全文获取类型
收费全文 | 7256篇 |
免费 | 701篇 |
国内免费 | 6篇 |
专业分类
7963篇 |
出版年
2021年 | 81篇 |
2020年 | 51篇 |
2019年 | 56篇 |
2018年 | 85篇 |
2017年 | 78篇 |
2016年 | 117篇 |
2015年 | 202篇 |
2014年 | 193篇 |
2013年 | 307篇 |
2012年 | 339篇 |
2011年 | 353篇 |
2010年 | 194篇 |
2009年 | 210篇 |
2008年 | 312篇 |
2007年 | 313篇 |
2006年 | 294篇 |
2005年 | 309篇 |
2004年 | 298篇 |
2003年 | 319篇 |
2002年 | 305篇 |
2001年 | 97篇 |
2000年 | 76篇 |
1999年 | 82篇 |
1998年 | 90篇 |
1997年 | 64篇 |
1996年 | 74篇 |
1995年 | 68篇 |
1994年 | 62篇 |
1993年 | 59篇 |
1992年 | 65篇 |
1991年 | 62篇 |
1990年 | 60篇 |
1989年 | 73篇 |
1988年 | 54篇 |
1987年 | 67篇 |
1986年 | 55篇 |
1985年 | 59篇 |
1984年 | 71篇 |
1983年 | 78篇 |
1982年 | 83篇 |
1981年 | 87篇 |
1980年 | 85篇 |
1979年 | 58篇 |
1978年 | 62篇 |
1977年 | 76篇 |
1976年 | 78篇 |
1975年 | 59篇 |
1974年 | 60篇 |
1973年 | 59篇 |
1961年 | 48篇 |
排序方式: 共有7963条查询结果,搜索用时 15 毫秒
61.
Selenium (Se) deficiency decreased by 8-fold the activity of type 1 iodothyronine 5'-deiodinase (ID-I) in hepatic microsomal fractions from rats. Solubilized hepatic microsomes from rats injected with 75Se-labelled Na2SeO3 4 days before killing were found by chromatography on agarose gels to contain a 75Se-containing fraction with ID-I activity. PAGE of this fraction under reducing conditions, followed by autoradiography, revealed a single 75Se-containing protein (Mr 27,400 +/- 300). This protein could also be labelled with 125I-bromoacetyl reverse tri-iodothyronine, an affinity label for ID-I. The results suggest that hepatic ID-I is a selenoprotein or has an Se-containing subunit essential for activity. 相似文献
62.
Effect of immune system imagery on secretory IgA 总被引:4,自引:0,他引:4
Dr. Mark S. Rider Jeanne Achterberg G. Frank Lawlis Arthur Goven Rafael Toledo J. Robert Butler 《Applied psychophysiology and biofeedback》1990,15(4):317-333
This study was an investigation of the effects of physiologically-oriented mental imagery on immune functioning. College students with normal medical histories were randomly selected to one of three groups. Subjects in Group 1 participated in short educational training on the production of secretory immunoglobulin A. They were then tested on salivary IgA, skin temperature, and the Profile of Mood States (POMS) before and after listening to a 17-minute tape of imagery instructions with specially composed background entrainment music designed to enhance imagery. Subjects in Group 2 (placebo controls) listened to the same music but received nor formal training on the immune system. Group 3 acted as a control and subjects were tested before and after 17 minutes of no activity. Treatment groups listened to their tapes at home on a bi-daily basis for six weeks All groups were again tested at Weeks 3 and 6. Secretory IgA was analyzed using standard radial immunodiffusion techniques. Repeated measures analyses of variance with planned orthogonal contrasts were used to evaluate the data. Significant overall increases (p<0.05) were found between pre- and posttests for all three trials. Groups 1 and 2 combined (treatment groups) yielded significantly greater increases in sIgA over Group 3 (control) for all three trials. Group 1 (imagery) was significantly higher than Group 2 (music) in antibody production for Trials 2 and 3. Symptomatology, recorded by subjects at Weeks 3 and 6, was significantly lower for three symptoms (rapid heartbeat, breathing difficulty, and jaw clenching), favoring both treatment groups over the control group. 相似文献
63.
