全文获取类型
收费全文 | 330151篇 |
免费 | 39152篇 |
国内免费 | 203篇 |
专业分类
369506篇 |
出版年
2016年 | 3709篇 |
2015年 | 4903篇 |
2014年 | 5748篇 |
2013年 | 8557篇 |
2012年 | 10568篇 |
2011年 | 10909篇 |
2010年 | 6911篇 |
2009年 | 6025篇 |
2008年 | 9616篇 |
2007年 | 9927篇 |
2006年 | 9360篇 |
2005年 | 9012篇 |
2004年 | 8741篇 |
2003年 | 8721篇 |
2002年 | 8394篇 |
2001年 | 12315篇 |
2000年 | 12617篇 |
1999年 | 9930篇 |
1998年 | 3925篇 |
1997年 | 4042篇 |
1996年 | 4098篇 |
1995年 | 3785篇 |
1994年 | 3797篇 |
1993年 | 3735篇 |
1992年 | 8489篇 |
1991年 | 8225篇 |
1990年 | 7995篇 |
1989年 | 7906篇 |
1988年 | 7451篇 |
1987年 | 7402篇 |
1986年 | 6849篇 |
1985年 | 7066篇 |
1984年 | 6031篇 |
1983年 | 5407篇 |
1982年 | 4360篇 |
1981年 | 4191篇 |
1980年 | 3809篇 |
1979年 | 6153篇 |
1978年 | 4863篇 |
1977年 | 4631篇 |
1976年 | 4428篇 |
1975年 | 4786篇 |
1974年 | 5224篇 |
1973年 | 5106篇 |
1972年 | 4690篇 |
1971年 | 4260篇 |
1970年 | 3736篇 |
1969年 | 3762篇 |
1968年 | 3366篇 |
1967年 | 2891篇 |
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
991.
Rachel M. Haywood Andrew W. D. Claxson Geoffrey E. Hawkes David P. Richardson Declan P. Naughton Gregory Coumbarides Jane Hawkes Edward J. Lynch Martin C. Grootveld 《Free radical research》1995,22(5):441-482
High field (400 and 600 MHz) proton NMR spectroscopy has been employed to investigate the thermally-induced autoxidation of glycerol-bound polyunsaturated fatty acids present in intact culinary frying oils and fats. Heating of these materials at 180°C for periods of 30, 60 and 90 min. generated a variety of peroxidation products, notably aldehydes (alkanals, trans-2-alkenals and alka-2, 4-dienals) and their conjugated hydroperoxydiene precursors. Since such aldehydes appear to be absorbed into the systemic circulation from the gut in vivo, the toxicological significance of their production during standard frying practices is discussed. 相似文献
992.
993.
OBJECTIVE--To determine the incidence of haemorrhagic disease of the newborn in the British Isles, study risk factors, and examine the effect of vitamin K prophylaxis. DESIGN--Prospective survey of all possible cases of haemorrhagic disease of the newborn as reported by consultant paediatricians using the monthly notification cards of the British Paediatric Surveillance Unit and a follow up questionnaire for each case to validate the diagnosis and accrue further data. SETTING--Britain (England, Scotland, and Wales) and Ireland (Northern Ireland and the Irish Republic) during December 1987 to March 1990. PATIENTS--27 infants classified as having confirmed (n = 25) or probable (n = 2) haemorrhagic disease of the newborn. RESULTS--24 of the 27 infants were solely breast fed. 10 suffered intracranial haemorrhage; two of these died and there was clinical concern about the remainder. 20 infants had received no vitamin K prophylaxis, and seven had received oral prophylaxis. Relative risk ratios for these groups compared with babies who had received intramuscular vitamin K were 81:1 and 13:1 respectively. Six infants had hepatitis (alpha 1 antitrypsin deficiency in four), unsuspected until presentation with haemorrhagic disease of the newborn, of whom four had received oral prophylaxis. One other baby had prolonged jaundice. One mother had taken phenytoin during pregnancy. CONCLUSIONS--All newborn infants should receive vitamin K prophylaxis. Intramuscular vitamin K is more effective than oral prophylactic regimens currently used in the British Isles. 相似文献
994.
