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   788786篇
  免费   94333篇
  国内免费   319篇
  2018年   6663篇
  2016年   8655篇
  2015年   11530篇
  2014年   13962篇
  2013年   19937篇
  2012年   23143篇
  2011年   23737篇
  2010年   15316篇
  2009年   14089篇
  2008年   21152篇
  2007年   22115篇
  2006年   20885篇
  2005年   20019篇
  2004年   19764篇
  2003年   19104篇
  2002年   18574篇
  2001年   36396篇
  2000年   37094篇
  1999年   28925篇
  1998年   9888篇
  1997年   10377篇
  1996年   9937篇
  1995年   9470篇
  1994年   9386篇
  1993年   9388篇
  1992年   24418篇
  1991年   24168篇
  1990年   23491篇
  1989年   22858篇
  1988年   21328篇
  1987年   20319篇
  1986年   18823篇
  1985年   19030篇
  1984年   15765篇
  1983年   13811篇
  1982年   10489篇
  1981年   9653篇
  1980年   9122篇
  1979年   15295篇
  1978年   11943篇
  1977年   10961篇
  1976年   10202篇
  1975年   11452篇
  1974年   11899篇
  1973年   11732篇
  1972年   10809篇
  1971年   9625篇
  1970年   8481篇
  1969年   8033篇
  1968年   7279篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
No guidelines are currently available that advise which soil properties of arable land can be used to suggest suitable locations for heathland reconstruction. This paper reviews studies comparing soil properties of heathland or semi-natural grassland with those of adjacent arable fields, investigations in the autecology of the dominant heathland plant, Calluna vulgaris (common heather), and long-term experiments of fertilizer inputs on arable soils. Three properties must be assessed before the suitability of a field can be determined: extractable phosphorus, exchangeable calcium, and pH. A number of other nutrients may also be important, but evidence is currently insufficient to substantiate this. Natural changes in levels of extractable phosphorus, exchangeable calcium, and pH appear to be very slow, so nutrient stripping and acidification will be necessary where recommended levels are exceeded to successfully restore heathland vegetation.  相似文献   
62.
OBJECTIVE--To examine the relation between bone density and indices of calcium metabolism including parathyroid hormone and 25-hydroxyvitamin D concentrations in middle aged women. DESIGN--A cross sectional study. SETTING AND SUBJECTS--138 women volunteers aged 45-65 with no known osteoporosis and unselected for disease status recruited for a dietary assessment study from the community using general practice registers. Volunteer rate was 20%. MAIN OUTCOME MEASURE--Bone mineral density measured with dual energy x ray absorptiometry. RESULTS--Bone density at the lumbar spine and neck and trochanteric regions of the femur was inversely related to serum intact parathyroid hormone concentrations and positively related to serum 25-hydroxyvitamin D concentrations. These associations were independent of possible confounding factors, including age, body mass index, cigarette smoking habit, menopausal status, and use of diuretics and postmenopausal hormone replacement therapy. These associations were apparent throughout the whole distribution of bone density and 25-hydroxyvitamin D and parathyroid hormone concentrations within the normal range, suggesting a physiological relation. CONCLUSIONS--The findings are consistent with the hypothesis that parathyroid hormone and 25-hydroxyvitamin D concentrations influence bone density in middle aged women. Findings from this study together with other work suggest that the role of vitamin D in osteoporosis should not be neglected. The associations with parathyroid hormone also indicate plausible biological mechanisms. The roughly 5-10% difference in bone density between top and bottom tertiles of serum 25-hydroxyvitamin D concentrations, though not large in magnitude, may have considerable public health implications in terms of prevention of osteoporosis and its sequelae, fractures.  相似文献   
63.
64.
65.
66.
Over the past 10 years the AIDS crisis has produced a large volume of writing. Much of this is documentary. Dozens of studies of AIDS from various clinical and political perspectives have been complemented by just as many published diaries, autobiographies, novels, plays, and poems. A few of these works have risen to the surface not only as extraordinarily valuable testimonies to the changes AIDS has wrought in individual and collective life but also as first-rate literary works, worth reading because beyond their immediate purposes they articulate with extraordinary lucidity and compassion some deep truths about the human--and the modern--condition. Paul Monette's Borrowed Time is among the most distinctive of those. It speaks not only for the community of people with AIDS and those who support them but for a generation.  相似文献   
67.
68.
Trigger finger is a relatively common clinical entity, most frequently caused by stenosing tenosynovitis. Several other conditions not related to tenosynovitis also have been described as a cause of triggering, and these have been reviewed. We present a rare anomaly of the fourth lumbrical muscle insertion as a cause of triggering of the right little finger. This was completely relieved following excision of the anomalous muscle. This rare anatomic variant should be added to the list of potential causes of trigger finger.  相似文献   
69.
70.
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