全文获取类型
收费全文 | 1168篇 |
免费 | 148篇 |
专业分类
1316篇 |
出版年
2021年 | 18篇 |
2019年 | 16篇 |
2018年 | 16篇 |
2017年 | 19篇 |
2016年 | 19篇 |
2015年 | 48篇 |
2014年 | 51篇 |
2013年 | 60篇 |
2012年 | 66篇 |
2011年 | 63篇 |
2010年 | 42篇 |
2009年 | 34篇 |
2008年 | 46篇 |
2007年 | 59篇 |
2006年 | 52篇 |
2005年 | 67篇 |
2004年 | 60篇 |
2003年 | 53篇 |
2002年 | 35篇 |
2001年 | 23篇 |
2000年 | 18篇 |
1999年 | 16篇 |
1998年 | 20篇 |
1997年 | 20篇 |
1996年 | 12篇 |
1995年 | 12篇 |
1993年 | 12篇 |
1992年 | 14篇 |
1991年 | 10篇 |
1990年 | 14篇 |
1989年 | 8篇 |
1988年 | 17篇 |
1987年 | 17篇 |
1986年 | 9篇 |
1985年 | 10篇 |
1984年 | 12篇 |
1983年 | 21篇 |
1982年 | 12篇 |
1981年 | 11篇 |
1980年 | 15篇 |
1979年 | 8篇 |
1978年 | 7篇 |
1977年 | 9篇 |
1976年 | 9篇 |
1975年 | 11篇 |
1973年 | 11篇 |
1972年 | 9篇 |
1971年 | 7篇 |
1969年 | 12篇 |
1964年 | 7篇 |
排序方式: 共有1316条查询结果,搜索用时 8 毫秒
61.
A. Bradford Hill 《BMJ (Clinical research ed.)》1947,2(4522):366-368
62.
Cortical synaptosomes fixed in unbuffered OsO4 and glutaraldehyde have been block-stained with phosphotungstic acid (PTA) in order to investigate the relationship between complex vesicles and dense projections. It is concluded that the shell of the complex vesicles contributes to the formation of dense projections and that, in addition, there is a correspondence between this shell and the previously described presynaptic network. The process by which complex vesicles become associated with dense projections appears to be accentuated by electrical stimulation of the synaptosomes. 相似文献
63.
Jennifer W. Hill Carol F. Elias Makoto Fukuda Kevin W. Williams Eric D. Berglund William L. Holland You-Ree Cho Jen-Chieh Chuang Yong Xu Michelle Choi Danielle Lauzon Charlotte E. Lee Roberto Coppari James A. Richardson Jeffrey M. Zigman Streamson Chua Philipp E. Scherer Bradford B. Lowell Jens C. Brüning Joel K. Elmquist 《Cell metabolism》2010,11(4):286-297
64.
Bradford Simcock 《American anthropologist》2000,102(4):957-958
Japanese Working Class Lives: An Ethnographic Study of Factory Workers. James E. Roberson. New York: Routledge, 1998. 227 pp. 相似文献
65.
Yersinia pestis Yop secretion protein F: purification, characterization, and protective efficacy against bubonic plague 总被引:1,自引:0,他引:1
Yersinia pestis is a gram-negative human pathogen that uses a type III secretion system to deliver virulence factors into human hosts. The delivery is contact-dependent and it has been proposed that polymerization of Yop secretion protein F (YscF) is used to puncture mammalian cell membranes to facilitate delivery of Yersinia outer protein effectors into host cells. To evaluate the potential immunogenicity and protective efficacy of YscF against Y. pestis, we used a purified recombinant YscF protein as a potential vaccine candidate in a mouse subcutaneous infection model. YscF was expressed and purified from Escherichia coli by immobilized metal-ion affinity chromatography and protein identity was confirmed by ion trap mass spectrometry. The recombinant protein was highly alpha-helical and formed relatively stable aggregates under physiological conditions. The properties were consistent with behavior expected for the native YscF, suggesting that the antigen was properly folded. Ten mice were inoculated subcutaneously, administered booster injections after one month, and challenged with 130 LD(50) of wild type Y. pestis CO92. Six animals in the vaccinated group but none in the control group survived the challenge. The vaccinated animals produced high levels of specific antibodies against YscF as determined by Western blot. The data were statistically significant (P = 0.053 by two-tailed Fisher's test), suggesting that the YscF protein can provide a protective immune response against lethal plague challenge during subcutaneous plague infection. 相似文献
66.
