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   1495776篇
  免费   142702篇
  国内免费   1479篇
  2021年   16167篇
  2019年   16198篇
  2018年   18178篇
  2017年   17043篇
  2016年   28491篇
  2015年   42753篇
  2014年   50815篇
  2013年   77093篇
  2012年   41417篇
  2011年   30778篇
  2010年   44956篇
  2009年   45907篇
  2008年   28994篇
  2007年   27218篇
  2006年   31826篇
  2005年   32860篇
  2004年   31928篇
  2003年   29583篇
  2002年   27463篇
  2001年   41900篇
  2000年   39601篇
  1999年   37014篇
  1998年   25277篇
  1997年   25120篇
  1996年   24495篇
  1995年   23919篇
  1994年   23794篇
  1993年   22987篇
  1992年   33017篇
  1991年   31238篇
  1990年   30036篇
  1989年   30740篇
  1988年   28261篇
  1987年   27133篇
  1986年   25556篇
  1985年   27274篇
  1984年   26007篇
  1983年   23039篇
  1982年   22433篇
  1981年   21357篇
  1980年   20102篇
  1979年   22819篇
  1978年   20578篇
  1977年   19448篇
  1976年   18669篇
  1975年   18581篇
  1974年   19349篇
  1973年   19518篇
  1972年   16878篇
  1971年   15288篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
993.
994.
995.
Free flaps in the elderly.   总被引:3,自引:0,他引:3  
Microsurgical transfer of tissue has become a primary tool of the reconstructive surgeon. The elderly, as a growing segment of our society, are requiring free-tissue transfers in proportion to their numbers. To investigate the potential morbidity of free-tissue transfers in the elderly, we studied consecutive populations of 31 patients above the age of 65 years and 90 patients below the age of 65 years. Complication rates were 65 and 49 percent, respectively. Premorbid medical conditions were present in 87 percent of patients 65 years and older and in 72 percent of those under 65 years. Medically related complications in free-tissue transfers, previously unreported in the literature, were 35 percent in the elderly group and 10 percent in the younger group. Wound-healing complications were seen in equal proportions between groups. The rates of wound and medically related complications observed in the elderly group were nearly double those observed in the younger group; however, after correction for the presence of preexisting medical conditions, no significant differences were seen between the two groups. These observations suggest that age alone is not a variable in risk for free-tissue transfers. Elective microsurgery can be performed in the elderly patient with a high expectation of success.  相似文献   
996.
A new species, Pseudopecoelus sewelli, is described from the deep-sea fish Hoplostethus atlanticus from off southeastern Australia. It is an elongate form with considerable variation in such important characters as the anterior vitelline extent, the sucker ratio and the egg-size. The features distinguishing this species from five related forms are combinations of the sucker ratio, egg-size, lobation of gonads, presence or absence of a cirrus-sac and the position of the genital pore.  相似文献   
997.
Magic angle spinning 31P-NMR (MAS 31P-NMR) spectra of bovine rod outer segments, unphosphorylated and phosphorylated, were obtained. In the phosphorylated samples the spectra showed new resonances not assignable to phospholipids. These signals were present only when stimulation of receptor phosphorylation occurred. These resonances were not due to exogenous, soluble phosphorus-containing compounds. Limited proteolysis to remove the carboxyl-terminal region of the photoreceptor that contains the phosphorylation sites removed these resonances. The chemical shifts were in the usual range for serine phosphate and threonine phosphate. The pKa obtained from a pH titration of the 31P chemical shift was typical of serine phosphate. Therefore, these 31P-NMR resonances were assigned to the phosphorylation sites on membrane proteins in the rod outer segment disk membranes. Static 31P-NMR measurements revealed that at least some of these sites gave rise to relatively narrow resonances, indicative of considerable motional freedom of the carboxyl-terminal segment of the photoreceptor when phosphorylated. These data indicate that it is possible to study phosphorylation sites on membrane proteins using MAS 31P-NMR, and that using in vivo 31P 'spin labelling' one can study directly and selectively regions of receptors crucial to receptor function.  相似文献   
998.
MURRAY  B. G. 《Annals of botany》1990,65(6):691-698
Observations on the floral biology of Luculia gratissima (Rubiaceae)showed that this species is distylous with complementary positioningof anthers and stigmas in the two floral forms. Unusual featuresof distyly in this species include the larger size of the corolla,the stigma surface and the stigmatic papillae in the thrum flowerscompared to the pin ones. Stigmatic surfaces have similar secretionsbut they appear more copious in thrum than pin. The floral dimorphismwas accompanied by a very effective self-incompatibility systemand no seed was set on selfing. Seed number per capsule on crossingwas significantly greater in thrum flowers compared to pin.Incompatible pollen tubes were inhibited within 24 h at thebase of the stigma/top of the style in both morphs. Amputationof this region of the gynoecium removed the self-incompatibilityreaction in thrum but not pin flowers. Pollination with a mixtureof compatible and incompatible pollen and sequential pollinationwith self followed by cross pollen showed that there were interactionsbetween the two types of pollen tube. The presence of compatibletubes was found to cause the excessive swelling of the pollen-tubetip of the incompatible ones. The incompatible tubes did notappear to have any effect on the growth of compatible ones. Luculia gratissima, distyly, floral biology, self-incompatibility, pollen-tube interactions  相似文献   
999.
1000.
Peculiarities of the axons growth in the culture of 14-day old chick embryo spinal cord after 24, 48 hr, 3, 5 and 7 days in the Maximov's chamber were observed. For the stimulation of axon growth the spinal cord was cultivated simultaneously with the explants of the muscle tissue and in the medium after the addition of supernatant of the somatic muscle. It has been demonstrated that the growth of the axons stimulated with the muscle explants or muscle supernatant takes place through the growth cones, while in the absence of growth stimulation effect glial cells can take part in the axons growth. It is supposed that the glial cells are capable of playing the role of the cells, which direct axons growth in the absence of influence of specific target factor.  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