首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   268篇
  免费   30篇
  国内免费   1篇
  2022年   8篇
  2021年   11篇
  2020年   4篇
  2019年   3篇
  2018年   3篇
  2017年   2篇
  2016年   9篇
  2015年   12篇
  2014年   14篇
  2013年   14篇
  2012年   15篇
  2011年   12篇
  2010年   12篇
  2009年   6篇
  2008年   7篇
  2007年   16篇
  2006年   15篇
  2005年   8篇
  2004年   14篇
  2002年   10篇
  2001年   7篇
  2000年   2篇
  1999年   2篇
  1998年   2篇
  1997年   2篇
  1995年   4篇
  1993年   6篇
  1992年   6篇
  1991年   3篇
  1990年   3篇
  1989年   2篇
  1987年   3篇
  1986年   4篇
  1985年   4篇
  1984年   5篇
  1983年   4篇
  1982年   4篇
  1981年   3篇
  1980年   3篇
  1977年   4篇
  1976年   3篇
  1975年   2篇
  1974年   3篇
  1972年   3篇
  1968年   2篇
  1967年   3篇
  1965年   2篇
  1962年   1篇
  1960年   1篇
  1939年   1篇
排序方式: 共有299条查询结果,搜索用时 15 毫秒
101.
We present here the results of a large-scale diachronic palaeodietary (carbon and nitrogen isotopic measurements of bone collagen) study of humans and animals from a single site, the city of York (U.K.), dating from the Roman period to the early 19th century. The human sample comprises 313 burials from the cemeteries of Trentholme Drive and Blossom Street (Roman), Belle Vue House (Anglo-Saxon), Fishergate (High and Later Medieval), and All Saints, Pavement (Later and Post-Medieval). In addition, 145 samples of mammal, fish and bird bone from the sites of Tanner Row and Fishergate were analyzed. The isotope data suggest dietary variation between all archaeological periods, although the most significant change was the introduction of significant quantities of marine foods in the Medieval periods. These are first evident in the diet of a small group of individuals from the High Medieval cemetery at Fishergate, although they were consumed almost universally in the following periods. The human isotope values are also remarkable due to unusually elevated delta(15)N ratios that are not sufficiently explained by the comparably small enrichment in (13)C that accompanies them. We discuss the possible reasons behind this and the archaeological significance of the data set.  相似文献   
102.
Stable isotope ratios (δ13C and δ15N) were measured in human burials from the post‐medieval (16th–18th c. AD) Carmelite friary burial grounds at Aalst, a town in Flanders, Belgium. Dietary patterns of 39 adult individuals were analyzed, from a mixed monastic and lay population buried in three different locations, reflecting groups with differing social status. The data show significant variation in the consumption of perhaps meat, but certainly also marine protein between females and males. This result represents a remarkable continuity with medieval dietary patterns, suggesting that the social and economic changes of the early modern period had a limited effect on everyday life. When both sexes were examined together, individuals buried in the cloister garth consumed significantly less marine protein compared to people buried in the church, likely reflecting social stratification. No statistical differences were observed between isotopic values from the church and the cloister alley, suggesting a similarly diverse diet of the monastic part of the buried population and that of the richer lay population. Finally, the hypothesis that diffuse idiopathic skeletal hyperostosis (DISH) is linked to a diet rich in animal protein was tested. No systematic or statistically significant differences between pathological and non‐pathological bones from the same individuals affected with DISH were observed, and no statistical differences were found between individuals with DISH and individuals without DISH. Am J Phys Anthropol 153:203–213, 2014. © 2013 Wiley Periodicals, Inc.  相似文献   
103.
Terpenoid emissions from ponderosa pine (Pinus ponderosa subsp. scopulorum) were measured in Colorado, USA over two growing seasons to evaluate the role of incident light, needle temperature, and stomatal conductance in controlling emissions of 2-methyl-3-buten-2-ol (MBO) and several monoterpenes. MBO was the dominant daylight terpenoid emission, comprising on average 87 % of the total flux, and diurnal variations were largely determined by light and temperature. During daytime, oxygenated monoterpenes (especially linalool) comprised up to 75 % of the total monoterpenoid flux from needles. A significant fraction of monoterpenoid emissions was dependent on light and 13CO2 labeling studies confirmed de novo production. Thus, modeling of monoterpenoid emissions required a hybrid model in which a significant fraction of emissions was dependent on both light and temperature, while the remainder was dependent on temperature alone. Experiments in which stomata were forced to close using abscisic acid demonstrated that MBO and a large fraction of the monoterpene flux, presumably linalool, could be limited at the scale of seconds to minutes by stomatal conductance. Using a previously published model of terpenoid emissions, which explicitly accounts for the physico-chemical properties of emitted compounds, we were able to simulate these observed stomatal effects, whether induced experimentally or arising under naturally fluctuation conditions of temperature and light. This study shows unequivocally that, under naturally occurring field conditions, de novo light-dependent monoterpenes comprise a significant fraction of emissions in ponderosa pine. Differences between the monoterpene composition of ambient air and needle emissions imply a significant non-needle emission source enriched in Δ-3-carene.  相似文献   
104.
Mounting evidence suggests that human pancreatic ribonuclease (RNase 1) plays important roles in vivo, ranging from regulating blood clotting and inflammation to directly counteracting tumorigenic cells. Understanding these putative roles has been pursued with continual comparisons of human RNase 1 to bovine RNase A, an enzyme that appears to function primarily in the ruminant gut. Our results imply a different physiology for human RNase 1. We demonstrate distinct functional differences between human RNase 1 and bovine RNase A. Moreover, we characterize another RNase 1 homolog, bovine brain ribonuclease, and find pronounced similarities between that enzyme and human RNase 1. We report that human RNase 1 and bovine brain ribonuclease share high catalytic activity against double-stranded RNA substrates, a rare quality among ribonucleases. Both human RNase 1 and bovine brain RNase are readily endocytosed by mammalian cells, aided by tight interactions with cell surface glycans. Finally, we show that both human RNase 1 and bovine brain RNase are secreted from endothelial cells in a regulated manner, implying a potential role in vascular homeostasis. Our results suggest that brain ribonuclease, not RNase A, is the true bovine homolog of human RNase 1, and provide fundamental insight into the ancestral roles and functional adaptations of RNase 1 in mammals.  相似文献   
105.