Population-based data have not been readily available on relatively short-term changes in weight. Therefore, we sought to determine the nature of self-reported substantial (> 10%) weight change over one year in a representative sample of the US population which participated in the 1989 National Health Interview Survey (NHIS). Across all ages, a larger proportion of women than men reported both weight loss as well as weight gain of any amount (18.9% vs. 16.1% for weight loss and 20.0% vs. 16.1% for weight gain). In sex-specific logistic regression analyses, significant risk factors common to both sexes for substantial weight loss included divorced/separated marital status, smoking, increased number of blood pressure checks, increased BMI (body mass index) and increased number of bed days. Black race reduced the risk of weight loss for both men and women. Sex-specific risk factors for weight loss in men only were widowhood or never married marital status, while increasing age was a protective factor in women only. Concerning weight gain > 10% over the past year, increased number of blood pressure checks and having one or more diabetic parents were significant risk factors among both men and women; while never being married, increased age, BMI, and education exerted a protective effect in both sexes. For women only, risk factors for weight gain included black race, increased number of contacts with a health professional, and being unemployed. Intention to lose weight was associated with both weight gain and weight loss in both sexes, although it did not serve as a confounder in any of these relationships. A greater likelihood of substantial weight loss among women relative to men was diminished for persons with higher BMI, higher number of blood pressure checks, being widowed, divorced or separated, and intention to lose weight. A greater likelihood of substantial weight gain among women relative to men was diminished for persons with low BMI. The results of this cross-sectional study of weight change, involving a one-year follow-up period, generally correspond with the results obtained by longitudinal studies involving a longer follow-up. 相似文献
64.
Dr. Arthur K. Satterley 《Facies》1994,30(1):119-149
Summary The Upper Triassic Dachsteinkalk of the Hochk?nig Massif, situated 50 km south of Salzburg in the Northern Calcareous Alps,
corresponds to a platform margin reef complex of exceptional thickness. The platform interior limestones form equally thick
sequences of the well known cyclic Lofer facies. Sedimentation in the reef complex was not so strongly controlled by low-amplitude
sea-level oscillations as was the Lofer facies. The westernmost of the 8 facies of the reef complex is an oncolite-dominated
lagoon, in which wave-resistant stromatolite mounds with a relief of a few metres were periodically developed. The transition
to the central reef area is accomplished across the back-reef facies. In the back-reef facies patch reefs and calcisponges
appear. The proportion of coarse bioclastic sediment increases rapidly over a few hundred metres before the central reef area
is encountered. The central reef area consists of relatively widely spaced small patch reefs that did not develop wave-resistant
reef framework structures. The bulk of the sediment in the central reef area is coarse bioclastic material, provided by the
dense growth of reef organisms and the wave-induced disintegration of patch reefs. Collapse of the reef margin is recorded
by the supply of large blocks of patch reef material to the upper reef slope. Additionally, coarse, loose bioclastic debris
was supplied to the upper reef slope and this was incorporated into debris flows on the reef slope and turbidites found at
the base of the slope and in the off-reef facies. Partially lithified packstones and wackestones of the lower to middle reef
slope were modified by mass movement to form breccia and rudstone sheets. The latter reach out hundreds of metres into the
off-reef facies environment. A reef profile is presented which was derived by the restoration of strike and dip information.
In conjunction with constraints imposed by sedimentary facies related to slope processes, the angle of slope in the reef margin
area ranged from 11° to 5°, forming a concave (dished downwards) slope. Water depth estimations require that the central reef
area did not develop in water of less than 10 metres depth. At the reef margin water depths were about 30 metres, at the base
of the reef slope 200 metres and deepening in the off-reef facies to 250 metres. While previous work on reef complexes from
this type of setting suggests growth in a heavily storm-dominated environment, the present author finds little evidence for
the storm generation of the fore reef breccias, although there is good evidence for storm-influenced sedimentation and reworking
in the central reef area.
Post-depositional processes were characterised by continued slope processes causing brecciation and hydraulic injection of
red internal sediments downwards into the reef slope and off-reef limestones. Hydrothermal circulation caused a number of
phases of post-depositional (diagenetic) brecciation. There appears not to have been an important period of emergence at the
Triassic/Jurassic boundary. 相似文献
65.
David Kipling Helen E. Wilson Arthur R. Mitchell Benjamin A. Taylor Howard J. Cooke 《Chromosoma》1994,103(1):46-55
Cytologically, the centromere is found at the very end of most Mus musculus chromosomes, co-localizing with an array of minor satellite sequences. It is separated from the euchromatin of the long arm by a large domain of heterochromatin, composed in part of arrays of major satellite sequences. We used oligonucleotide probes that specifically detect regions of sequence variation found in certain cloned minor satellite sequences. They detect a limited subset of the minor satellite arrays in the mouse genome, based on both pulsed-field gel electrophoresis and in situ hybridization data, and provide direct molecular genetic markers for individual centromeres in some inbred mouse strains. Array size polymorphisms detected by these probes map to positions consisten with the centromeres of chromosomes 1 and 14 in the BXD recombinant inbred (RI) strains. The genetic distances between these minor satellite arrays and loci on the long arms of chromosomes 1 and 14 are consistent with repression of meiotic recombination in the heterochromatic domains separating them. The existence of chromosome-specific minor satellite sequences implies that the rate of sequence exchange between non-homologous chromosomes relative to the rate between homologous chromosomes is much lower than has previously been postulated. We suggest that the high degree of sequence homogeneity of mouse satellite sequences may instead reflect recent common ancestry. 相似文献
66.
