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   1079731篇
  免费   72518篇
  国内免费   1595篇
耳鼻咽喉   15791篇
儿科学   30136篇
妇产科学   27359篇
基础医学   153702篇
口腔科学   32556篇
临床医学   91372篇
内科学   208971篇
皮肤病学   23427篇
神经病学   85886篇
特种医学   44657篇
外国民族医学   103篇
外科学   176760篇
综合类   19009篇
一般理论   271篇
预防医学   68445篇
眼科学   25617篇
药学   83140篇
  3篇
中国医学   2457篇
肿瘤学   64182篇
  2018年   11580篇
  2017年   9236篇
  2016年   10804篇
  2015年   12175篇
  2014年   15708篇
  2013年   22710篇
  2012年   31789篇
  2011年   32279篇
  2010年   19142篇
  2009年   17712篇
  2008年   31454篇
  2007年   33306篇
  2006年   34058篇
  2005年   33375篇
  2004年   32134篇
  2003年   31122篇
  2002年   30504篇
  2001年   59417篇
  2000年   61298篇
  1999年   51046篇
  1998年   11597篇
  1997年   10087篇
  1996年   9952篇
  1995年   9571篇
  1994年   8879篇
  1993年   8177篇
  1992年   37445篇
  1991年   35850篇
  1990年   34695篇
  1989年   33684篇
  1988年   30598篇
  1987年   29848篇
  1986年   27805篇
  1985年   26616篇
  1984年   18971篇
  1983年   16055篇
  1982年   8411篇
  1981年   7322篇
  1979年   17050篇
  1978年   11525篇
  1977年   9822篇
  1976年   8537篇
  1975年   9636篇
  1974年   11501篇
  1973年   10809篇
  1972年   10314篇
  1971年   9790篇
  1970年   9248篇
  1969年   8693篇
  1968年   7949篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
Objective: Nucleolar organizer regions (NORs) are DNA coils that transcribe to ribosomal RNA. The NOR-associated protein, termed argyrophilic NOR (AgNOR), was visible within the nucleus by staining with silver nitrate examination via the light microscope. AgNOR counting is a proliferation marker and may help in the diagnosis and prognosis of various neoplastic lesions. Aneuploidy (abnormal DNA content) can predict the progression, survival and prognosis of the tumors. The aim of this study was to evaluate the role of AgNORs, DNA ploidy status, and total S-phase fraction (TSPF) as prognostic parameters in malignant salivary gland tumors (MSGTs). Methods: The current study is a retrospective study on a cohort of MSGTs (N=47), to assess AgNORs using Silver Nitrate stain, DNA index (DI), and TSPF using flow cytometry (FCM). Data including tumor size and site, lymphovascular invasion (LVI), lymph node metastasis (LNM) were collected. Results: The AgNORs count was statistically significant with MSGT type. DI was found to have a significant association with tumor site, tumor size and MSGT type. In addition, TSPF was found to be significantly associated with LVI. A moderate positive correlation was noted between AgNORs count and TSPF. LNM, tumor site, high AgNORs and low DI were all associated with short disease-free survival (DFS) and poor overall survival (OS). Conclusion: The present study revealed that high AgNORs count, DNA aneuploidy and TSPF had a poor influence on MSGTs prognosis.  相似文献   
3.
4.
Thomas Behrens  Calvin Ge  Roel Vermeulen  Benjamin Kendzia  Ann Olsson  Joachim Schüz  Hans Kromhout  Beate Pesch  Susan Peters  Lützen Portengen  Per Gustavsson  Dario Mirabelli  Pascal Guénel  Danièle Luce  Dario Consonni  Neil E. Caporaso  Maria Teresa Landi  John K. Field  Stefan Karrasch  Heinz-Erich Wichmann  Jack Siemiatycki  Marie-Elise Parent  Lorenzo Richiardi  Lorenzo Simonato  Karl-Heinz Jöckel  Wolfgang Ahrens  Hermann Pohlabeln  Guillermo Fernández-Tardón  David Zaridze  John R. McLaughlin  Paul A. Demers  Beata Świątkowska  Jolanta Lissowska  Tamás Pándics  Eleonora Fabianova  Dana Mates  Vladimir Bencko  Lenka Foretova  Vladimír Janout  Paolo Boffetta  Bas Bueno-de-Mesquita  Francesco Forastiere  Kurt Straif  Thomas Brüning 《International journal of cancer. Journal international du cancer》2023,152(4):645-660
There is limited evidence regarding the exposure-effect relationship between lung-cancer risk and hexavalent chromium (Cr(VI)) or nickel. We estimated lung-cancer risks in relation to quantitative indices of occupational exposure to Cr(VI) and nickel and their interaction with smoking habits. We pooled 14 case-control studies from Europe and Canada, including 16 901 lung-cancer cases and 20 965 control subjects. A measurement-based job-exposure-matrix estimated job-year-region specific exposure levels to Cr(VI) and nickel, which were linked to the subjects' occupational histories. Odds ratios (OR) and associated 95% confidence intervals (CI) were calculated by unconditional logistic regression, adjusting for study, age group, smoking habits and exposure to other occupational lung carcinogens. Due to their high correlation, we refrained from mutually adjusting for Cr(VI) and nickel independently. In men, ORs for the highest quartile of cumulative exposure to CR(VI) were 1.32 (95% CI 1.19-1.47) and 1.29 (95% CI 1.15-1.45) in relation to nickel. Analogous results among women were: 1.04 (95% CI 0.48-2.24) and 1.29 (95% CI 0.60-2.86), respectively. In men, excess lung-cancer risks due to occupational Cr(VI) and nickel exposure were also observed in each stratum of never, former and current smokers. Joint effects of Cr(VI) and nickel with smoking were in general greater than additive, but not different from multiplicative. In summary, relatively low cumulative levels of occupational exposure to Cr(VI) and nickel were associated with increased ORs for lung cancer, particularly in men. However, we cannot rule out a combined classical measurement and Berkson-type of error structure, which may cause differential bias of risk estimates.  相似文献   
5.
Journal of Pharmacokinetics and Pharmacodynamics - Clinical trials in patients with ulcerative colitis (UC) face the challenge of high and variable placebo response rates. The Mayo Clinical Score...  相似文献   
6.
7.
The notarium is a rigid bony structure, which resulted from the fusion of thoracic vertebrae of some pterosaurs and birds. It is high variable, ranging from two to six fused thoracic vertebrae. In this study, we reviewed and analyzed approximately 270 specimens of neornithine birds (representing 80% of the living orders) and some fossils in order to identify the number of fused vertebrae, degree and sites of vertebral fusion, occurrence of sutures, and other structures of potential phylogenetic and functional significance. These data were analyzed using a recent time-calibrated molecular phylogenetic tree and principal component analyses analysis evaluating the relationship with long bones in order to reconstruct macroevolutionary trends related to the evolution of the notarium. The occurrence of this structure shows a mosaic distribution over neornithine phylogeny, originating several times independently, especially during the Paleogene, in predominantly ground-dwelling forms. The notarium of these groups is characterized by: neural spines fused into single structure, intervertebral openings small to absent, large ventral keels forming ventral plates, and fused transverse processes. Derived neornithines, such as aquatic forms and long-legged birds, have a tendency to display a decreased degree of fusion between the vertebrae, which may indicate a reduction or disappearance of the notarium.  相似文献   
8.
Clinical Oral Investigations - Mechanisms of wound healing are often impaired in patients with osteonecrosis of the jaw (ONJ). According to the guidelines for the treatment of this disease, early...  相似文献   
9.
10.

