全文获取类型
收费全文 | 15947篇 |
免费 | 1766篇 |
国内免费 | 212篇 |
专业分类
17925篇 |
出版年
2023年 | 73篇 |
2022年 | 135篇 |
2021年 | 275篇 |
2020年 | 229篇 |
2019年 | 255篇 |
2018年 | 304篇 |
2017年 | 286篇 |
2016年 | 433篇 |
2015年 | 606篇 |
2014年 | 681篇 |
2013年 | 793篇 |
2012年 | 931篇 |
2011年 | 909篇 |
2010年 | 572篇 |
2009年 | 569篇 |
2008年 | 723篇 |
2007年 | 714篇 |
2006年 | 638篇 |
2005年 | 557篇 |
2004年 | 509篇 |
2003年 | 463篇 |
2002年 | 424篇 |
2001年 | 1173篇 |
2000年 | 1005篇 |
1999年 | 759篇 |
1998年 | 251篇 |
1997年 | 266篇 |
1996年 | 205篇 |
1995年 | 181篇 |
1994年 | 172篇 |
1993年 | 118篇 |
1992年 | 379篇 |
1991年 | 336篇 |
1990年 | 290篇 |
1989年 | 231篇 |
1988年 | 205篇 |
1987年 | 147篇 |
1986年 | 149篇 |
1985年 | 114篇 |
1984年 | 68篇 |
1983年 | 72篇 |
1981年 | 32篇 |
1979年 | 39篇 |
1978年 | 32篇 |
1976年 | 43篇 |
1975年 | 41篇 |
1973年 | 42篇 |
1972年 | 52篇 |
1971年 | 46篇 |
1970年 | 34篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
91.
92.
93.
Zhuang XC Sun YZ Cui J Zhu JM Jiang C Xiang QL Li CS 《Journal of gravitational physiology : a journal of the International Society for Gravitational Physiology》1994,1(1):P61-P63
In the environment of microgravity, the disused atrophy of skeletal muscle, especially leg's muscle, would occur. The three purposes of this study were: 1. To observe the dynamic changes of disused atrophy of skeletal muscle under simulated weightlessness; 2. To approach the mechanism of disused atrophy of muscle; 3. To approach the countermeasures for reducing the degree of atrophy of muscle. 相似文献
94.
Conrad GW Luer CA Paulsen AQ Funderburgh JL 《Transactions of the Kansas Academy of Science. Kansas Academy of Science》1993,96(1-2):62-68
Morphogenesis of the clearnose skate, Raja eglanteria, was not significantly inhibited as a result of 7 days of exposure to 1-2 mM selenate in the sea water during Days 59-69 of embryonic development (hatching would normally have occurred at 82 +/- 4 days of incubation). Although corneal transparency appeared normal in the eye, preliminary measurements of the thickness of Bowman's layer of the cornea suggested that it was significantly thinner in the corneas of embryos exposed to 1-2 mM selenate. Selenate is an ion reported to inhibit sulfation of glycosaminoglycans in connective tissue. 相似文献
95.
Induction of B-A transitions of deoxyoligonucleotides by multivalent cations in dilute aqueous solution. 总被引:4,自引:1,他引:3 下载免费PDF全文
Circular dichroism (CD) spectra of d(CCCCGGGG) in the presence of Co(NH3)6(3+) are very similar to spectra of r(CCCCGGGG). In contrast, B-form characteristics are observed for d(CCCCGGGG) in the presence of Na+ and Mg2+, even at high salt concentrations. Spermidine induces modest changes of the CD of d(CCCCGGGG). The NMR chemical shifts of the nonexchangeable protons of d(CCCCGGGG) in the absence and presence of Co(NH3)6(3+) were assigned by proton two-dimensional (2D) NOESY and COSY measurements. The chemical shifts of the GH8 protons of d(CCCCGGGG) move upfield upon titration with Co(NH3)6Cl3. The sums of the sugar H1' coupling constants decrease with added Co(NH3)6Cl3. Cross peak intensities in the 2D proton NOESY spectra show a transformation from B-DNA to A-DNA characteristics upon the addition of Co(NH3)6Cl3. The temperature-dependent 59Co transverse and longitudinal relaxation rates demonstrate that Co(NH3)6(3+) is site-bound to the oligomer. Such localization is not a general feature of Co(NH3)6(3+) binding to oligonucleotides. 59Co NMR relaxation and CD measurements demonstrate chiral discrimination by d(CCCCGGGG) for the two stereoisomers of Co(en)3(3+). Both stereoisomers bind tightly as judged by 59Co NMR, and both cause large (but nonequivalent) changes in the CD of this oligomer. 相似文献
96.