Lauren B. Buckley T. Jonathan Davies David D. Ackerly Nathan J. B. Kraft Susan P. Harrison Brian L. Anacker Howard V. Cornell Ellen I. Damschen John-Avid Grytnes Bradford A. Hawkins Christy M. McCain Patrick R. Stephens John J. Wiens 《Proceedings. Biological sciences / The Royal Society》2010,277(1691):2131-2138
Biologists have long searched for mechanisms responsible for the increase in species richness with decreasing latitude. The strong correlation between species richness and climate is frequently interpreted as reflecting a causal link via processes linked to energy or evolutionary rates. Here, we investigate how the aggregation of clades, as dictated by phylogeny, can give rise to significant climate–richness gradients without gradients in diversification or environmental carrying capacity. The relationship between climate and species richness varies considerably between clades, regions and time periods in a global-scale phylogenetically informed analysis of all terrestrial mammal species. Many young clades show negative richness–temperature slopes (more species at cooler temperatures), with the ages of these clades coinciding with the expansion of temperate climate zones in the late Eocene. In carnivores, we find steeply positive richness–temperature slopes in clades with restricted distributions and tropical origins (e.g. cat clade), whereas widespread, temperate clades exhibit shallow, negative slopes (e.g. dog–bear clade). We show that the slope of the global climate–richness gradient in mammals is driven by aggregating Chiroptera (bats) with their Eutherian sister group. Our findings indicate that the evolutionary history should be accounted for as part of any search for causal links between environment and species richness. 相似文献
67.
Dhillon H Zigman JM Ye C Lee CE McGovern RA Tang V Kenny CD Christiansen LM White RD Edelstein EA Coppari R Balthasar N Cowley MA Chua S Elmquist JK Lowell BB 《Neuron》2006,49(2):191-203
Leptin, an adipocyte-derived hormone, acts directly on the brain to control food intake and energy expenditure. An important question is the identity of first-order neurons initiating leptin's anti-obesity effects. A widely held view is that most, if not all, of leptin's effects are mediated by neurons located in the arcuate nucleus of the hypothalamus. However, leptin receptors (LEPRs) are expressed in other sites as well, including the ventromedial hypothalamus (VMH). The possible role of leptin acting in "nonarcuate" sites has largely been ignored. In the present study, we show that leptin depolarizes and increases the firing rate of steroidogenic factor-1 (SF1)-positive neurons in the VMH. We also show, by generating mice that lack LEPRs on SF1-positive neurons, that leptin action at this site plays an important role in reducing body weight and, of note, in resisting diet-induced obesity. These results reveal a critical role for leptin action on VMH neurons. 相似文献
68.
Bradford A. Hawkins 《Journal of Biogeography》2012,39(1):1-9
Biogeography is spatial by nature. Over the past 20 years, the literature related to the analysis of spatially structured data has exploded, much of it focused on a perceived problem of spatial autocorrelation and ways to deal with it. However, there are a number of other issues that permeate the biogeographical and macroecological literature that have become entangled in the spatial autocorrelation web. In this piece I discuss some of the assumptions that are often made in the analysis of spatially structured data that can lead to misunderstandings about the nature of spatial data, the methods used to analyse them, and how results can be interpreted. 相似文献
69.
70.
Community and ecosystem responses to global environmental change are contingent on the magnitude of change and interacting global change factors. To reveal whether responses are also contingent on the magnitude of each interacting factor, multifactor, multilevel experiments are required, but are rarely conducted. We exposed model grassland ecosystems to six levels of atmospheric CO(2) and six levels of nitrogen enrichment, applying the latter both chronically (simulating deposition) and acutely (simulating fertilization). The 66 treatments were maintained for 6?months under controlled growing conditions, with biomass harvested every 28?d and sorted to species. Aboveground plant productivity responses to CO(2) were contingent on nitrogen amount, and the responses to nitrogen amount were dependent on whether applications were chronic or acute. Specifically, productivity responses to increasing CO(2) concentrations were accentuated with higher nitrogen enrichments, and productivity was greater when higher nitrogen enrichments were applied acutely. Plant community composition was influenced only by nitrogen enrichment, where the co-dominant grass species with the greatest leaf trait plasticity increasingly dominated with higher nitrogen amounts. Community processes are considered to be unpredictable, but our data suggest that the prediction of the impacts of simultaneous global changes is more complex for ecosystem processes, given that their responses are contingent on the levels of interacting factors. 相似文献