全文获取类型
收费全文 | 11099篇 |
免费 | 1089篇 |
国内免费 | 17篇 |
出版年
2022年 | 69篇 |
2021年 | 110篇 |
2020年 | 62篇 |
2019年 | 105篇 |
2018年 | 121篇 |
2017年 | 138篇 |
2016年 | 232篇 |
2015年 | 367篇 |
2014年 | 453篇 |
2013年 | 537篇 |
2012年 | 758篇 |
2011年 | 759篇 |
2010年 | 522篇 |
2009年 | 453篇 |
2008年 | 692篇 |
2007年 | 727篇 |
2006年 | 658篇 |
2005年 | 680篇 |
2004年 | 669篇 |
2003年 | 664篇 |
2002年 | 677篇 |
2001年 | 119篇 |
2000年 | 90篇 |
1999年 | 120篇 |
1998年 | 194篇 |
1997年 | 135篇 |
1996年 | 116篇 |
1995年 | 117篇 |
1994年 | 120篇 |
1993年 | 109篇 |
1992年 | 88篇 |
1991年 | 87篇 |
1990年 | 72篇 |
1989年 | 85篇 |
1988年 | 81篇 |
1987年 | 72篇 |
1986年 | 79篇 |
1985年 | 75篇 |
1984年 | 92篇 |
1983年 | 87篇 |
1982年 | 104篇 |
1981年 | 115篇 |
1980年 | 104篇 |
1979年 | 62篇 |
1978年 | 59篇 |
1977年 | 46篇 |
1976年 | 57篇 |
1974年 | 50篇 |
1973年 | 45篇 |
1972年 | 42篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
921.
Carmichael SL Rasmussen SA Shaw GM 《Birth defects research. Part A, Clinical and molecular teratology》2010,88(10):804-810
Obesity is associated with increased risk of many adverse health conditions. During pregnancy, obesity presents particularly important challenges for both mother and baby. Over the last 20 years, studies have emerged indicating an association between prepregnancy weight and risks of birth defects. However, few studies have examined the mechanisms through which this association occurs. Understanding the underlying mechanisms may provide clues to public health strategies for the prevention of birth defects associated with maternal obesity. This article briefly reviews existing literature on the association between maternal obesity and birth defects, discusses potential underlying mechanisms, and suggests research needed to improve our understanding of this important association. 相似文献
922.
Karen Kuehl Christopher Loffredo Edward J. Lammer David M. Iovannisci Gary M. Shaw 《Birth defects research. Part A, Clinical and molecular teratology》2010,88(2):101-110
INTRODUCTION: Clark ( 1996 ) proposed that abnormal blood flow is related to some congenital cardiovascular malformations (CCVMs), particularly CCVM with obstruction to blood flow. Our hypothesis is that CCVMs may relate to genes that affect blood coagulation or flow. We studied whether polymorphisms of such genes are related to CCVMs; previous association of these SNPs to conotruncal CCVMs is described. METHODS: We assessed risk of pulmonary stenosis (PS, N = 120), atrial septal defect (ASD, N = 108), aortic stenosis (AS, N = 36), and coarctation of the aorta (CoAo, N = 64), associated with 33 candidate genes, selected for their relationship to blood flow affected by homocysteine metabolism, coagulation, cell‐cell interaction, inflammation, or blood pressure regulation. RESULTS: Effects were specific to cardiac phenotype and race. CoAo was associated with MTHFR (?667) C>T (odds ratio [OR] for TT 3.5, 95% confidence limits [CI] 1.4–8.6). AS was associated with a polymorphism of SERPINE1, G5>G4, OR = 5.6 for the homozygote with 95% CI 1.4–22.9. Unique polymorphisms were associated with increased risk of ASD and PS: NPPA 664G>A with ASD (OR of 2.4, 95%CI 1.3–4.4) and NOS3 (?690) C>T with PS (OR 6.1; 95% CI 1.6–22.6 in the African American population only). For ASD, the NPPA (?664) G>A SNP there was increased risk from the variant genotype only in maternal smokers (OR 2.6; 95% CI 1.0–7.2). CONCLUSIONS: Genes affecting vascular function and coagulation appear to be promising candidates for the etiology of cardiac malformations and warrant further study. Birth Defects Research (Part A), 2010. © 2009 Wiley‐Liss, Inc. 相似文献
923.