995.
W R Engels 《BioEssays : news and reviews in molecular, cellular and developmental biology》1992,14(10):681-686
The P family of transposable genetic elements is thought to be a recent addition to the Drosophila melanogaster genome. New evidence suggests that the elements came from another Drosophila species, possibly carried by parasitic mites. The transposition mechanism of P elements involves DNA gap repair which may have facilitated their rapid spread through D. melanogaster worldwide. These results provide new insight into the process of a transposon's invasion into a new species and the potential risk of extinction such an invasion might entail. 相似文献
996.
I C Zambrano A T Kowal L E Mortenson M W Adams M K Johnson 《The Journal of biological chemistry》1989,264(35):20974-20983
The two iron-only hydrogenases (I and II) from Clostridium pasteurianum have been investigated by variable temperature magnetic circular dichroism (MCD) and electron paramagnetic resonance (EPR) spectroscopies. Samples were studied both reduced with dithionite under an atmosphere of H2 and after oxidation with thionine. The results are consistent with four and two [4Fe-4S]1+,2+ (F)-clusters in hydrogenases I and II, respectively. All four F-clusters are reduced and paramagnetic in reduced hydrogenase I, with up to one exhibiting an S = 3/2 ground state and the remainder having conventional S = 1/2 ground states. Both F-clusters have S = 1/2 ground states in reduced hydrogenase II; however, one appears to be only partially reduced under the conditions used for reduction. MCD studies of the oxidized enzymes show no temperature-dependent features in the visible region which can be attributed to the EPR-active S = 1/2 hydrogen-activating cluster, suggesting predominantly oxygen and nitrogen coordination for the iron atoms of this center. However, temperature-dependent MCD transitions arising from a hitherto undetected S greater than 1/2 Fe-S clusters are apparent in both oxidized hydrogenases. Detailed EPR studies of oxidized hydrogenase I revealed resonances from an S = 3/2 species, however, spin quantitation reveals this to be a trace component that is unlikely to be responsible for the observed low temperature MCD spectrum. The nature and origin of these S greater than 1/2 Fe-S clusters are discussed in light of the available spectroscopic data for these and other iron-only hydrogenases. 相似文献
997.
998.
A method by which virus penetration through condoms can be tested with simple, inexpensive equipment is described. The method uses chi X174 bacteriophage as the challenge virus and physiologically relevant pressure. Penetration by 0.1 microliters (or less) of challenge suspension can be readily detected. As examples, latex and natural-membrane condoms were examined. 相似文献
999.
Cytokine production in a model of wound healing: the appearance of MIP-1, MIP-2, cachectin/TNF and IL-1 总被引:9,自引:1,他引:8
T J Fahey B Sherry K J Tracey S van Deventer W G Jones J P Minei S Morgello G T Shires A Cerami 《Cytokine》1990,2(2):92-99
Macrophages are essential for normal wound repair and many of their effects on healing wounds are likely to be mediated by the secretion of cytokines. This study examines the appearance of messenger RNA (mRNA) for cachectin/tumor necrosis factor (TNF), IL 1, and macrophage inflammatory proteins 1 and 2 (MIP-1 and MIP-2), as well as the mature peptides, in a model of wound healing using wound chambers. RNA for all four cytokines can be detected in wound inflammatory cells by polymerase chain reaction amplification throughout the first 7 days. Cachectin/TNF and IL 1 protein levels peaked on the first day after wound chamber implantation, and MIP-1 and MIP-2 were detected only on day 3. The data suggest that these cytokines participate in the early inflammatory response to wounding. 相似文献
1000.
W. T. Brown 《American journal of human genetics》1996,58(5):903