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   3858篇
  免费   278篇
  国内免费   413篇
  2023年   95篇
  2022年   150篇
  2021年   198篇
  2020年   130篇
  2019年   201篇
  2018年   143篇
  2017年   141篇
  2016年   124篇
  2015年   117篇
  2014年   179篇
  2013年   262篇
  2012年   150篇
  2011年   147篇
  2010年   116篇
  2009年   172篇
  2008年   157篇
  2007年   194篇
  2006年   163篇
  2005年   142篇
  2004年   122篇
  2003年   115篇
  2002年   122篇
  2001年   95篇
  2000年   76篇
  1999年   68篇
  1998年   57篇
  1997年   69篇
  1996年   49篇
  1995年   47篇
  1994年   50篇
  1993年   51篇
  1992年   38篇
  1991年   30篇
  1990年   23篇
  1989年   30篇
  1988年   23篇
  1987年   19篇
  1986年   25篇
  1985年   30篇
  1984年   63篇
  1983年   58篇
  1982年   63篇
  1981年   34篇
  1980年   36篇
  1979年   35篇
  1978年   31篇
  1977年   31篇
  1976年   26篇
  1975年   14篇
  1974年   16篇
排序方式: 共有4549条查询结果,搜索用时 46 毫秒
61.
不同产地苍术药材化学成分的比较   总被引:13,自引:2,他引:11  
江苏、安徽及湖北5个产地苍术(Atractylodeslancea(Thunb.)DC.)商品药材的化学分析结果表明:它们在某些药材性状、一些常规化学成分、挥发油成分和含量上存在显著差异。江苏茅苍术根茎挥发油的化学成分复杂,由此探讨了它作为地道药材的特殊性。同时对苍术的“断面暴露稍久可析出结晶”的性状和《药典》(1990年版)规定的“层析谱上应显有苍术素斑点”的鉴别实验进行讨论。  相似文献   
62.
Heinz Löffler 《Hydrobiologia》1993,264(3):169-174
The northwestern area of the Pannonian Lowland extends into Austria. The climatic and hydrologic attributes of this biographic region promote the existence of extremely astatic bodies water lacking any fish and hence the occurrence of Anostraca, Notostraca, Laevicaudata and Spinicaudata. Zoogeographical and ecological features as well as the extinction of species due to anthropogenic influence are described.Dedicated to Prof. Dr F. Berger, Lunz, Austria, on the occasion of his 90th birthdayDedicated to Prof. Dr F. Berger, Lunz, Austria, on the occasion of his 90th birthday  相似文献   
63.
A previous survey on pepper lines (Capsicum annuum L.) indicated that a susceptible cultivar, Yolo Wonder, reacted to cucumber mosaic virus (CMV) by producing a systemic yellow mosaic. By contrast, CMV caused no symptoms on lines Perennial and Vania. The virus is recoverable from the uninoculated leaves of Perennial, while in Vania CMV is restricted to the inoculated leaves. To interpret these phenomena, a comparative study on CMV multiplication rates, yield, specific infectivity and relative proportion of RNAs was made in the inoculated leaves of the three pepper varieties. The rate of CMV multiplication, as estimated by the double antibody sandwich form of enzyme-linked immu-nosorbent assay, was lower in Perennial than in Vania or Yolo Wonder. The yield of virus purified from Perennial was very low when compared with Vania or Yolo Wonder. The specific infectivity of the virus extracted from Perennial was less than that from Vania or Yolo Wonder. These results suggest that Perennial is resistant to CMV multiplication, while restriction of the virus in inoculated leaves of Vania is not due to the inhibition of the virus replication. However, polyacrylamide gel electrophoresis revealed that the RNA profiles of CMV purified from the three pepper lines were similar.  相似文献   
64.
Kutikova  L. A.  Markevich  G. I. 《Hydrobiologia》1993,255(1):545-549
Phylogenetic relations among the main groups of Monimotrochida are considered. The principal directions of monimotrochid evolution were defined by comparative investigations of mastax morphology (SEM), basic body structures, and general biology. On the basis of these results we propose a revision of previous rotifer taxonomy. We suggest to place the Monimotrochida in the order Protoramida divided into two suborders Flosculariina and Conochilina.  相似文献   
65.
66.
Introduction and objectivesThere is a considerable lack of structure in training health professionals in geriatrics. The narratives can promote a collaborative reflection on different topics and might be used as a pedagogic strategy for undergraduate health students. This study aimed to explore the adoption of new perspectives on ageing after the implementation of dynamic narratives in the first graduate year for physiotherapy students.Materials and methodsAn exploratory qualitative study was performed. Participants were included if they were: ≥18 years old, physiotherapy students and agreed to participate. Physiotherapy students (n = 44) were recruited from the School of Health Sciences, Polytechnic Institute of Leiria. Two gaming sessions were performed to help students, as narrators, express their visions and ways to deal with the geriatrics field. Students’ perspectives about ageing at baseline (T1) and after the narratives experience (T2) were collected by answering the following question: What is your perception regarding ageing? Two evaluators were involved in qualitative data analysis by performing: (i) an individual analysis of themes/subthemes and a discussion meeting to evaluate discrepancies and to reach a consensus.ResultsNegative perceptions about ageing were referred 39 times at T1 (most of them related to restriction; deterioration subthemes). There were no negative perceptions registered at T2. Positive perceptions increased at T2, from n = 39 to n = 52, and three new subthemes emerged (beginning of something, fighting ageism, challenge).ConclusionThis study demonstrated the potential of narrative-based experiences as a desirable pedagogic methodology (board games-oriented) for geriatric education in undergraduate health students.  相似文献   
67.
