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   22829篇
  免费   1604篇
  国内免费   9篇
  24442篇
  2023年   126篇
  2022年   313篇
  2021年   587篇
  2020年   354篇
  2019年   473篇
  2018年   598篇
  2017年   488篇
  2016年   814篇
  2015年   1237篇
  2014年   1359篇
  2013年   1681篇
  2012年   1982篇
  2011年   1917篇
  2010年   1187篇
  2009年   1035篇
  2008年   1393篇
  2007年   1310篇
  2006年   1202篇
  2005年   1095篇
  2004年   994篇
  2003年   950篇
  2002年   781篇
  2001年   153篇
  2000年   147篇
  1999年   159篇
  1998年   134篇
  1997年   123篇
  1996年   95篇
  1995年   99篇
  1994年   97篇
  1993年   101篇
  1992年   69篇
  1991年   70篇
  1990年   88篇
  1989年   64篇
  1988年   71篇
  1987年   51篇
  1986年   54篇
  1985年   76篇
  1984年   65篇
  1983年   54篇
  1982年   47篇
  1981年   67篇
  1980年   55篇
  1979年   49篇
  1978年   72篇
  1977年   50篇
  1976年   53篇
  1975年   46篇
  1974年   44篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
191.
13C-N.m.r. spectra of all possible 1,6:2,3- and 1,6:3,4-dianhydro-β-D-hexo-pyranoses and their O-acetyl and deoxy derivatives are presented. Relations between chemical shifts of certain carbon atoms and the structure of the dianhydrides are outlined, and their application in structural analysis is discussed. Inversion of configuration of the oxirane ring from the endo to the exo position is associated with typical upfield-shifts for oxirane-ring carbon atoms C-2 or C-4, respectively. Possible inter-relationships between 13C-chemical shifts and steric and polar interactions in the dianhydro derivatives are discussed.  相似文献   
192.
Studies examining an association between artificial cranial deformation and the presence of wormian bones in the lambdoid suture have been inconclusive. Cranial deformation, however, does not seem to have a direct effect of increasing the sutural complexity of the pars lambdica of the lambdoid suture and also increasing the mean number of lambdoidal wormian bones, given their presence.  相似文献   
193.
Summary Normal mouse spleen cells take up in vitro radioactively labeled immune RNA. RNA taken up is present in nuclei, polysomes, membranes and cytoplasm. About 20–40% of immune RNA is nonspecifically associated with cell surface. 45% of RNA taken up is degraded and reutilized inside the cells within 2 hours.This work was supported by the Polish Academy of Sciences within the project 09.7.4.1.1.  相似文献   
194.
Rats poisoned with abrin (2.5 micrograms/100 g body weight) died within 36 h with severe necrosis of acinar pancreatic cells. Incorporation in vivo of labelled amino acids into pancreatic protein was greatly impaired 6 h after poisoning. Microsomes isolated from the pancreas of poisoned rats at 6 h had a reduced capacity for protein synthesis in vitro. Incorporation in vivo of orotic acid into pancreatic RNA was decreased 12 h after poisoning.  相似文献   
195.
196.
Sheila S.  Anderson  John R.  Baker  John H.  Prime  Anna  Baird 《Journal of Zoology》1979,189(3):407-417
Pup production and mortality of beach breeding Grey seals was studied at two contrasting types of rookery, the cliff-bound island of Ramsey, Dyfed, and the low, grassy island of Auskerry, Orkney. Thirty-five per cent of pups died on Ramsey compared with 14 % on Auskerry. Almost half of the Ramsey mortality was accounted for by animals being lost from the beaches so that they were not available for analysis. The main proximate causes of death revealed by post-mortem examination were starvation and infections. The other conditions which accounted for a small proportion of deaths were drowning, trauma, non-viability, stillbirths, atelectasis, dystokia and heart abnormality. Certain pathogens associated with disease were common to both sites, while others were specific to Ramsey or Auskerry.
