全文获取类型
收费全文 | 565623篇 |
免费 | 66665篇 |
国内免费 | 977篇 |
专业分类
633265篇 |
出版年
2016年 | 6084篇 |
2015年 | 8634篇 |
2014年 | 10200篇 |
2013年 | 14059篇 |
2012年 | 15931篇 |
2011年 | 16091篇 |
2010年 | 10833篇 |
2009年 | 10211篇 |
2008年 | 14279篇 |
2007年 | 14581篇 |
2006年 | 14029篇 |
2005年 | 13714篇 |
2004年 | 13348篇 |
2003年 | 13275篇 |
2002年 | 12789篇 |
2001年 | 26374篇 |
2000年 | 26628篇 |
1999年 | 21151篇 |
1998年 | 7153篇 |
1997年 | 7472篇 |
1996年 | 7182篇 |
1995年 | 6622篇 |
1994年 | 6554篇 |
1993年 | 6588篇 |
1992年 | 17440篇 |
1991年 | 17138篇 |
1990年 | 16425篇 |
1989年 | 16223篇 |
1988年 | 15272篇 |
1987年 | 14555篇 |
1986年 | 13353篇 |
1985年 | 13426篇 |
1984年 | 10923篇 |
1983年 | 9673篇 |
1982年 | 7321篇 |
1981年 | 6700篇 |
1980年 | 6245篇 |
1979年 | 10647篇 |
1978年 | 8308篇 |
1977年 | 7779篇 |
1976年 | 7176篇 |
1975年 | 7987篇 |
1974年 | 8571篇 |
1973年 | 8424篇 |
1972年 | 7623篇 |
1971年 | 7015篇 |
1970年 | 6187篇 |
1969年 | 6121篇 |
1968年 | 5447篇 |
1967年 | 4704篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
321.
A. J. Edwards J. D. Hamilton W. D. Nichol G. W. Taylor A. M. Dawson 《BMJ (Clinical research ed.)》1970,1(5692):342-345
Seven patients with “coeliac axis compression syndrome” are reported. Five were treated surgically, but only two did well. A survey of 200 healthy adults showed epigastric bruits in 6·5%; only one of these had dyspepsia, though dyspepsia was present in 12·5% overall.Caution is urged in attributing a causal relationship between coeliac axis compression and pain and in proceeding to arteriography when compression is suspected on clinical grounds. 相似文献
322.
T S Il'ina 《Genetika》1986,22(11):2572-2582
The published data on molecular mechanisms of transposons movement inside and between genomes are reviewed. The replicative mechanism of transposition of the family of Tn3-like elements is discussed, as well as the modes of bacteriophage Mu, Tn9, Tn10, Tn903 transposition. The factors affecting the choice of transposition pathways are analysed. 相似文献
323.
The [3H]ketanserin binding characteristics in the apomorphine-induced aggressive and nonaggressive adult male Wistar rats were studied. Repeated apomorphine (0.5 mg/kg, once daily) treatment gradually induced aggressive behaviour in sixteen animals from twenty. Thereafter the animals were retrospectively divided into apomorphine-induced aggressive and nonaggressive group. The maximal number of the [3H]ketanserin binding sites was increased in the apomorphine-treated animals in the frontal (233.9+/-26.5, 364.6+/-31.7, and 367.0+/-34.8 fmol/mg protein for the vehicle, apomorphine-nonaggressive, and apomorphine-aggressive group, respectively) and cerebral cortex (164.2+/-6.7, 289.7+/-29.3, and 249.0+/-15.4 fmol/mg protein for the vehicle, apomorphine-nonaggressive, and apomorphine-aggressive group, respectively). In conclusion, our experiments demonstrate that repeated apomorphine treatment upregulates the maximal number of the 5-HT2A receptors in rat frontal and cerebral cortex as measured by [3H]ketanserin binding and this phenomenon is independent from the development of aggressive behaviour. 相似文献
324.
Genetic programming for development in Drosophila 总被引:2,自引:0,他引:2
325.
W. H. Trethowan 《BMJ (Clinical research ed.)》1970,4(5727):109-111
At Birmingham University since 1966 the major medical examinations began to be replaced by a process of continuous assessment. This is based on fairly frequent tests, usually at the end of each course, including short essay questions, multiple-choice questions, and tests of ability in wards and laboratory. At the end of the first final-year course 96 students underwent assessment. Of these 77 passed; 19 were referred for re-examination, of whom a further 12 passed subsequently. The scheme was found suitable by both teachers and external examiners. 相似文献
326.
327.
Winnie W.Y. Lau Marlene M. Martinez 《Journal of experimental marine biology and ecology》2003,295(1):1-21
Hydrodynamic forces can affect survival as well as limit the movement of motile benthic animals. An animal's danger of dislodgement depends on the hydrodynamic forces it experiences in its microhabitat relative to the force required to dislodge it (tenacity) from the substratum. We measured water flow and substratum characteristics in two different habitats of the shore crab Pachygrapsus crassipes: a wave-swept rocky shore and an intertidal mudflat. The maximum water velocities and accelerations in the microhabitats of the crabs at the wave-swept site were three times and two times greater, respectively, than at the mudflat site. In the laboratory, we measured the tenacity of crabs of various sizes on different substrata, and also measured their drag, lift and added-mass coefficients. Using these data, we calculated the flow conditions under which crabs would be overturned or sheared off the substratum in their two habitats. The net horizontal force (drag plus acceleration reaction) required to dislodge a crab on a rugose rock substratum was an order of magnitude greater than on smooth rock and two orders of magnitude greater than on mud. Our calculations indicate that, under non-storm conditions, crabs will not be dislodged from the substratum in either the mudflat or the wave-swept habitat when grasping the substratum with maximum tenacity. Moving crabs have lower tenacity and our calculations predict that hydrodynamic forces will restrict the mobility of large crabs more than that of small ones on smooth, but not on rugose rock. 相似文献
328.
329.
The Vascular System of Maize Stems Revisited: Implications for Water Transport and Xylem Safety 总被引:3,自引:0,他引:3
The plexus of vascular bundles in the nodes of grasses is notoriouslycomplex, where long axial bundles pass through a network oftransverse bundles. The xylem pathways for water in maize stemshave been investigated anatomically and with dye and particulatetracers, revealing some of the details of this complexity. Onlyapprox. 3% of axial vessels pass through nodes without beinginterrupted by end walls. Axial bundles at nodes differ fromthose in internodes in having the metaxylem and protoxylem vesselsconnected by small tracheary elements. So it is only at nodesthat exchange of sap occurs between the large vessels withina bundle. End walls, acting as filters for particles and gasbubbles, always separate axial vessels from vessels in transversebundles. The high redundancy of bundle connections in the nodalplexus is interpreted as providing alternative water pathwaysto bypass embolisms and damaged or diseased sections of thexylem. The pores in the filters at the base of nodes and betweenaxial and transverse vessels within nodes are <20 nm in diameter.Where axial vessels connect to transverse vessels, a varietyof unusual shapes of vessel elements mediate two- and three-wayconnections within the plexus.Copyright 2000 Annals of BotanyCompany Zea mays, cryoSEM, maize, node, pits, pit membranes, vessel ends, vessels, xylem embolism, xylem pathogens 相似文献
330.