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   29591篇
  免费   2520篇
  国内免费   23篇
  32134篇
  2022年   182篇
  2021年   386篇
  2020年   238篇
  2019年   296篇
  2018年   519篇
  2017年   466篇
  2016年   754篇
  2015年   1202篇
  2014年   1251篇
  2013年   1562篇
  2012年   2088篇
  2011年   2001篇
  2010年   1270篇
  2009年   1017篇
  2008年   1547篇
  2007年   1692篇
  2006年   1563篇
  2005年   1482篇
  2004年   1466篇
  2003年   1350篇
  2002年   1331篇
  2001年   318篇
  2000年   236篇
  1999年   273篇
  1998年   325篇
  1997年   228篇
  1996年   216篇
  1995年   255篇
  1994年   218篇
  1993年   204篇
  1992年   219篇
  1991年   199篇
  1990年   174篇
  1989年   158篇
  1988年   172篇
  1987年   156篇
  1986年   163篇
  1985年   199篇
  1984年   219篇
  1983年   199篇
  1982年   252篇
  1981年   253篇
  1980年   246篇
  1979年   170篇
  1978年   172篇
  1977年   154篇
  1976年   206篇
  1975年   151篇
  1974年   157篇
  1973年   179篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
41.
42.
43.
Via Oswalt’s system of classification, I compare the tool-kits of wild and captive capuchins with those of Tanzanian chimpanzees and Tasmanian aborigines. The results indicate that capuchins have tool-kits that are smaller, and have lower ratios of artifacts to naturefacts, than those of Tanzanian chimpanzees and Tasmanian aborigines. Accordingly, Oswalt’s system can be used productively to assess the relative technological skills of monkeys versus those of apes and humans.  相似文献   
44.
The enzymatic activity of salivary amylase bound to the surface of several species of oral streptococci was determined by the production of acid from starch and by the degradation of maltotetraose to glucose in a coupled, spectrophotometric assay. Most strains able to bind amylase exhibited functional enzyme on their surface and produced acid from the products of amylolytic degradation. These strains were unable to utilise starch in the absence of salivary amylase. Two strains failed to produce acid from starch, despite the presence of functional salivary amylase, because they could not utilise maltose. Strains that could not bind salivary amylase failed to produce acid from starch. In no case was all the bound salivary amylase active, and two strains of Streptococcus mitis which bound amylase did not exhibit any enzyme activity on their cell surface. The ability to bind amylase may confer a survival advantage on oral bacteria which inhabit hosts that consume diets containing starch.  相似文献   
45.
46.
Objectives To understand how the policy of user involvement is interpreted in health service organisations and to identify factors that influence how user involvement is put into practice.Design Ethnographic study using participant observation, interviews, and collection of documentary evidence.Setting A multiagency modernisation programme to improve stroke services in two London boroughs.Participants Service users, National Health Service managers, and clinicians.Results User involvement in the programme was initiated and led by professionals. Professionals determined the areas of service improvement service users could participate in. A wide range of activities were considered “user involvement,” from patient satisfaction surveys to service users delivering peer support. Involvement tended to be most active in the least technical areas and areas with least input from clinicians. Factors that might explain this included organisational structure, the vagueness of the concept of user involvement, the value attributed to service users’ experiential knowledge, and variations in professional and service user understandings of and commitment to involvement. The gains of involvement were harder to identify in terms of impact on services. More evident were the personal gains for those involved: satisfaction of feeling listened to by professionals, social opportunities of meeting others in a similar situation, and increased knowledge about stroke and services available.Conclusions User involvement may not automatically lead to improved service quality. Healthcare professionals and service users understand and practise user involvement in different ways according to individual ideologies, circumstances, and needs. Given the resource implications of undertaking user involvement in service development there is a need for critical debate on the purpose of such involvement as well as better evidence of the benefits claimed for it.  相似文献   
47.
Following the programme started at Janssen Research Foundation searching for 5-HT(2A/2C) antagonists, we now report on the synthesis of a series of substituted 2-(Dimethylaminomethyl)-2,3,3a,8-tetrahydrodibenzo[c,f]isoxazolo[2,3-a]azepine derivatives. The 5-HT(2A), 5-HT(2C) and H(1) receptor affinities as well as the mCPP antagonistic activity of the compounds synthesised is described.  相似文献   
48.
Numerous functional ergatoid replacement reproductives were found in one colony of Nasutitermes columbicus in Panama. Their morphology was mainly workerlike, although several imaginal characters such as the compound eyes and variable wing buds were more or less developed. The sex organs were fully mature and the fat body of the females, not of the males, was of the “royal” type. The development of the eyes was not accompanied by the differentiation of the optic lobes of the brain, nor was the presence of wing buds correlated with a development of the wing muscles.  相似文献   
49.
50.
Medical history     
Charles G. Roland 《CMAJ》1983,129(11):1234-1235
  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