of this auxiliary subunit decreases Ca2+ current density in myotubes from γ1 null mice. (3) In the current report, using Western blotting we show that the expression of the CaV1.1 protein is significantly lower when it is heterologously co-expressed with γ1. Consistent with this, patch-clamp recordings showed that transient transfection of γ1 drastically inhibited macroscopic currents through recombinant N-type (CaV2.2/α2δ−1/β3) channels expressed in HEK-293 cells. (4) These findings provide evidence that co-expression of the auxiliary γ1 subunit results in a decreased expression of the ion-conducting subunit, which may help to explain the reduction in Ca2+ current density following γ1 transfection. 相似文献
94.
ThuyTien Nguyen I. Ricardo Argueta-Morales Stephen Guimond William Clark Andres Ceballos Ruben Osorio 《Computer methods in biomechanics and biomedical engineering》2016,19(7):789-799
Stroke is the most devastating complication after ventricular assist device (VAD) implantation with a 19% incidence and 65% mortality in the pediatric population. Current pediatric VAD technology and anticoagulation strategies alone are suboptimal. VAD implantation assisted by computational methods (CFD) may contribute reducing the risk of cerebral embolization. Representative three-dimensional aortic arch models of an infant and a child were generated. An 8 mm VAD outflow-graft (VAD-OG) anastomosed to the aorta was rendered and CFD was applied to study blood flow patterns. Particle tracks, originating in the VAD, were computed with a Lagrangian phase model and the percentage of particles entering the cerebral vessels was calculated. Eight implantation configurations (infant = 5 and child = 3) and 5 particle sizes (0.5, 1, 2, 3, and 4 mm) were considered. For the infant model, percentage of particles entering the cerebral vessels ranged from 15% for a VAD-OG anastomosed at 90° to the aorta, to 31% for 30° VAD-OG anastomosis (overall percentages: X2 = 10,852, p < 0.0001). For the child model, cerebral embolization ranged from 9% for the 30° VAD-OG anastomosis to 15% for the 60° anastomosis (overall percentages: χ2 = 10,323, p < 0.0001). Using detailed CFD calculations, we demonstrate that the risk of stroke depends significantly on the VAD implantation geometry. In turn, the risk probably depends on patient-specific anatomy. CFD can be used to optimize VAD implantation geometry to minimize stroke risk. 相似文献
95.
96.
97.
98.
Engineering grapevine for increased resistance to fungal pathogens without compromising wine stability 总被引:3,自引:0,他引:3
The vast majority of wine proteins have recently been identified as pathogenesis-related (PR) proteins. During the growing season, these proteins are expressed in developmentally dependent and inducible manners in grapevine leaves and grape berries, in which they are believed to play an important role in protection against fungal pathogens and possibly other stresses. Because of their inherent resistance to proteolytic attack and to the low pH values characteristic of wines, vinification can be seen as a "purification strategy" for grape PR proteins. The inevitable consequent accumulation of these proteins in wines becomes a technological nuisance because they adversely affect the clarity and stability of wines. Genetically modified vines underexpressing PR proteins would certainly lead to stable wines but would increase the plant susceptibility to fungal attack, and the actual trend seems to be in the opposite direction, that is overexpressing these proteins to obtain plants with enhanced resistance to pathogens--a trend that will probably augment problems associated with protein instability in the resulting wines. 相似文献
99.
100.
Efficiency of the use of pedigree and molecular marker information in conservation programs 总被引:1,自引:0,他引:1 下载免费PDF全文
The value of molecular markers and pedigree records, separately or in combination, to assist in the management of conserved populations has been tested. The general strategy for managing the population was to optimize contributions of parents to the next generation for minimizing the global weighted coancestry. Strategies differed in the type of information used to compute global coancestries, the number and type of evaluated individuals, and the system of mating. Genealogical information proved to be very useful (at least for 10 generations of management) to arrange individuals' contributions via the minimization of global coancestry. In fact, the level of expected heterozygosity after 10 generations yielded by this strategy was 88-100% of the maximum possible improvement obtained if the genotype for all loci was known. Marker information was of very limited value if used alone. The amount and degree of polymorphism of markers to be used to compute molecular coancestry had to be high to mimic the performance of the strategy relying on pedigree, especially in the short term (for example, >10 markers per chromosome with 10 alleles each were needed if only the parents' genotype was available). When both sources of information are combined to calculate the coancestry conditional on markers, clear increases in effective population size (Ne) were found, but observed diversity levels (either gene or allelic diversity) in the early generations were quite similar to the ones obtained with pedigree alone. The advantage of including molecular information is greater when information is available on a greater number of individuals (offspring and parents vs. parents only). However, for realistic situations (i.e., large genomes) the benefits of using information on offspring are small. The same conclusions were reached when comparing the use of the different types of information (genealogical or/and molecular) to perform minimum coancestry matings. 相似文献