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   44308篇
  免费   4137篇
  国内免费   34篇
  48479篇
  2022年   271篇
  2021年   604篇
  2020年   336篇
  2019年   411篇
  2018年   554篇
  2017年   513篇
  2016年   928篇
  2015年   1593篇
  2014年   1738篇
  2013年   2205篇
  2012年   2760篇
  2011年   2841篇
  2010年   1856篇
  2009年   1792篇
  2008年   2606篇
  2007年   2621篇
  2006年   2558篇
  2005年   2519篇
  2004年   2456篇
  2003年   2424篇
  2002年   2373篇
  2001年   498篇
  2000年   391篇
  1999年   568篇
  1998年   673篇
  1997年   479篇
  1996年   435篇
  1995年   419篇
  1994年   401篇
  1993年   410篇
  1992年   341篇
  1991年   312篇
  1990年   331篇
  1989年   244篇
  1988年   298篇
  1987年   274篇
  1986年   269篇
  1985年   331篇
  1984年   375篇
  1983年   337篇
  1982年   434篇
  1981年   432篇
  1980年   413篇
  1979年   267篇
  1978年   298篇
  1977年   265篇
  1976年   274篇
  1975年   209篇
  1974年   280篇
  1973年   251篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
Summary Anthraquinones produced by suspension cultures of Cinchona ledgeriana are released into the medium, which becomes saturated with products late in the growth cycle. When a high affinity polymeric adsorbent, such as the macro-reticular Amberlite XAD-7, is added to the culture the concentration of anthraquinone in the medium is maintained at a low level and production may be stimulated 15-fold, yielding up to 20 mg/1/day. More than 90% of the product is released from the cells. For maximal yields it is shown that both the amount of adsorbent used and the time after sub-culture at which it is added to the system are critical. The value of such a method for product recovery from immobilised cells is discussed.  相似文献   
92.
93.
Increased longevity and recently intensified emphasis on gerodontology mandate that dental students'exposure to clinical care of the elderly be enhanced. The extent to which individual students can be exposed to geriatric dental care depends on the availability of elderly patients to dental school clinics and the treatment needs of these patients. The purposes of this study were (1) to determine the dental treatment needs of geriatric patients who seek care at a dental school, and (2) to ascertain if differences exist between the needs of older versus relatively young geriatric patients. Data collected on 128 elderly patients during a three month period indicate that 57% of the aged were either edentulous at examination or treatment planned to become edentulous and receive two full dentures. The remaining 43% were treatment planned to remain dentulous and receive therapies other than full upper and lower dentures. More older geriatric patients required full dentures than their younger cohorts. More young elderly required prophylaxes, scalings, root planing therapy, dental restorations, and partial dentures. Additional to their denture requirements, aged patients appear to have sufficient non-prosthetic needs to allow for meaningful gerodontic experience by students.  相似文献   
94.
95.
Translation of encephalomyocarditis virus RNA in rabbit reticulocyte lysates in the presence of N-formyl-[(35)S]methionine-tRNA(f) (Met) revealed that a small polypeptide is cleaved from the N-terminus of the capsid protein precursor, preA, by virus-coded protease activity. Therefore, this N-terminal segment comprising the translation initiation site is not conserved in any of the mature capsid proteins.  相似文献   
96.
Abstract: The combined techniques of HPLC and radioimmunoassay were used to identify and quantitate enkephalin-related peptides in the guinea pig hippocampus. Both met- and leu-enkephalin were identified, in approximately a 2:1 ratio, as well as a third enkephalin-like molecule that is neither met- nor leu-enkephalin. The third enkephalin elutes earlier than met- or leu-enkephalin from a reversed-phase column, has a molecular weight similar to the other enkephalins, and is as active as these enkephalins are in inhibiting binding of labeled opiates to rat brain membranes. All regions of the hippocampus (dentate gyrus, CA1–2, CA3–4, and subiculum) contain all three immunoreactive peptides. Immunocytochemical techniques, using antisera raised against met-enkephalin, show with one antiserum immunoreactivity in the granule cell-mossy fiber system, and with the other scattered immunoreactive cells mostly in the CA2 region. Enkephalins are not confined to the mossy fiber system, as previously suggested, but may be a component of another hippocampal innervation.  相似文献   
97.
98.
Inbred mice and lines congenic to them for the major histocompatibility complex were similar in susceptibility to EAE except for moderate differences in two pairs. TheH-2 haplotypesq, s, andb occurred in inbred strains and in congenic lines of high, medium, or low susceptibility. It is concluded that the major histocompatibility complex does not control susceptibility to EAE in mice. Furthermore, the low susceptibility of DBA/1J mice was not enhanced by poly A-U.  相似文献   
99.
100.
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