Background

While drinking alcohol, one must choose between the immediate rewarding effects and the delayed reward of a healthier lifestyle. Individuals differ in their devaluation of a delayed reward based on the time required to receive it, i.e., delay discounting (DD). Previous studies have shown that adolescents discount more steeply than adults and that steeper DD is associated with heavier alcohol use in both groups.

Methods

In a large-scale longitudinal study, we investigated whether higher rates of DD are an antecedent or a consequence of alcohol use during adolescent development. As part of the IMAGEN project, 2220 adolescents completed the Monetary Choice Questionnaire as a DD measure, the Alcohol Use Disorders Identification Test, and the Timeline Follow Back interview at ages 14, 16, 18, and 22. Bivariate latent growth curve models were applied to investigate the relationship between DD and drinking. To explore the consequences of drinking, we computed the cumulative alcohol consumption and correlated it with the development of discounting. A subsample of 221 participants completed an intertemporal choice task (iTeCh) during functional magnetic resonance imaging at ages 14, 16, and 18. Repeated-measures ANOVA was used to differentiate between high-risk and low-risk drinkers on the development of neural processing during intertemporal choices.

Results

Overall, high rates of DD at age 14 predicted a greater increase in drinking over 8 years. In contrast, on average, moderate alcohol use did not affect DD from ages 14 to 22. Of note, we found indicators for less brain activity in top-down control areas during intertemporal choices in the participants who drank more.

Conclusions

Steep DD was shown to be a predictor rather than a consequence of alcohol use in low-level drinking adolescents. Important considerations for future longitudinal studies are the sampling strategies to be used and the reliability of the assessments.
  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