全文获取类型
收费全文 | 120536篇 |
免费 | 6555篇 |
国内免费 | 10045篇 |
专业分类
137136篇 |
出版年
2023年 | 1669篇 |
2022年 | 3271篇 |
2021年 | 3827篇 |
2020年 | 2656篇 |
2019年 | 3279篇 |
2018年 | 3077篇 |
2017年 | 2669篇 |
2016年 | 3160篇 |
2015年 | 5008篇 |
2014年 | 8568篇 |
2013年 | 9206篇 |
2012年 | 6472篇 |
2011年 | 7527篇 |
2010年 | 5488篇 |
2009年 | 5677篇 |
2008年 | 5743篇 |
2007年 | 6281篇 |
2006年 | 4913篇 |
2005年 | 4096篇 |
2004年 | 3173篇 |
2003年 | 2919篇 |
2002年 | 2635篇 |
2001年 | 2094篇 |
2000年 | 1825篇 |
1999年 | 1733篇 |
1998年 | 1555篇 |
1997年 | 1425篇 |
1996年 | 1406篇 |
1995年 | 1569篇 |
1994年 | 1446篇 |
1993年 | 1389篇 |
1992年 | 1332篇 |
1991年 | 1282篇 |
1990年 | 1055篇 |
1989年 | 1070篇 |
1988年 | 986篇 |
1987年 | 889篇 |
1986年 | 797篇 |
1985年 | 1304篇 |
1984年 | 1823篇 |
1983年 | 1311篇 |
1982年 | 1531篇 |
1981年 | 1301篇 |
1980年 | 1138篇 |
1979年 | 1062篇 |
1978年 | 759篇 |
1977年 | 742篇 |
1976年 | 689篇 |
1974年 | 482篇 |
1973年 | 490篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
Abstract: Slices cut from five frozen human brains were dissected into 2-mm cubes and assayed for choline acetyltransferase (ChAT) activity and protein content. A pattern of enrichment of ChAT activity was found ventral to the anterior commissure; this finding is consistent with the location of the enzyme in the cells of the nucleus basalis of Meynert. The region beneath the anterior commissure was the only place a discrete enrichment of activity could be found, and the precise topography of the enrichment was somewhat variable from brain to brain. The results are discussed in the light of recent knowledge concerning the source of the cortical cholinergic innervation. 相似文献
112.
The uptake of two different preparations of99mTechnetium-methylene diphosphonate in fetal rat calvaria is compared. The localization of99mTc after administration of99mTc(Sn)-MDP and99mTc-MDP showed equal distribution in autoradiography. 相似文献
113.
Restriction mapping of the rRNA genes from Artemia larvae 总被引:4,自引:0,他引:4
J Cruces J Sebastián J Renart 《Biochemical and biophysical research communications》1981,98(2):404-409
A restriction endonuclease analysis of the genes coding for the ribosomal RNA from Artemia larvae has shown that these genes consist of a repeat unit of 16.2 kilobase pairs (10.7 Mdaltons) and that the repeat unit seems to be homogeneous in size. 相似文献
114.
Abstract Both the intracellular and the extracellular concentration of cyclic AMP increases logarithmically in synchronously growing cultures of Escherichia coli . Thus, cyclic AMP by itself cannot regulate growth and division of the bacterium during the cell cycle. 相似文献
115.
116.
117.
The synthesis and characterization of rare-earth (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y) thiocyanate adducts with tripiperidinophosphine oxide (tpppO) with general formula (RE)(SCN)3(tpppO)3 are reported. Conductance measurements in acetonitrile indicate the non-electrolytic nature of the complexes. Infrared absorption spectra evidence that the SCN− ion coordinates through the nitrogen atom (isothiocyanate form) and that tpppO coordinates through the phosphoryl oxygen. X-ray powder patterns suggest the existence of three different crystal forms: (1) La; (2) an isomorphous series including Ce, Nd and Pr; and (3) another isomorphous series, including Sm, Gd, Eu, Ho, Er, Tb, Lu and Y. The visible spectra of the Nd adduct and the calculated parameters β = 0.98, b1/2 = 0.072 and δ = 1.06 indicate that the metal-ligand bonds are essentially electrostactic. The emission spectra of the Eu compound showed 5D0 → 7FJ bands (J = 0, 1, 2), suggesting a C3v symmetry for the coordination polyhedron. The lifetime of the 5D0 state is 1.28 ms. The emission spectra of the Tb complex presented 5D4 → 7FJ bands (J = 4, 5, 6) and the Dy complex showed the 4F9/2 → 6H13/2 band. The structure of the Pr complex showed that the coordination polyhedron is a trigonal antiprism, with the isothiocyanate anions in one base and three tpppO ligands in the other. Thermal analyses (TG-DTG) were carried out for the Ce, Nd and Gd adducts. Mass losses start between 250 and 334 °C. The final residues at 1300 °C are the corresponding phosphates. 相似文献
118.
119.
Jan Dirk van Elsas Alexander V Semenov Rodrigo Costa Jack T Trevors 《The ISME journal》2011,5(2):367-183
In this review, our current understanding of the species Escherichia coli and its persistence in the open environment is examined. E. coli consists of six different subgroups, which are separable by genomic analyses. Strains within each subgroup occupy various ecological niches, and can be broadly characterized by either commensalistic or different pathogenic behaviour. In relevant cases, genomic islands can be pinpointed that underpin the behaviour. Thus, genomic islands of, on the one hand, broad environmental significance, and, on the other hand, virulence, are highlighted in the context of E. coli survival in its niches. A focus is further placed on experimental studies on the survival of the different types of E. coli in soil, manure and water. Overall, the data suggest that E. coli can persist, for varying periods of time, in such terrestrial and aquatic habitats. In particular, the considerable persistence of the pathogenic E. coli O157:H7 is of importance, as its acid tolerance may be expected to confer a fitness asset in the more acidic environments. In this context, the extent to which E. coli interacts with its human/animal host and the organism''s survivability in natural environments are compared. In addition, the effect of the diversity and community structure of the indigenous microbiota on the fate of invading E. coli populations in the open environment is discussed. Such a relationship is of importance to our knowledge of both public and environmental health. 相似文献
120.
A new nematode genus, Nothacrobeles, is proposed in the subfamily Acrobelinae. Four new species are described and one new combination made. The five species exhibit a progression from short-to-long, bifurcate, elaborately fringed labial probolae, the longest probolae resembling those of species of Acrobeles. 相似文献