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   436836篇
  免费   48332篇
  国内免费   494篇
  2016年   4649篇
  2015年   6955篇
  2014年   8000篇
  2013年   11221篇
  2012年   12697篇
  2011年   12646篇
  2010年   8274篇
  2009年   7529篇
  2008年   11084篇
  2007年   11565篇
  2006年   11059篇
  2005年   10688篇
  2004年   10538篇
  2003年   9975篇
  2002年   9985篇
  2001年   19231篇
  2000年   19840篇
  1999年   15777篇
  1998年   5298篇
  1997年   5472篇
  1996年   5116篇
  1995年   4875篇
  1994年   4840篇
  1993年   4821篇
  1992年   12607篇
  1991年   12154篇
  1990年   11792篇
  1989年   11410篇
  1988年   10760篇
  1987年   10246篇
  1986年   9739篇
  1985年   9797篇
  1984年   8138篇
  1983年   7006篇
  1982年   5626篇
  1981年   5238篇
  1980年   4712篇
  1979年   7932篇
  1978年   6399篇
  1977年   5856篇
  1976年   5524篇
  1975年   6235篇
  1974年   6846篇
  1973年   6760篇
  1972年   6300篇
  1971年   5715篇
  1970年   4929篇
  1969年   4912篇
  1968年   4506篇
  1967年   3780篇
排序方式: 共有10000条查询结果,搜索用时 565 毫秒
941.
The importance of functional parameters for evaluating the severity of low back pain is gaining clinical recognition, with evidence suggesting that the angular velocity of lordosis is critical for identification of musculoskeletal deficits. However, there is a lack of data regarding the range of functional kinematics (RoKs), particularly which include the changing shape and curvature of the spine. We address this deficit by characterising the angular velocity of lordosis throughout the thoracolumbar spine according to age and gender. The velocity of lumbar back shape changes was measured using Epionics SPINE during maximum flexion and extension activities in 429 asymptomatic volunteers. The difference between maximum positive and negative velocities represented the RoKs. The mean RoKs for flexion decreased with age; 114°/s (20–35 years), 100°/s (36–50 years) and 83°/s (51–75 years). For extension, the corresponding mean RoKs were 73°/s, 57°/s and 47°/s. ANCOVA analyses revealed that age and gender had the largest influence on the RoKs (p<0.05). The Epionics SPINE system allows the rapid assessment of functional kinematics in the lumbar spine. The results of this study now serve as normative data for comparison to patients with spinal pathology or after surgical treatment.  相似文献   
942.
943.
944.
945.
The genotoxicity of ptaquiloside (PT), recently isolated from bracken fern and shown to be carcinogenic, was examined by means of the hepatocyte primary culture/DNA-repair test. PT elicited clear unscheduled DNA synthesis with a dose-response effect. The result indicates that PT is a genotoxic carcinogen.  相似文献   
946.
Lethal mutations in the T/t complex cause stage-specific morphologic abnormalities during early embryogenesis of mice. Although mutant embryos are lethal at the early stages of development, we have succeeded in establishing several cell lines from one of these mutants ( T/T ). Mutant-specific abnormality was not observed in gross morphology and growth patterns of cells. They, however, retained the characters of freshly dissociated embryonic cells to form smaller aggregates than the wild-type in rotation-mediated aggregation.
One of the T/T cell lines (T-1) formed tumors when injected into one-day-old syngeneic and allogeneic host, Expression of H-2 antigens was serologically studied with H-2 specificity 5 as a marker antigen. All lines except T-1 were shown to have this specificity.  相似文献   
947.
948.
949.
950.
Only 20-25% of families screened for BRCA1/2 mutations are found positive. Because only a positive result is informative, we studied the role of BRCA1/2 immunohistochemistry as an additional method for patient selection. From 53 high-risk-affected probands, 18 (34%) had available paraffin blocks of their tumors and were selected for this study. Mutation screening was done by conformation-sensitive gel electrophoresis and multiplex ligation-dependent probe amplification. For immunohistochemistry, 21 neoplastic specimens (15 breast carcinomas, 5 ovary neoplasms, and 1 rectal adenocarcinoma) were analyzed with BRCA1 (monoclonal antibody, Ab-1, oncogene) and BRCA2 (polyclonal antibody, Ab-2, oncogene) antibodies. Absence of the BRCA1 protein was confirmed in negative tumors by Western blotting. Seven patients were positive for BRCA1/2 mutations: 5 for BRCA1 and 2 for BRCA2. Four out of five positive patients had tumors negative for BRCA1 immunostaining, and the remaining 13 BRCA1-negative patients had positive BRCA1 immunostaining in all tumor samples. Sensitivity to predict for BRCA1 mutation carriers was 80%, and specificity was 100%, with a positive predictive value of 100% and a negative predictive value of 93%. This correlation was statistically significant (p=0.001). No correlation was observed for BRCA2. If larger studies confirm these results, high-risk patients with BRCA1-negative tumors should be screened first for this gene.  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