Background:

Limited evidence suggests that adiposity and lack of physical activity may increase the risk of chronic obstructive pulmonary disease (COPD). We investigated the relation of body size and physical activity with incidence of COPD.

Methods:

We obtained data on anthropometric measurements and physical activity from 113 279 participants in the National Institutes of Health–AARP Diet and Health Study who reported no diagnosis of COPD at baseline (1995–1996). We estimated associations between these measurements and subsequent diagnosis of COPD between 1996 and 2006, with extensive adjustment for smoking and other potentially confounding variables.

Results:

Participants reported 3648 new COPD diagnoses during follow-up. The incidence of COPD was higher in both severely obese (body mass index [BMI]D≥ 35) and underweight (BMID< 18.5) participants, but after adjustment for waist circumference, only underweight remained positively associated with COPD (relative risk [RR]D1.56, 95% confidence interval [CI]D1.15–2.11). Larger waist circumference (highest v. normal categories, adjusted RRD1.72, 95% CID1.37–2.16) and higher waist–hip ratio (highest v. normal categories, adjusted RRD1.46, 95% CID1.23–1.73) were also positively associated with COPD. In contrast, hip circumference (highest v. normal categories, adjusted RR 0.78, 95% CI 0.62–0.98) and physical activity (≥ 5 v. 0 times/wk, adjusted RRD0.71, 95% CID0.63–0.79) were inversely associated with COPD.

Interpretation:

Obesity, in particular abdominal adiposity, was associated with an increased risk of COPD, and increased hip circumference and physical activity were associated with a decreased risk of COPD. These findings suggest that following guidelines for a healthy body weight, body shape and physical activity decrease the risk of COPD.Chronic obstructive pulmonary disease (COPD) is a progressive, irreversible condition that severely affects quality of life1 and ability to work.2 Direct and indirect annual costs of COPD, including inpatient and outpatient care, medication and loss of productivity, sum to $50 billion in the United States3 and R39 billion (about US$50 billion) in Europe.4Chronic obstructive pulmonary disease may be prevented by avoidance of tobacco smoke, occupational dust and other environmental air pollution.5 Body mass index (BMI) and physical activity are established correlates of disease progression among patients with COPD,6,7 but data relating body size or physical activity to incident COPD are sparse. The few studies available are based on small samples and show inverse relations of both BMI8,9 and physical activity10,11 to incidence of COPD. Data are lacking regarding waist or hip circumference in relation to COPD incidence. We therefore examined BMI, waist circumference, hip circumference, waist–hip ratio and physical activity in relation to incidence of COPD in a large cohort of women and men in the US.  相似文献   
106.
Immunity and protection against Brucella abortus   总被引:2,自引:0,他引:2  
Brucella abortus is an intracellular pathogen that causes disease in cattle and in humans. The response against B. abortus involves the whole gamut of the immune system, from innate to adaptive immunity resulting from stimulation of antigen-presenting cells, NK cells, CD4(+) and CD8(+) T cells, and B cells.  相似文献   
107.

