全文获取类型
收费全文 | 25348篇 |
免费 | 1905篇 |
国内免费 | 7篇 |
出版年
2023年 | 229篇 |
2022年 | 280篇 |
2021年 | 636篇 |
2020年 | 482篇 |
2019年 | 549篇 |
2018年 | 837篇 |
2017年 | 775篇 |
2016年 | 906篇 |
2015年 | 1068篇 |
2014年 | 1149篇 |
2013年 | 1532篇 |
2012年 | 2078篇 |
2011年 | 2052篇 |
2010年 | 1095篇 |
2009年 | 878篇 |
2008年 | 1569篇 |
2007年 | 1571篇 |
2006年 | 1466篇 |
2005年 | 1300篇 |
2004年 | 1239篇 |
2003年 | 1160篇 |
2002年 | 982篇 |
2001年 | 355篇 |
2000年 | 408篇 |
1999年 | 289篇 |
1998年 | 136篇 |
1997年 | 125篇 |
1996年 | 111篇 |
1995年 | 106篇 |
1994年 | 104篇 |
1993年 | 97篇 |
1992年 | 165篇 |
1991年 | 121篇 |
1990年 | 99篇 |
1989年 | 97篇 |
1988年 | 84篇 |
1987年 | 99篇 |
1986年 | 71篇 |
1985年 | 75篇 |
1984年 | 98篇 |
1983年 | 52篇 |
1982年 | 69篇 |
1981年 | 68篇 |
1980年 | 63篇 |
1979年 | 62篇 |
1978年 | 51篇 |
1977年 | 48篇 |
1976年 | 52篇 |
1975年 | 59篇 |
1973年 | 46篇 |
排序方式: 共有10000条查询结果,搜索用时 587 毫秒
31.
32.
33.
In this paper, we present a model for the development of connections between muscle afferents and motoneurones in the human spinal cord. The model consists of a limb with six muscles, one motoneurone pool, one pooled (Ia-like) afferent for each muscle and a central programme generator. The weights of the connections between the afferents and the motoneurone pools are adapted during centrally induced movements of the limb. The connections between the afferents and the motoneurone pools adapt in a hebbian way, using only local information present at the synapses. This neural network is tested in two examples of a limb with two degrees of freedom and six muscles. Despite the simplifications, the model predicts the pattern of autogenic and heterogenic monosynaptic reflexes quite realistically. 相似文献
34.
35.
David W. Thomas Amabile K. Matida Célio L. Silva Thuioshi Ioneda 《Chemistry and physics of lipids》1979,23(3):267-282
The purification procedure of 6,6′-diesters of trehalose from Corynebacterium diphtheriae was modified and the isolated substance was analysed by mass spectrometry as its permethylated derivative. The fatty acid moiety released from the glycolipid after alkaline hydrolysis was studied by mass spectral analysis of the O-methylated and O-acetylated methyl ester derivatives. By argentation thin-layer chromatography, three species of O-acetylated methyl esters were recognized, corresponding to saturated, mono-unsaturated and di-unsaturated α-branched-β-hydroxylated fatty acids. The double bond was located by ozonolysis of the O-acetylated methyl ester derivatives, by gas chromatography of the reaction product and mass spectrometry of the effluent from the gas chromatograph. The main components of each species of α-branched-β-hydroxylated fatty acids found in the gly colipid fraction of C. diphtheriae were 2-tetradecyl-3-hydroxyoctadecanoic acid (C32H64O3, corynomycolic acid), 2-tetradecyl-3-hydroxy-11-octadecenoic acid (C32H62O3, corynomycolenic acid), 2-tetradec-7′-enyl-3-hydroxy octadecanoic acid (C32H62O3) and 2-tetradec-7′-enyl-3-hydroxy-11-octadecenoic acid (C32H60O3, corynomycoldienic acid). The glycolipid fraction from C. diphtheriae is obviously a complex mixture of 6,6′-diesters of trehalose. 相似文献
36.
37.
Catherine Bonaïti-Pellié Françoise Clerget-Darpoux Marie-Claude Babron 《Human genetics》1990,86(2):203-208
Summary Although the retinoblastoma gene has been isolated and sequenced, the difference in penetrance and expressivity among families has not yet been fully explained. Balanced chromosomal insertion involving the 13q14 regions has been shown to account for some families with several unaffected carriers. Since there could be cases with karyotypically undetectable insertions, we tested whether this mechanism was general enough to explain the whole difference in expressivity among families. Using 166 pedigrees, reported in nine series available in the literature (including our own), we conclude that balanced insertion cannot entirely explain the familial data, even if we allow for a reduced viability of unbalanced gametes. Other mechanisms are proposed and discussed in this paper. 相似文献
38.
39.
Gonçalo Matias Luís Miguel Rosalino José Luís Rosa Pedro Monterroso 《Population Ecology》2021,63(3):247-259
Population density data on depleted and endangered wildlife species are essential to assure their effective management and, ultimately, conservation. The European wildcat is an elusive and threatened species inhabiting the Iberian Peninsula, with fragmented populations and living in low densities. We fitted spatial capture–recapture models on camera-trap data, to provide the first estimate of wildcat density for Portugal and assess the most influential drivers determining it. The study was implemented in Montesinho Natural Park (NE Portugal), where we identified nine individuals, over a total effort of 3,477 trap-nights. The mean density estimate was 0.032 ± 0.012 wildcat/km2, and density tended to increase with distance to humanized areas, often linked to lower human disturbance and domestic cat presence, with forest and herbaceous vegetation cover and with European rabbit abundance. Although, this density estimate is within the range of values estimated for protected areas elsewhere in the Iberian Peninsula, our estimates are low at the European level. When put in context, our results highlight that European wildcats may be living in low population densities across the Iberian Mediterranean biogeographic region. No phenotypic domestic or hybrid cats were detected, suggesting potentially low admixture rates between the two species, although genetic sampling would be required to corroborate this assertion. We provide evidence that Montesinho Natural Park may be a suitable area to host a healthy wildcat population, and thus be an important protected area in this species' conservation context. 相似文献
40.
Christian Alvarez Glaucia Gon?alves Barbosa Raquel de Vasconcellos Carvalhaes de Oliveira Bernardina Penarrieta Morales Bodo Wanke Márcia dos Santos Lazéra 《Memórias do Instituto Oswaldo Cruz》2013,108(1):126-129
In this study, we evaluated several techniques for the detection of the yeast form of Cryptococcus in decaying wood and measured the viability of these fungi in environmental samples stored in the laboratory. Samples were collected from a tree known to be positive for Cryptococcus and were each inoculated on 10 Niger seed agar (NSA) plates. The conventional technique (CT) yielded a greater number of positive samples and indicated a higher fungal density [in colony forming units per gram of wood (CFU.g-1) ] compared to the humid swab technique (ST). However, the difference in positive and false negative results between the CT-ST was not significant. The threshold of detection for the CT was 0.05.103 CFU.g-1, while the threshold for the ST was greater than 0.1.103 CFU-1. No colonies were recovered using the dry swab technique. We also determined the viability of Cryptococcus in wood samples stored for 45 days at 25ºC using the CT and ST and found that samples not only continued to yield a positive response, but also exhibited an increase in CFU.g-1, suggesting that Cryptococcus is able to grow in stored environmental samples. The ST.1, in which samples collected with swabs were immediately plated on NSA medium, was more efficient and less laborious than either the CT or ST and required approximately 10 min to perform; however, additional studies are needed to validate this technique. 相似文献