The most important ultimate cause of pup deaths appeared to be failure of the mother/pup bond. The differences in levels of mortality between the sites is thought to be due to differences in the topography of the beaches. Pup survival on the narrow cliff-bound beaches, or in caves, on Ramsey was reduced either directly through pups being washed off, or indirectly by overcrowding of animals at high tide.  相似文献   
197.
Phosphoethanolaminetransferase of high specific activity was solubilized from rat liver microsomes with the non-ionic detergent octyl glucoside. The solubilization method is fast and simple, allowing for processing of large amounts of material. The solubilized enzyme is stable. It contains virtually no phosphocholinetransferase activity. A preliminary characterization of the enzyme, with both diacyl- and alkylacyl-glycerol as substrate, is given. For the reaction, the lipid substrates were incorporated into artificial phospholipid bilayers (liposomes).  相似文献   
198.
A benzene extract of the trunk of an Aniba species (Lauraceae) contained benzyl benzoate, benzyl salicylate, sitosterol and the neolignans (2S,3S,3aR)-3a-allyl-5-methoxy-3-methyl-2-piperonyl-2,3,3a,6-tetrahydro-6-oxobenzofuran (burchellin); (2S,3S,3aR)-3a-allyl-5-methoxy-3-methyl-2-veratryl-2,3,3a,6-tetrahydro-6-oxobenzofuran; (2S,3S,3aR)-3a-allyl-5,7-dimethoxy-3-methyl-2-veratryl-2,3,3a,6-tetrahydro-6-oxobenzofuran; (2S,3S,5S)-5-allyl-5-methoxy-3-methyl-2-veratryl-2,3,5,6-tetrahydro-6-oxo-benzofuran; (2R,3R)-7-methoxy-3-methyl-5-propenyl-2-veratryl-2,3-dihydrobenzofuran; rel-(1R,5R,6R,7R,8S)-1-allyl-8-hydroxy-3-methoxy-7-methyl-4-oxo-6-piperonylbicyclo[3,2,1]oct-2-ene (guianin); rel-(1S,5S,6S,7R,8R)-1-allyl-8-hydroxy-3,5-dimethoxy-7-methyl-4-oxo-6-piperonylbicyclo[3,2,1]oct-2-ene; rel-(1S,5S,6S,7R,8R)-8-acetoxy-1-allyl-3-hydroxy-5-methoxy-7-methyl-4-oxo-6-piperonyl-bicyclo[3,2,1]oct-2-ene; rel-1S,5S,6S,7R,8R)-8-acetoxy-3,5-dimethoxy-7-methyl-4-oxo-6-piperonylbicyclo[3,2,1]oct-2-ene; rel-(1R,5S,6R,7R)-1-allyl-3-methoxy-7-methyl-4,8-dioxo-6-piperonylbicyclo[3,2,1]oct-2-ene.  相似文献   
199.
200.
Forty-five patients with disseminated breast cancer were given a trial of combination chemotherapy consisting of fluorouracil, adriamycin, and cyclophosphamide (FAC) and immunotherapy with BCG given by scarification. The results were compared with those in a comparable group of 44 patients treated with FAC alone immediately before the chemoimmunotherapy study. The remission rates (73% and 76% for FAC and FAC-BCG respectively) were similar in both studies. The durations of remission for patients on FAC-BCG (medium 12 months) were longer than remissions achieved for patients given FAC alone (median 8 months) (P = 0.068). The most notable effect of BCG was on survival. Thus 21 out of 34 patients achieving remission on FAC-BCG were alive at the time of the last follow-up examination (median over 22 months) compared with 11 out of 32 patients achieving remission on FAC (median 15 months) (P = 0.01). Twenty-six of the 45 patients given FAC-BCG were alive at the time of the last follow-up examination (median over 22 months) compared with 12 of the 44 patients given FAC (median 15 months) (P = 0.005). Although the apparent benefit of BCG could be explained by a maldistribution of some prognostic factors, the data suggest that further trial of chemoimmunotherapy of breast cancer should be carried out.  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