全文获取类型
收费全文 | 19276篇 |
免费 | 813篇 |
国内免费 | 44篇 |
专业分类
20133篇 |
出版年
2022年 | 158篇 |
2021年 | 347篇 |
2020年 | 196篇 |
2019年 | 312篇 |
2018年 | 470篇 |
2017年 | 474篇 |
2016年 | 804篇 |
2015年 | 1084篇 |
2014年 | 1001篇 |
2013年 | 1678篇 |
2012年 | 1239篇 |
2011年 | 995篇 |
2010年 | 1042篇 |
2009年 | 932篇 |
2008年 | 1032篇 |
2007年 | 956篇 |
2006年 | 974篇 |
2005年 | 767篇 |
2004年 | 633篇 |
2003年 | 553篇 |
2002年 | 536篇 |
2001年 | 323篇 |
2000年 | 402篇 |
1999年 | 296篇 |
1998年 | 175篇 |
1997年 | 140篇 |
1996年 | 102篇 |
1995年 | 82篇 |
1994年 | 95篇 |
1993年 | 90篇 |
1992年 | 190篇 |
1991年 | 145篇 |
1990年 | 156篇 |
1989年 | 167篇 |
1988年 | 129篇 |
1987年 | 125篇 |
1986年 | 117篇 |
1985年 | 105篇 |
1984年 | 105篇 |
1983年 | 78篇 |
1982年 | 71篇 |
1981年 | 63篇 |
1980年 | 64篇 |
1979年 | 89篇 |
1978年 | 64篇 |
1977年 | 60篇 |
1976年 | 50篇 |
1975年 | 51篇 |
1974年 | 49篇 |
1972年 | 52篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
Kinship and friendship are key human relationships. Increasingly, data suggest that people are not less altruistic toward friends than close kin. Some accounts suggest that psychologically we do not distinguish between them; countering this is evidence that kinship provides a unique explanatory factor. Using the Implicit Association Test, we examined how people implicitly think about close friends versus close kin in three contexts. In Experiment 1, we examined generic attitudinal dispositions toward friends and family. In Experiment 2, attitude similarity as a marker of family and friends was examined, and in Experiments 3 and 4, strength of in-group membership for family and friends was examined. Findings show that differences exist in implicit cognitive associations toward family and friends. There is some evidence that people hold more positive general dispositions toward friends, associate attitude similarity more with friends, consider family as more representative of the in-group than friends, but see friends as more in-group than distant kin. 相似文献
142.
Memory impairment and morphological changes in rats induced by active fragment of anti-nerve growth factor-antibody 总被引:2,自引:0,他引:2
T Nabeshima S Ogawa H Ishimaru T Kameyama T Fukuta R Takeuchi K Hayashi 《Biochemical and biophysical research communications》1991,175(1):215-219
Treatment of rats with a specific Fab' fragment of anti-nerve growth factor (NGF)-antibody (anti-NGF, 12, 120 and 400 micrograms/4 weeks, i.c.v.) impaired their learning ability. The distance of swimming of anti-NGF-treated rats in a water maze was shortened more slowly by training than that of control rats. Anti-NGF treatment altered the staining of nuclei of cells in the hippocampus, parietal cortex and dentate gyrus with hematoxylin. It is suggested that the anti-NGF-induced amnesia could be due to change in nuclear morphology. 相似文献
143.
This paper deals with global statistical unsupervised magnetic resonance image (MRI) segmentation based on a Markovian model combined with the family of mixture identification algorithms EM (Hidden Markov Random Field [HMRF-EM]) and using the Bootstrap sampling, which allows a greater respect to the data independence assumption and a reduction of algorithmic complexity. This leads not only to a flexibility in the control of running time but also an improvement in the classification quality relatively to the performance shown by an earlier version of the bootstrapped HMRF-EM. 相似文献
144.
145.
146.
Neurovascular injury comprises a wide spectrum of pathophysiology that underlies the progression of brain injury after cerebral
ischemia. Recently, it has been shown that activation of the integrin-associated protein CD47 mediates the development of
blood–brain barrier injury and edema after cerebral ischemia. However, the mechanisms that mediate these complex neurovascular
effects of CD47 remain to be elucidated. Here, we compare the effects of CD47 signaling in brain endothelial cells, astrocytes,
and pericytes. Exposure to 4N1 K, a specific CD47-activating peptide derived from the major CD47 ligand thrombospondin-1,
upregulated two major neurovascular mediators, vascular endothelial growth factor (VEGF) and matrix metalloproteinase-9 (MMP-9),
in brain endothelial cells and astrocytes. No changes were detected in pericytes. These findings may provide a potential mechanism
for CD47-induced changes in blood–brain barrier homeostasis, and further suggest that CD47 may be a relevant neurovascular
target in stroke. 相似文献
147.
148.
Yu. V. Kharitonov E. E. Shul’ts M. M. Shakirov M. A. Pokrovsky A. G. Pokrovsky G. A. Tolstikov 《Russian Journal of Bioorganic Chemistry》2012,38(1):107-115
Coupling of methyl 16-aminomethyllambertianate with N-Boc-protected ω-amino acids resulted in 16-(N-Boc-aminononan)- and 16-(N-Boc-aminoundecan)amidomethyllabdanoids. Interaction of methyl aminomethyllambertianate with bicyclo[2.2.1]hept-5-en-2,3-dicarboxylic acid anhydride led to the amide of bicyclo[2.2.1]heptan-1,2-dicarboxylic acid with a labdanoid substituent. Reaction of methyl 16-aminomethyllambertianate with chloroacetyl chloride resulted in methyl 16-(chloroacetylaminomethyl)lambertianate; coupling of the latter with methyl esters of amino acids gave the corresponding amides of methyl lambertianate. The compounds obtained were more cytotoxic toward CEM-13, MT-4, and U-937 tumor cell lines as compared with lambertianic acid; the dose inhibiting tumor cell viability by 50% (CCID50) of the more active compounds was 3.9–9.9 μM. 相似文献
149.
A mathematical model has been constructed to describe the growth dynamics of various plant communities of halophytic meadows depending on the temperature factor and degree of soil salinity. Field investigation of the yields of halophytic meadow plant communities were performed in the coastal area of Kurinka Lake in the Altaiskii district of the Republic of Khakasia in 2004 and 2006. The results of field investigations and model studies show that there is a correlation between plant growth and air temperature for plant communities growing on soils with the lowest and medium salinity levels. It was proven in model studies that for the plant communities that grow on highly saline (3.58%) soils, not only air temperature but also the salinity level of the soil should be taken into account. 相似文献
150.
I. D. Konstantinova I. V. Fateev G. A. Galegov P. G. Deryabin A. G. Botikov I. S. Muzyka D. K. L’vov A. I. Miroshnikov 《Russian Journal of Bioorganic Chemistry》2013,39(5):530-538
The biotechnological method of synthesis of the antiviral drug ribavirin based on the transglycosylation reaction was improved due to the addition of catalytic amounts of sodium arsenate. This approach allows us to hydrolyze the excess natural nucleoside guanosine, a ribose donor, and, hence, made the composition of the reaction mixture less complicated, thus facilitating the process of ribavirin isolation. It was shown that in cell cultures the combination of ribavirin and oseltamivir carboxylate inhibited the replication of the influenza A virus more effectively than each of them alone. Similar results were obtained in experiments on laboratory animals (mouse Balb/c) infected with the influenza A virus H3N2/Aichi/68 strain. 相似文献