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   20479篇
  免费   1936篇
  国内免费   21篇
  22436篇
  2022年   139篇
  2021年   268篇
  2020年   138篇
  2019年   212篇
  2018年   267篇
  2017年   208篇
  2016年   386篇
  2015年   678篇
  2014年   750篇
  2013年   932篇
  2012年   1234篇
  2011年   1294篇
  2010年   786篇
  2009年   691篇
  2008年   984篇
  2007年   1114篇
  2006年   1022篇
  2005年   1007篇
  2004年   1002篇
  2003年   973篇
  2002年   945篇
  2001年   242篇
  2000年   162篇
  1999年   228篇
  1998年   277篇
  1997年   198篇
  1996年   184篇
  1995年   215篇
  1994年   193篇
  1993年   178篇
  1992年   184篇
  1991年   162篇
  1990年   151篇
  1989年   138篇
  1988年   145篇
  1987年   143篇
  1986年   146篇
  1985年   166篇
  1984年   184篇
  1983年   158篇
  1982年   218篇
  1981年   228篇
  1980年   226篇
  1979年   151篇
  1978年   153篇
  1977年   134篇
  1976年   171篇
  1975年   127篇
  1974年   143篇
  1973年   158篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
31.
Prostaglandins E1, F and A2 were covalently joined to the surface of Sepharose as carboxamide linkages. The insolubilized prostaglandins were shown to function effectively in the purification of 15(S) -hydroxyprostaglandin dehydrogenase.  相似文献   
32.
Mutants of Escherichia coli K12 which accumulated the haem precursor porphobilinogen are described. The mutants grew very slowly on carbon and energy sources which K12 uses only oxidatively, and they had low catalase activities, suggesting that they were deficient in haem. Extracts had one-tenth of the parental activity of the enzyme porphobilinogen deaminase. In transduction, the mutation mapped close to genes ilvD and metE at minute 84. The gene was tentatively identified as hemC, coding for porphobilinogen deaminase. The gene symbol hemC replaces the earlier and temporary symbol popE.  相似文献   
33.
34.
35.
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   
36.
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   
37.
38.
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   
39.
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   
40.
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