全文获取类型
收费全文 | 1552篇 |
免费 | 202篇 |
国内免费 | 52篇 |
专业分类
1806篇 |
出版年
2022年 | 21篇 |
2021年 | 40篇 |
2020年 | 19篇 |
2019年 | 28篇 |
2018年 | 39篇 |
2017年 | 29篇 |
2016年 | 40篇 |
2015年 | 56篇 |
2014年 | 58篇 |
2013年 | 90篇 |
2012年 | 68篇 |
2011年 | 76篇 |
2010年 | 55篇 |
2009年 | 60篇 |
2008年 | 57篇 |
2007年 | 80篇 |
2006年 | 67篇 |
2005年 | 62篇 |
2004年 | 45篇 |
2003年 | 40篇 |
2002年 | 61篇 |
2001年 | 33篇 |
2000年 | 34篇 |
1999年 | 31篇 |
1998年 | 26篇 |
1997年 | 21篇 |
1996年 | 12篇 |
1995年 | 16篇 |
1994年 | 17篇 |
1993年 | 13篇 |
1992年 | 25篇 |
1991年 | 23篇 |
1990年 | 27篇 |
1988年 | 24篇 |
1987年 | 18篇 |
1986年 | 17篇 |
1985年 | 15篇 |
1984年 | 13篇 |
1983年 | 12篇 |
1982年 | 14篇 |
1980年 | 14篇 |
1979年 | 14篇 |
1978年 | 21篇 |
1977年 | 12篇 |
1976年 | 15篇 |
1974年 | 14篇 |
1972年 | 11篇 |
1971年 | 19篇 |
1970年 | 14篇 |
1968年 | 15篇 |
排序方式: 共有1806条查询结果,搜索用时 0 毫秒
1.
This study has demonstrated that aneurysms occur in about 50 percent of the rat femoral arteries subjected to microvascular anastomosis. These aneurysms are consistent histologically--being characterized by medial necrosis, loss of elastic lamellae, and subintimal hyperplasia. Mechanical trauma is implicated as a factor in their pathogenesis, though further study in necessary to define the exact etiology. 相似文献
2.
3.
BACKGROUND: Concern has been raised about the potential for adverse cognitive effects associated with the use of calcium-channel blockers (CCBs) in older people. This study was undertaken to examine prospectively the association between the use of these and other antihypertensive drugs and cognitive function. METHODS: The authors examined data from the Canadian Study of Health and Aging (CSHA), a population-based, prospective 5-year investigation of the epidemiology of dementia and other health problems in Canadians 65 years of age and older. The risk of cognitive decline, as indicated by a decline in performance on the Modified Mini-Mental State (3MS) examination over the 5-year period, was assessed in relation to the use of antihypertensive and diuretic drugs by 205 subjects with a history of hypertension and no evidence of dementia at baseline. RESULTS: The proportion of subjects whose cognitive performance declined over the study period was significantly higher in the group using CCBs than in the group using other antihypertensive agents (75% v. 59%). The adjusted odds ratio (OR) for a significant decline in cognitive performance (defined as a decrease in 3MS score of 10 points or more) was 2.28 (95% confidence interval [CI] 1.12-4.66) for subjects using CCBs. The adjusted ORs (and 95% CIs) for cognitive decline in subjects using selected antihypertensive agents or diuretics relative to those exposed to beta-blockers were as follows: angiotensin-converting-enzyme inhibitor, OR 1.36 (95% CI 0.41-4.55); diuretic or other antihypertensive drug, OR 1.45 (95% CI 0.51-4.14); dihydropyridine CCB (nifedipine), OR 1.94 (95% CI 0.52-7.27) and non-dihydropyridine CCB (diltiazem or verapamil), OR 3.72 (95% CI 1.22-11.36). INTERPRETATION: Older people taking CCBs were significantly more likely than those using other agents to experience cognitive decline. These findings are consistent with the results of previous cross-sectional research and emphasize the need for further trials to examine the associations between CCB use, blood pressure and cognitive impairment in elderly patients. 相似文献
4.
Background
Ubiquitin (Ub)-mediated signaling is one of the hallmarks of all eukaryotes. Prokaryotic homologs of Ub (ThiS and MoaD) and E1 ligases have been studied in relation to sulfur incorporation reactions in thiamine and molybdenum/tungsten cofactor biosynthesis. However, there is no evidence for entire protein modification systems with Ub-like proteins and deconjugation by deubiquitinating enzymes in prokaryotes. Hence, the evolutionary assembly of the eukaryotic Ub-signaling apparatus remains unclear. 相似文献5.
6.
Assay of DNA-RNA hybrids by S1 nuclease digestion and adsorption to DEAE-cellulose filters. 总被引:11,自引:1,他引:10 下载免费PDF全文
A fast and accurate assay procedure for DNA-RNA hybrids is described in which exhaustive digestion of unhybridized DNA with S1 nuclease is followed by binding of hybrids to filter discs of DEAE-cellulose. The digested DNA can be efficiently washed from the filters so that background levels of 0.1-0.2% of input tracer DNA can be achieved, in contrast to the much higher (approximately 1-5%) backgrounds obtained using TCA precipitation procedures. Short duplexes, as small as 36 nucleotides in length, which are inefficiently bound to hydroxyapatite, are quantitatively bound to the DEAE-cellulose filters. 相似文献
7.
8.
9.
The binding of a ligand to a one-dimensional lattice in the presence of a second ("rider") ligand, which binds only to the first ligand (piggy-back binding), is studied. A model derived from this study is used to analyze the effects of co-operativity on the reaction rates of enzymes activated by polymeric cofactors that provide multiple binding sites for the enzyme. It is found that in the presence of strong co-operativity, the steady-state reaction rates of polymer-activated enzymes can be very different from the Michaelis-Menten paradigm. By adjusting the co-operativity parameters and the binding constants of the ligands, the model can generate apparent auto-catalytic enhancement by substrates at low substrate concentrations and apparent substrate inhibition at high substrate concentrations. The model is shown to be able to explain the differences in the rates of ATP hydrolysis by DNA gyrase in the presence of long versus short DNA molecules and in the presence of long DNA molecules at different gyrase to DNA ratios. 相似文献
10.