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   1960篇
  免费   256篇
  2020年   14篇
  2019年   14篇
  2018年   21篇
  2017年   26篇
  2016年   35篇
  2015年   59篇
  2014年   53篇
  2013年   67篇
  2012年   99篇
  2011年   117篇
  2010年   55篇
  2009年   75篇
  2008年   116篇
  2007年   87篇
  2006年   90篇
  2005年   86篇
  2004年   73篇
  2003年   80篇
  2002年   74篇
  2001年   70篇
  2000年   70篇
  1999年   56篇
  1998年   34篇
  1997年   25篇
  1996年   18篇
  1995年   23篇
  1994年   28篇
  1993年   31篇
  1992年   36篇
  1991年   47篇
  1990年   34篇
  1989年   38篇
  1988年   29篇
  1987年   35篇
  1986年   29篇
  1985年   23篇
  1984年   23篇
  1983年   27篇
  1982年   23篇
  1980年   14篇
  1979年   16篇
  1977年   13篇
  1976年   12篇
  1975年   16篇
  1974年   13篇
  1973年   15篇
  1972年   15篇
  1970年   20篇
  1969年   12篇
  1966年   15篇
排序方式: 共有2216条查询结果,搜索用时 15 毫秒
1.
K K Carroll 《CMAJ》1991,144(5):572
  相似文献   
2.
Climate connectivity, the ability of a landscape to promote or hinder the movement of organisms in response to a changing climate, is contingent on multiple factors including the distance organisms need to move to track suitable climate over time (i.e. climate velocity) and the resistance they experience along such routes. An additional consideration which has received less attention is that human land uses increase resistance to movement or alter movement routes and thus influence climate connectivity. Here we evaluate the influence of human land uses on climate connectivity across North America by comparing two climate connectivity scenarios, one considering climate change in isolation and the other considering climate change and human land uses. In doing so, we introduce a novel metric of climate connectivity, ‘human exposure’, that quantifies the cumulative exposure to human activities that organisms may encounter as they shift their ranges in response to climate change. We also delineate potential movement routes and evaluate whether the protected area network supports movement corridors better than non‐protected lands. We found that when incorporating human land uses, climate connectivity decreased; climate velocity increased on average by 0.3 km/year and cumulative climatic resistance increased for ~83% of the continent. Moreover, ~96% of movement routes in North America must contend with human land uses to some degree. In the scenario that evaluated climate change in isolation, we found that protected areas do not support climate corridors at a higher rate than non‐protected lands across North America. However, variability is evident, as many ecoregions contain protected areas that exhibit both more and less representation of climate corridors compared to non‐protected lands. Overall, our study indicates that previous evaluations of climate connectivity underestimate climate change exposure because they do not account for human impacts.  相似文献   
3.
4.
5.
6.
Females of Zeiraphera canadensis Mut. & Free., the spruce bud moth, were reared in the laboratory at constant and alternating temperatures, and in an outdoor insectary, to (1) determine the effects of temperature, age and size on several reproductive parameters and, (2) to test the hypothesis that body size-temperature interactions influence longevity and realized fecundity. Egg maturation was linearly related to age and large moths developed eggs at a higher rate than small ones. Mcan lifetime oviposition rate reached a maximum and remained stable at temperatures 20° C while the mean lifetime rate of egg maturation increased linearly with temperature, indicating that higher temperatures adversely affect oviposition. The production of nonviable eggs increased with age but also with temperature, suggesting high temperature (25° C) reduces egg quality and/or hinders fertilization. The realized fecundity and longevity of females reared under an alternating temperature regime (mean 20° C) was significantly less than that of females reared at constant 20° C. Similar realized fecundity, longevity and mean lifetime oviposition rates for females reared at temperatures alternating between 10 and 25° C (mean 20° C) and those at constant 25° C reflected the inability of females to recover from elevated diurnal temperatures. Longevity was positively related to female body size at constant 15 and 20° C but the relationships were negative for moths exposed to diurnal temperatures equal to or exceeding 25° C. Due to the reduced longevity of large moths at high temperatures, linear regressions between size and realized fecundity were only significant at constant temperatures 20° C. At higher temperatures, the size-fecundity relationship became curvilinear as a result of the diminished reproductive output of large individuals. Reduced fecundity and longevity of large females at high temperatures may have been due to elevated internal temperatures of large-bodied moths. Large females in a controlled-environment chamber maintained at 25° C developed an internal temperature excess (i.e. temperature above ambient) of nearly 2° C while small-bodied females exceeded ambient by only 0.3° C. However, when held at 20° C, the temperature excess of large-bodied moths was much less than 1° C and small-bodied females did not differ from ambient. Such interactions between temperature and body size suggest that there should be stabilizing selection toward moderate-sized individuals and may explain the absence of size-related effects on fecundity and longevity previously reported for several other lepidopterans.  相似文献   
7.
A minimal lentivirus Tat.   总被引:7,自引:4,他引:3       下载免费PDF全文
  相似文献   
8.
J C Carroll  J B Brown  A J Reid 《CMAJ》1995,153(9):1283-1289
OBJECTIVE: To describe the experiences of female family physicians who practise obstetrics in balancing professional obligations with personal and family needs, given the unique challenges that such practice poses for these physicians. DESIGN: Qualitative study. SETTING: Ontario. PARTICIPANTS: A purposefully selected sample of nine female family physicians who met the criteria of being married, having children and currently practising obstetrics. OUTCOME MEASURES: Experiences of female family physicians and their strategies in their personal, family and professional lives that enable them to continue practising obstetrics. RESULTS: All participants continued to practise obstetrics because of the pleasure they derived from it, despite the challenges of balancing the unpredictable demands of obstetrics with their personal and family needs. To continue in obstetrics, they needed to make changes in their lives, either through a gradual, evolutionary process or in response to a critical event. Alterations to work and family arrangements permitted them to meet the challenges and led to increased satisfaction. Changes included making supportive call-group arrangements, limiting work hours and the number of births attended and securing help with household duties. CONCLUSIONS: An in-depth examination, through the use of qualitative methods, showed the reasons why some female family physicians continue to practise obstetrics despite the stressful aspects of doing so. This knowledge may be useful for women who are residents or experienced clinicians and who are considering including obstetrics in their practice.  相似文献   
9.
S B Carroll 《Cell》1990,60(1):9-16
  相似文献   
10.

