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   702663篇
  免费   80545篇
  国内免费   856篇
  784064篇
  2018年   6381篇
  2016年   8662篇
  2015年   12289篇
  2014年   14367篇
  2013年   19975篇
  2012年   22962篇
  2011年   23562篇
  2010年   15816篇
  2009年   14258篇
  2008年   20617篇
  2007年   21471篇
  2006年   20095篇
  2005年   19301篇
  2004年   19407篇
  2003年   18473篇
  2002年   17992篇
  2001年   29561篇
  2000年   29562篇
  1999年   23687篇
  1998年   8832篇
  1997年   8882篇
  1996年   8475篇
  1995年   8155篇
  1994年   7881篇
  1993年   7811篇
  1992年   20076篇
  1991年   19608篇
  1990年   19302篇
  1989年   18726篇
  1988年   17391篇
  1987年   16571篇
  1986年   15494篇
  1985年   15376篇
  1984年   12750篇
  1983年   11197篇
  1982年   8590篇
  1981年   7730篇
  1980年   7247篇
  1979年   12379篇
  1978年   9715篇
  1977年   8869篇
  1976年   8288篇
  1975年   9335篇
  1974年   10202篇
  1973年   10066篇
  1972年   9084篇
  1971年   8498篇
  1970年   7214篇
  1969年   7044篇
  1968年   6371篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
993.
Craniofacial fibrous dysplasia, a skeletal disorder most often affecting the mandible or maxilla, can cause a variety of complications. Nasal obstruction secondary to this unusual disorder presents a diagnostic and therapeutic challenge. The nasal obstruction in this case is due to expansion of fibrous lesions in adjacent bones as well as direct involvement of the turbinates with fibrous dysplasia. We describe the surgical correction of nasal obstruction in this unique setting and present a brief update of this intriguing disorder.  相似文献   
994.
In addition to the immunostimulating activity, bone marrow mediators, myelopeptides (MP) show the dose-dependent effect on the development of pain sensitivity in mice. When injected in nanogram amounts, MP induce hyperalgesia and 3-9 fold higher production of antibodies against SREC. When injected in milligram amounts, they exhibit hypoalgesic effect and no influence on antibody production. Immunostimulating effect in MP (mol, mass less than 1 KD, fraction 3) is accompanied with hypoalgesia. Bone marrow factors of mol. masses 40-150 KD (fraction 1) eluted at Sephadex G-25 gel-filtration before MP enhance the pain sensitivity tHreshold and show a potent immunodepressive effect. Thus the bone marrow factors are capable of exhibiting the opposite effects on the immune system in the pain control system that evidences the tight interrelation between these systems.  相似文献   
995.
The structure and mathematical model of PARATI, a detailed computer programme developed for the assessment of the radiological consequences of an accidental contamination of urban areas, is described with respect to the scenarios used for the estimation of exposure fields in a village or town, the models for the initial and secondary contamination with the radionuclide 137Cs, the concepts for calculating the resulting radiation exposures and the changes with time of the contamination and radiation fields. Kerma rates at various locations in tropical urban areas are given, and the contribution of different contaminated surfaces to these rates after dry or wet deposition are discussed. Received: 12 April 1996 / Accepted in revised form: 30 August 1996  相似文献   
996.
A full length cDNA encoding human pro-alpha 2(V) collagen was constructed. Partial sequencing of the cDNA and primer extension analysis of mRNA from fibroblasts found that pro-alpha 2(V) mRNA differs from the mRNAs of other fibrillar collagens in the increased length of its 5'-untranslated region. The pro-alpha 2(V) cDNA was placed downstream of the human cytomegalovirus immediate early promoter/regulatory sequences for expression studies in cultured Chinese hamster lung cells. These cells have been shown previously to synthesize large quantities of pro-alpha 1(V) homotrimers as their only collagenous product. Transfection resulted in a number of clonal cell lines that express human alpha 2(V) RNA at levels comparable to, and in some cases greater than, levels found in normal human skin fibroblasts. Pro-alpha 2(V) chains produced in the majority of clonal lines were of sufficient quantity to complex all available endogenous pro-alpha 1(V) chains. Chimeric heterotrimers, composed of hamster alpha 1(V) and human alpha 2(V) chains in a 2:1 ratio, were stable to pepsin digestion and were found predominantly associated with the cell layer. Surprisingly, pro-alpha 2(V) chains, in excess to pro-alpha 1(V) chains, were found in the extracellular matrix and, in much greater abundance, in media. These chains were pepsin sensitive, indicating that pro-alpha 2(V) chains can be secreted as nonstable homotrimers or as free chains.  相似文献   
997.
In this study we investigated whether alpha-tocopherol can be spared by ubiquinol-3 during autoxidation of multilamellar liposome. A lipophilic azocompound, 2,2'-azobis-(2,4-dimethyl-valeronitrile), was chosen to initiate liposome autoxidation. The effect of either alpha-tocopherol, ubiquinol-3, or a mixture of them was compared. Rates of conjugated diene formation and concomitant disappearance of the two antioxidants was measured. Since the inhibition rate constant for the scavenging of peroxyl radical for alpha-tocopherol was higher than that for quinol-3, it was concluded that alpha-tocopherol is regenerated by ubiquinol-3.  相似文献   
998.
999.
1000.
The self-association in aqueous solution of folic acid (FA), 7,8-dihydrofolic acid (DHFA) and 5,6,7,8-tetrahydrofolic acid (THFA) has been studied by the use of proton magnetic resonance (1H NMR) spectroscopy. At concentrations below 10 mM, all three folates exist in (monomer)2 in equilibrium dimer equilibria with association constants (Ka) equal to 400, 66 and 14 M-1 for FA, DHFA and THFA respectively. These values decreased markedly to 157, 18 and 3 M-1, for FA, DHFA and THFA respectively, in the presence of 0.8 M KCl. The high extent of dimerization of FA is believed to impede the interaction with the active site of dihydrofolate reductase (DHFR) rendering it a poor substrate. In contrast, the DHFA with a much lower Ka is a better substrate. Conditions that lower the Ka of both FA and DHFA, (i.e., 0.8M KCl) turn them into better substrates. Based on the findings of the present study, it is also predicted that dihydro MTX may be a better inhibitor of DHFR than MTX.  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