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   31277篇
  免费   2756篇
  国内免费   22篇
  2023年   143篇
  2022年   310篇
  2021年   776篇
  2020年   415篇
  2019年   549篇
  2018年   654篇
  2017年   537篇
  2016年   965篇
  2015年   1631篇
  2014年   1709篇
  2013年   2000篇
  2012年   2632篇
  2011年   2679篇
  2010年   1632篇
  2009年   1353篇
  2008年   2009篇
  2007年   1985篇
  2006年   1930篇
  2005年   1655篇
  2004年   1679篇
  2003年   1476篇
  2002年   1439篇
  2001年   275篇
  2000年   201篇
  1999年   264篇
  1998年   298篇
  1997年   214篇
  1996年   197篇
  1995年   167篇
  1994年   149篇
  1993年   143篇
  1992年   114篇
  1991年   140篇
  1990年   102篇
  1989年   92篇
  1988年   82篇
  1987年   66篇
  1986年   82篇
  1985年   89篇
  1984年   92篇
  1983年   92篇
  1982年   111篇
  1981年   110篇
  1980年   87篇
  1979年   60篇
  1978年   45篇
  1977年   45篇
  1976年   47篇
  1975年   46篇
  1974年   46篇
排序方式: 共有10000条查询结果,搜索用时 156 毫秒
161.
Cholest-5-ene-3beta,26-diol, isolated from human brain, was further characterized by oxidation to 3-oxocholest-4-en-26-ol and to 3-oxocholest-4-en-26-oic acid. Identification was achieved by comparison (by t.l.c., g.l.c. and g.l.c.-mass spectrometry) with corresponding reference compounds derived from kryptogenin.  相似文献   
162.
Polyphenol oxidase (PPO) oxidizes, due to their tyrosine content, the proteins histone, casein and human serum albumin. These oxidations are inhibited by ascorbate which lowers the redox potential of the medium. Serum albumin in its native state is only moderately oxidized. If, however, prior to oxidation, the albumin is subjected to denaturation, involving unfolding of the chain, the attack by the enzyme is markedly increased. Such denaturation was effected by either the action of dodecyl sulfate or heating to 60°C. The implications of these findings to the problem of senescence are discussed.  相似文献   
163.
164.
165.
The karyotypes of the three diploid (n = 10) species of the subg. Diplothrix (Zinnia—Compositae) were compared to determine whether there were any demonstrable differences which could then be sought in their polyploid derivatives. Because many of the chromosomes in a set were too similar to distinguish confidently between them, a method of analysis was developed which measures the similarity of whole sets of chromosomes rather than individual ones. The method consists of measuring the distances between graph-plotted vertices representing arm lengths of chromosomes of real or paper hybrids and then comparing these distances by means of U tests with those similarly derived for the “parents.” This procedure obviates the need of attempting to identify morphologues (morphologically similar chromosomes) in a somatic diploid root-tip cell and to equate corresponding pairs of chromosomes from different cells of a single plant or from different species or hybrids. No demonstrable differences in the karyotypes of diploid cespitose zinnias were found. Analysis of previously published data by this method indicated that there has been a general non-objectivity and non-operationalism in the determination of homologous chromosomes, and a general but unwarranted assumption that morphologues are in reality genologues (genetically corresponding chromosomes).  相似文献   
166.
In a series of 240 cases of perforated peptic ulcer prior to 1952 there was a total mortality of 17 per cent. This included cases treated conservatively as a planned procedure or because of other morbid conditions and undiagnosed cases, all ending in death.In cases treated by simple suture, mortality was 2.6 per cent for patients under 50 years of age and 17 per cent for those over 50, the rate rising very steeply with each decade over 50.From 1953 through 1959 selective gastric resection was carried out, the operation being done in all cases of perforated gastric ulcers, of chronic duodenal ulcers and of perforations associated with hemorrhage.During this period 303 perforated ulcers were treated, with a mortality of 15.5 per cent. Patients with associated morbid conditions and those admitted moribund and died, or died undiagnosed were again included.In this series also the mortality increased steeply from age 50 onward.Gastric resection was carried out in 148 cases of perforated peptic ulcer with a mortality of 6 per cent. In 105 cases under 60 years of age there were no deaths but in those over 60 the mortality rate was 21.8 per cent. Results in the patients who had resection, as determined after three years of observation, compared favorably with the results in patients who were treated by suture.The mortality of perforated ulcers in females was higher than in males.Emergency gastric resection has a definite place in the treatment of perforated peptic ulcer.  相似文献   
167.
168.
169.
170.
Birefringence in endosperm mitosis   总被引:10,自引:0,他引:10  
  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