Background

HCC is diagnosed in approximately half a million people per year, worldwide. Staging is a more complex issue than in most other cancer entities and, mainly due to unique geographic characteristics of the disease, no universally accepted staging system exists to date. Focusing on survival rates we analyzed demographic, etiological, clinical, laboratory and tumor characteristics of HCC-patients in our institution and applied the common staging systems. Furthermore we aimed at identifying the most suitable of the current staging systems for predicting survival.

Methodology/Principal Findings

Overall, 405 patients with HCC were identified from an electronic medical record database. The following seven staging systems were applied and ranked according to their ability to predict survival by using the Akaike information criterion (AIC) and the concordance-index (c-index): BCLC, CLIP, GETCH, JIS, Okuda, TNM and Child-Pugh. Separately, every single variable of each staging system was tested for prognostic meaning in uni- and multivariate analysis. Alcoholic cirrhosis (44.4%) was the leading etiological factor followed by viral hepatitis C (18.8%). Median survival was 18.1 months (95%-CI: 15.2–22.2). Ascites, bilirubin, alkaline phosphatase, AFP, number of tumor nodes and the BCLC tumor extension remained independent prognostic factors in multivariate analysis. Overall, all of the tested staging systems showed a reasonable discriminatory ability. CLIP (closely followed by JIS) was the top-ranked score in terms of prognostic capability with the best values of the AIC and c-index (AIC 2286, c-index 0.71), surpassing other established staging systems like BCLC (AIC 2343, c-index 0.66). The unidimensional scores TNM (AIC 2342, c-index 0.64) and Child-Pugh (AIC 2369, c-index 0.63) performed in an inferior fashion.

Conclusions/Significance

Compared with six other staging systems, the CLIP-score was identified as the most suitable staging system for predicting prognosis in a large German cohort of predominantly non-surgical HCC-patients.  相似文献   
108.
109.
This article reports the first known study on dogs in Port-au-Prince. Interviews with 1,290 residents provided information on 1,804 dogs. More than 57.7% of homes kept dogs. Not all the dogs received vaccinations for rabies (41.6%), even though 28.2% of households had had a household member bitten by a dog. Although the "owned" dog population had decreased as a result of the earthquake in January 2010, the number of roaming dogs appeared to have been uninfluenced by the disaster. Given that 64.8% of dogs probably had access to the street and only 6.0% of the females were spayed, to humanely contain the dog population will require both confinement and neutering. Although roaming dogs were considered a nuisance by 63.3% of respondents, 42.6% of households fed dogs they did not own.  相似文献   
110.
The N-end rule pathway of protein degradation targets proteins with destabilizing N-terminal residues. Ubr2 is one of the E3 ubiquitin ligases of the mouse N-end rule pathway. We have previously shown that Ubr2-/- male mice are infertile, owing to the arrest of spermatocytes between the leptotene/zygotene and pachytene of meiosis I, the failure of chromosome pairing, and subsequent apoptosis. Here, we report that mouse fibroblast cells derived from Ubr2-/- embryos display genome instability. The frequency of chromosomal bridges and micronuclei were much higher in Ubr2-/- fibroblasts than in +/+ controls. Metaphase chromosome spreads from Ubr2-/- cells revealed a high incidence of spontaneous chromosomal gaps, indicating chromosomal fragility. These fragile sites were generally replicated late in S phase. Ubr2-/- cells were hypersensitive to mitomycin C, a DNA cross-linking agent, but displayed normal sensitivity to gamma-irradiation. A reporter assay showed that Ubr2-/- cells are significantly impaired in the homologous recombination repair of a double strand break. In contrast, Ubr2-/- cells appeared normal in an assay for non-homologous end joining. Our results therefore unveil the role of the ubiquitin ligase Ubr2 in maintaining genome integrity and in homologous recombination repair.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号