Introduction

One in three people will be diagnosed with diabetes by 2050, and the proportion will likely be higher among Native Americans. Diabetes control is currently suboptimal in underserved populations despite a plethora of new therapies. Patient empowerment is a key determinant of diabetes control, but such empowerment can be difficult to achieve due to resource limitation and cultural, language and health literacy barriers. We describe a home-based educational intervention using Community Health Representatives (CHRs), leading to improvement in Patient Activation Measures scores and clinical indicators of diabetes control.

Methods

Sixty participants with type 2 diabetes (T2D) completed a baseline evaluation including physical exam, Point of Care (POC) testing, and the Patient Activation Measure (PAM) survey. Participants then underwent a one hour group didactic session led by Community Health Representatives (CHRs) who subsequently carried out monthly home-based educational interventions to encourage healthy lifestyles, including diet, exercise, and alcohol and cigarette avoidance until follow up at 6 months, when clinical phenotyping and the PAM survey were repeated.

Results

PAM scores were increased by at least one level in 35 (58%) participants, while 24 participants who started at higher baseline score did not change. Six months after intervention, mean levels of A1C decreased by 0.7 ± 1.2%; fasting blood glucose decreased by 24.0 ± 38.0 mg/dl; BMI decreased by 1.5 ± 2.1 kg/m2; total cholesterol decreased by 12.0± 28.0 mg/dl; and triglycerides decreased by 52.0 ± 71.0 mg/dl. All of these changes were statistically significant (p<0.05).

Conclusion

This six month, CHR led and community-oriented educational intervention helps inform standards of practice for the management of diabetes, engages diabetic populations in their own care, and reduces health disparities for the underserved population of Zuni Indians.

Trial Registration

ClinicalTrials.gov NCT02339311  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