Adam R. Boyko Pascale Quignon Lin Li Jeffrey J. Schoenebeck Jeremiah D. Degenhardt Kirk E. Lohmueller Keyan Zhao Abra Brisbin Heidi G. Parker Bridgett M. vonHoldt Michele Cargill Adam Auton Andy Reynolds Abdel G. Elkahloun Marta Castelhano Dana S. Mosher Nathan B. Sutter Gary S. Johnson John Novembre Melissa J. Hubisz Adam Siepel Robert K. Wayne Carlos D. Bustamante Elaine A. Ostrander 《PLoS biology》2010,8(8)
Domestic dogs exhibit tremendous phenotypic diversity, including a greater
variation in body size than any other terrestrial mammal. Here, we generate a
high density map of canine genetic variation by genotyping 915 dogs from 80
domestic dog breeds, 83 wild canids, and 10 outbred African shelter dogs across
60,968 single-nucleotide polymorphisms (SNPs). Coupling this genomic resource
with external measurements from breed standards and individuals as well as
skeletal measurements from museum specimens, we identify 51 regions of the dog
genome associated with phenotypic variation among breeds in 57 traits. The
complex traits include average breed body size and external body dimensions and
cranial, dental, and long bone shape and size with and without allometric
scaling. In contrast to the results from association mapping of quantitative
traits in humans and domesticated plants, we find that across dog breeds, a
small number of quantitative trait loci (≤3) explain the majority of
phenotypic variation for most of the traits we studied. In addition, many
genomic regions show signatures of recent selection, with most of the highly
differentiated regions being associated with breed-defining traits such as body
size, coat characteristics, and ear floppiness. Our results demonstrate the
efficacy of mapping multiple traits in the domestic dog using a database of
genotyped individuals and highlight the important role human-directed selection
has played in altering the genetic architecture of key traits in this important
species. 相似文献
924.
Katherine E. Smith Gary Fooks Jeff Collin Heide Weishaar Sema Mandal Anna B. Gilmore 《PLoS medicine》2010,7(1)
Background
Impact assessment (IA) of all major European Union (EU) policies is now mandatory. The form of IA used has been criticised for favouring corporate interests by overemphasising economic impacts and failing to adequately assess health impacts. Our study sought to assess how, why, and in what ways corporations, and particularly the tobacco industry, influenced the EU''s approach to IA.Methods and Findings
In order to identify whether industry played a role in promoting this system of IA within the EU, we analysed internal documents from British American Tobacco (BAT) that were disclosed following a series of litigation cases in the United States. We combined this analysis with one of related literature and interviews with key informants. Our analysis demonstrates that from 1995 onwards BAT actively worked with other corporate actors to successfully promote a business-oriented form of IA that favoured large corporations. It appears that BAT favoured this form of IA because it could advance the company''s European interests by establishing ground rules for policymaking that would: (i) provide an economic framework for evaluating all policy decisions, implicitly prioritising costs to businesses; (ii) secure early corporate involvement in policy discussions; (iii) bestow the corporate sector with a long-term advantage over other actors by increasing policymakers'' dependence on information they supplied; and (iv) provide businesses with a persuasive means of challenging potential and existing legislation. The data reveal that an ensuing lobbying campaign, largely driven by BAT, helped secure binding changes to the EU Treaty via the Treaty of Amsterdam that required EU policymakers to minimise legislative burdens on businesses. Efforts subsequently focused on ensuring that these Treaty changes were translated into the application of a business orientated form of IA (cost–benefit analysis [CBA]) within EU policymaking procedures. Both the tobacco and chemical industries have since employed IA in apparent attempts to undermine key aspects of European policies designed to protect public health.Conclusions
Our findings suggest that BAT and its corporate allies have fundamentally altered the way in which all EU policy is made by making a business-oriented form of IA mandatory. This increases the likelihood that the EU will produce policies that advance the interests of major corporations, including those that produce products damaging to health, rather than in the interests of its citizens. Given that the public health community, focusing on health IA, has largely welcomed the increasing policy interest in IA, this suggests that urgent consideration is required of the ways in which IA can be employed to undermine, as well as support, effective public health policies. Please see later in the article for the Editors'' Summary 相似文献925.
926.