我国山水林田湖草沙生命共同体及其保护和修复工程的理论研究和实践正逐渐开展,需要系统的学科理论支撑,景观生态学作为地理学和生态学的交叉科学,能够以其宏观空间理论和技术体系满足这一需求。本文将景观生态学作为山水林田湖草沙一体化保护和修复工程的支撑学科,首先,明确了山水林田湖草沙生命共同体是镶嵌的异质景观、具有景观的所有特征并遵循景观生态学原理;其次,阐述了景观生态建设理论如何应用于山水林田湖草沙一体化保护和修复工程的规划和评价;最后,总结景观生态建设研究的新趋势,提出待解决的理论和实践问题,并论述山水林田湖草沙一体化保护和修复工程如何为解决这些问题提供广阔的研究空间。景观生态学和山水林田湖草沙一体化保护和修复工程实践相结合,将为实现我国乃至全球生态、经济、社会可持续发展提供极为有效的途径。  相似文献   
68.
Cancer is one of the main challenges of the health system around the world. This disease is increasing in developing countries and imposes heavy costs on patients and governments. On the other hand, despite various drugs, the death rate among cancer patients is still high and the current treatments have many harmful effects. In the traditional medicine of different countries, there are many medicinal plants that can be effective in the treatment of cancer. Ferula plants are traditionally used as spices and food or for medicinal purposes. Ferula assa-foetida is one of the famous plants of this genus, which has been used for the treatment of various diseases since ancient times. Among the main compounds of this plant, we can mention monoterpenes, sulfide compounds and polyphenols, which can show different therapeutic effects. This article has been compiled with the aim of collecting evidence and articles related to the anti-cancer effects of extracts, derived compounds, essential oils and nanoparticles containing Ferula assa-foetida. This review article was prepared by searching the terms Ferula assa-foetida and cancer, and relevant information was collected through searching electronic databases such as ISI Web of Knowledge, PubMed, and Google Scholar. Fortunately, the results of this review showed that relatively comprehensive studies have been conducted in this field and shown that Ferula assa-foetida can be very promising in the treatment of cancer.  相似文献   
69.
Projection of land use and land-cover change is highly uncertain yet drives critical estimates of carbon emissions, climate change, and food and bioenergy production. We use new, spatially explicit land availability data in conjunction with a model sensitivity analysis to estimate the effects of additional land protection on land use and land cover. The land availability data include protected land and agricultural suitability and is incorporated into the Moirai land data system for initializing the Global Change Analysis Model. Overall, decreasing land availability is relatively inefficient at preserving undeveloped land while having considerable regional land-use impacts. Current amounts of protected area have little effect on land and crop production estimates, but including the spatial distribution of unsuitable (i.e., unavailable) land dramatically shifts bioenergy production from high northern latitudes to the rest of the world, compared with uniform availability. This highlights the importance of spatial heterogeneity in understanding and managing land change. Approximately doubling the current protected area to emulate a 30% protected area target may avoid land conversion by 2050 of less than half the newly protected extent while reducing bioenergy feedstock land by 10.4% and cropland and grazed pasture by over 3%. Regional bioenergy land may be reduced (increased) by up to 46% (36%), cropland reduced by up to 61%, pasture reduced by up to 100%, and harvested forest reduced by up to 35%. Only a few regions show notable gains in some undeveloped land types of up to 36%. Half of the regions can reach the target using only unsuitable land, which would minimize impacts on agriculture but may not meet conservation goals. Rather than focusing on an area target, a more robust approach may be to carefully select newly protected land to meet well-defined conservation goals while minimizing impacts to agriculture.  相似文献   
70.
摘要 目的:观察右美托咪定联合芬太尼对心脏瓣膜置换术患者细胞免疫功能的影响及在心脑保护中的作用。方法:纳入海南医学院第二附属医院2019年4月~2021年6月间接收的体外循环(CPB)下心脏瓣膜置换术患者97例,根据信封抽签法将患者分为对照组(芬太尼,48例)和观察组(右美托咪定联合芬太尼,49例)。对比两组血流动力学指标[平均动脉压(MAP)、心率(HR)]、心肌损伤指标[肌钙蛋白(cTnI)、肌酸激酶同工酶(CK-MB)、心型脂肪酸结合蛋白(HFABP)]、脑损伤指标[神经元特异性烯醇化酶(NSE)、S-100β]、细胞免疫功能、镇痛情况及不良反应发生率。结果:观察组CPB开始后10 min(T2)时间点HR、MAP低于对照组,CPB结束(T3)、术毕(T4)时间点HR、MAP高于对照组(P<0.05)。两组术前(T1)、术后24 h、术后48 h视觉模拟评分法(VAS)评分升高后降低(P<0.05),观察组术后24 h、术后48 h VAS评分低于对照组(P<0.05)。观察组术后48 h CD4+、CD4+/CD8+高于对照组,CD8+低于对照组(P<0.05)。观察组术后48 h cTnI、CK-MB、HFABP低于对照组(P<0.05)。观察组术后48 h NSE、S100B低于对照组(P<0.05)。观察组的不良反应发生率低于对照组(P<0.05)。结论:心脏瓣膜置换术患者选用右美托咪定联合芬太尼麻醉方案,有助于减轻疼痛,稳定血流动力学,减轻免疫抑制,同时还可发挥心脑保护效果,降低不良反应发生率,是一种可靠的麻醉方案。  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