The effect of hydration on the dynamics of trimethoprim bound to dihydrofolate reductase. A deuterium NMR study. 下载免费PDF全文
To determine the effect of hydration on the dynamics of a protein complex, we used deuterium nuclear magnetic resonance (NMR) techniques to examine a trimethoprim (TMP)/E. coli dihydrofolate reductase (DHFR) complex in its lyophilized, partially hydrated, polycrystalline, and ammonium sulfate-precipitated states. The results indicate that TMP is rigid in the lyophilized powder state. The dynamic behavior could be restored by partial rehydration. At 30 wt% hydration the deuterium spectrum of the partially hydrated sample was indistinguishable from that of the polycrystalline and ammonium sulfate-precipitated samples, suggesting that the structure of the protein/TMP complex is similar in the three physical states. Furthermore, we found that the para- and meta-methoxyl groups have very different dynamical behavior. 相似文献
97.
The localization of neuropeptide Y (NPY) and atrial natriuretic peptide (ANP) in the endothelial cells of human umbilical blood vessels was studied using the pre-embedding peroxidase-antiperoxidase (PAP) technique for electron microscopy and avidin-biotin-complex (ABC) immunostaining for endothelial cells cultured from umbilical vein. Subpopulations of NPY- and ANP-immunoreactive endothelial cells were present in term umbilical vein and artery. The umbilical vein contained more positive cells than the artery. The percentage of NPY- and ANP-immunoreactive umbilical vein cells in culture was 32% and 44%, respectively, out of a total of 3013 cells examined. The possibility that these potent vasoactive substances located in the endothelial cells of the non-innervated umbilical vessels are involved in the local regulation of blood flow is discussed. 相似文献
98.
Alyssa C. Bader Aimée E. Carbaugh Jenny L. Davis Krystiana L. Krupa Ripan S. Malhi 《American journal of physical anthropology》2023,181(4):527-534
In the United States, the Native American Graves Protection and Repatriation Act (NAGPRA) provides a specific framework for the disposition of Native American Ancestral remains within its purview. However, samples such as a bone fragment, tooth, or other biological tissue taken from the remains of these Ancestors have been treated by institutions and researchers as independent of the individual from whom they were removed and used in destructive research such as paleogenomic and other archaeometric analyses without consultation, consent, and collaboration from Native American communities; are not cared for in keeping with the current best practices for Indigenous Ancestors; and are not likely to be repatriated to their communities. Here, we demonstrate that any biological samples removed from Ancestors who are covered under NAGPRA must also be handled according to the stipulations defined for “human remains” within the legislation. As such, we are not proposing a change to existing legislation, but rather best practices, specific to the context of the United States and NAGPRA, relating to the use of and care for biological samples taken from Native American Ancestors. 相似文献
99.
Leukocyte CD18 monoclonal antibody worsens endotoxemia and cardiovascular injury in canines with septic shock 总被引:5,自引:0,他引:5
100.
Le Q Darcel C Merriman M Beauregard M Avery RJ Kasting R 《Canadian journal of comparative medicine and veterinary science》1963,27(4):81-84
Homogenized brain, extracts, and residues of brain from normal and scrapie sheep were inoculated into 116 sheep. Of 72 sheep inoculated with scrapie material 27 developed the disease, whereas four of 44 inoculated with ;normal' brain material showed symptoms similar to those of scrapie. The scrapie agent survived extraction for 18 hours with diethyl ether followed by water for 24 hours. Similarly the agent survived extraction in a Soxhlet apparatus with 95 per cent ethanol containing 2.0 per cent 4 M HC1 for 24 hours. There was no evidence of demyelination in sections of brain and spinal cord supporting the view that allergic encephalomyelitis is not responsible for the scrapie symptoms. Liver function, as measured by the bromsulfalein test, remained normal until just before death. These results preclude that liver dysfunction contributes to the disease but do not exclude the possibilities of other metabolic derangements. 相似文献