Alex T. Nielsen Nadia A. Dolganov Thomas Rasmussen Glen Otto Michael C. Miller Stephen A. Felt Stéphanie Torreilles Gary K. Schoolnik 《PLoS pathogens》2010,6(9)
A fundamental, but unanswered question in host-pathogen interactions is the timing, localization and population distribution of virulence gene expression during infection. Here, microarray and in situ single cell expression methods were used to study Vibrio cholerae growth and virulence gene expression during infection of the rabbit ligated ileal loop model of cholera. Genes encoding the toxin-coregulated pilus (TCP) and cholera toxin (CT) were powerfully expressed early in the infectious process in bacteria adjacent to epithelial surfaces. Increased growth was found to co-localize with virulence gene expression. Significant heterogeneity in the expression of tcpA, the repeating subunit of TCP, was observed late in the infectious process. The expression of tcpA, studied in single cells in a homogeneous medium, demonstrated unimodal induction of tcpA after addition of bicarbonate, a chemical inducer of virulence gene expression. Striking bifurcation of the population occurred during entry into stationary phase: one subpopulation continued to express tcpA, whereas the expression declined in the other subpopulation. ctxA, encoding the A subunit of CT, and toxT, encoding the proximal master regulator of virulence gene expression also exhibited the bifurcation phenotype. The bifurcation phenotype was found to be reversible, epigenetic and to persist after removal of bicarbonate, features consistent with bistable switches. The bistable switch requires the positive-feedback circuit controlling ToxT expression and formation of the CRP-cAMP complex during entry into stationary phase. Key features of this bistable switch also were demonstrated in vivo, where striking heterogeneity in tcpA expression was observed in luminal fluid in later stages of the infection. When this fluid was diluted into artificial seawater, bacterial aggregates continued to express tcpA for prolonged periods of time. The bistable control of virulence gene expression points to a mechanism that could generate a subpopulation of V. cholerae that continues to produce TCP and CT in the rice water stools of cholera patients. 相似文献
927.
Haw Chuan Lim Fasheng Zou Sabrina S. Taylor Ben D. Marks Robert G. Moyle Gary Voelker Frederick H. Sheldon 《Journal of Biogeography》2010,37(10):1894-1906
Aim Magpie‐robins and shamas are forest and woodland birds of south Asia. There are two genera: Trichixos for the monotypic T. pyrrhopygus, and Copsychus for other species. Two species are widespread, whereas the others are restricted to specific islands. Endemicity is highest in the Philippines. Using phylogenetic methods, we examined how this group came to its unusual distribution. Location Mainland Asia from India to southern China, and islands from Madagascar to the Philippines. Particular emphasis is placed on the Greater Sundas and Philippines. Methods The phylogeny was estimated from DNA sequences of 14 ingroup taxa representing all nine currently recognized Copsychus and Trichixos species. The entire mitochondrial ND2 gene and portions of nuclear myoglobin intron 2 (Myo2) and transforming growth factor beta 2 intron 5 (TGFβ2‐5) were sequenced for all but two species. The phylogeny was reconstructed using maximum likelihood and Bayesian methods. The timing of divergence events was estimated using a relaxed molecular clock approach, and ancestral areas were examined using stochastic modelling. Results The group comprises three main clades corresponding to ecological types: Trichixos, a primary‐forest specialist; Copsychus magpie‐robins, open‐woodland and coastal species; and Copsychus shamas, thick‐forest species. Trichixos appears to be sister to the magpie‐robins, rendering Copsychus polyphyletic. The dating of phylogenetic nodes was too ambiguous to provide substantial insight into specific geographical events responsible for divergence within the group. Some patterns are nevertheless clear. Copsychus shamas reached the Philippines, probably in two separate invasions, and split into endemic species. Copsychus malabaricus and C. saularis expanded widely in the Greater Sundas and mainland Southeast Asia without species‐level diversification. Main conclusions Magpie‐robins are excellent dispersers and have diversified into distinct species only on isolated oceanic islands. Trichixos, a poor disperser, is restricted to mature forests of the Malay Peninsula, Sumatra and Borneo. Copsychus shamas are intermediate in habitat preference and dispersal capabilities. Their endemism in the Philippines may be attributed to early colonization and specialization to interior forests. In the Greater Sundas, C. malabaricus and C. saularis populations split and came together on Borneo to form two separate subspecies (of each species), which now hybridize. 相似文献
928.
Advantages of using different data sources in assessment of landscape change and its effect on visual scale 总被引:1,自引:0,他引:1
This paper presents a data-flexible indicator framework for analysis of visual landscape character; the VisuLands framework. The theory-based framework encompasses currently used indicators for visual assessment based on four different data sources: land cover data, aerial photographs, landscape photographs and field observations. This paper presents a study applying the VisuLands framework in analysis of landscape change and its effect on visual scale in a landscape in Southeast Sweden. The paper provides a critical assessment of the pros and cons of the approach. It identifies the advantages and disadvantages of using different data sources as well as the applicability and sensitivity of existing indicators in detecting visible landscape change. The results show that while some of the VisuLands indicators are relatively easily applied, others are more complex and demanding in terms of interpretation. The flexibility of the VisuLands framework makes it applicable and user-friendly as it helps meet the requirements and restrictions of the users. The assessment has shown that the different data sources complement each other and that applying indicators using various data sources, when available, will enhance the comprehensiveness of visual landscape assessment. The experience of this study is that the VisuLands framework is a useful tool in landscape analysis, monitoring and planning, which provides a repeatable, systematic and transparent approach with strong links to landscape theory. 相似文献
929.
930.