Arthur O. Tucker Michael J. Maciarello Pearl W. Burbage George Sturtz 《Economic botany》1994,48(3):333-336
The essential oils of fresh leaves, twigs, and/or fruits of spicebush cultivated in Oregon (field distilled) and Delaware (laboratory distilled) are examined by GC/MS. The oil of leaves of spicebush is notably high in 6-methyl-5-hepten-2-one (1.94% in Oregon to 34.83 ± 9.69% in Delaware). β-caryophyllene (15.26% in Oregon to 48.44 ± 1.35% in Delaware), and/or (E)-nerolidol (10.20% in Oregon to 12.05 ± 2.02% in Delaware). The oil of the twigs of spicebush is notably high in 1,8-cineole (45.41 ± 0.35% in Delaware), while the oil of the fruits is notably high in a-phellandrene (64.62 ± 0.66% in Delaware) and β-phellandrene (11.23+0.17% in Delaware). 相似文献
67.
E. L. Lubbers L. Arthur W. W. Hanna P. Ozias-Akins 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1994,89(5):636-642
Two molecular markers, a RAPD (randomly amplified polymorphic DNA) and a RFLP/STS (restriction fragment length polymorphism/sequence-tagged site), previously were found associated with apomictic reproductive behavior in a backcross population produced to transfer apomixis from Pennisetum squamulatum to pearl millet. The occurrence of these molecular markers in a range of 29 accessions of Pennisetum comprising 11 apomictic and 8 sexual species was investigated. Both markers were specific for apomictic species in Pennisetum. The RFLP/STS marker, UGT 197, was found to be associated with all taxa that displayed apomictic reproductive behavior except those in section Brevivalvula. Neither UGT197 nor the cloned RAPD fragment OPC-04600 hybridized with any sexually reproducing representatives of the genus. The cloned C04600 was associated with 3 of the 11 apomictic species, P. ciliare, P. massaicum, and P. squamulatum. UGT197 was more consistently associated with apomictic reproductive behavior than OPC04600 or cloned C04600, thus it could be inferred that UGT197 is more closely linked to the gene(s) for apomixis than the cloned C04600. The successful use of these probes to survey other Pennisetum species indicates that apomixis is a trait that can be followed across species by using molecular means. This technique of surveying species within a genus will be useful in determining the relative importance of newly isolated markers and may facilitate the identification of the apomixis gene(s). 相似文献
68.
The natural service fertility of yearling bulls on pasture was compared with that of 2-year-old bulls in a study (2 experiments) which involved a total of 748 composite breed cows over 3 breeding seasons. Other factors such as age of the cows, breed, year effects, and the sequence in which bulls of a particular age were assigned to cows were also evaluated. Pregnancy and calving rates and calving date were used as criteria for assessing fertility. The age of the bull, breed, year, or bull sequence did not significantly affect any of the traits studied. The age of cow was important in all the traits studied, with mature cows (5 years and older at breeding) calving earlier (P < 0.05) than the youngest group (2 years old at breeding) of cows, in both experiments. The pregnancy rate was also lower in the younger cows than in the mature cows in the first experiment. The results suggest that the age range of the cow herd is more important in determining herd fertility than the age of the bull or any of the parameters studied. 相似文献
69.
Arthur M. Brown 《The Journal of membrane biology》1993,131(2):93-104
Summary Ion channels are signaling molecules and by them-selves perform no work. In this regard they are un like the usual membrane
enzyme effectors for G proteins. The pathways of G protein receptor, G protein and ion channels are, therefore, purely infor
mational in function. Because a single G protein may have several ion channels as effectors, the effects should be coordinated
and this seems to be the case. Inhibition of Ca2+ current and stimulation of K+ currents would have a greater impact than either alone. Additional flexibility is provided by spontane ous noise in the complexes
of G protein receptor, G protein, and ion channel. By having a non-zero setpoint, the range of control is extended and the
responses become bi-directional. 相似文献
70.
Tihomir Paul Obrenovitch Jutta Urenjak Douglas Arthur Richards † Yukihiko Ueda Gerald Curzon † Lindsay Symon 《Journal of neurochemistry》1993,61(1):178-186
Abstract: Changes in the extracellular levels of excitatory and inhibitory amino acid transmitters were studied in the rat striatum during penumbral ischaemia using intracerebral microdialysis. Effects of penumbral forebrain ischaemia were compared with those of ischaemia with sustained anoxic depolarisation and K+ (100 m M ). Comparisons were also made between different groups of animals at 2 and 24 h after dialysis probe implantation. The K+ stimulus did not provoke any release of excitatory amino acids in the 24-h group, probably reflecting a decrease of functional synapses adjacent to the probe. During 30 min of penumbral ischaemia, excitatory amino acids did not reach critical concentrations in the extracellular fluid, and increases in levels of inhibitory/modulatory amino acids were similar. On the other hand, severe transient ischaemia resulted in massive synchronous release of many neuroactive excitatory and inhibitory compounds, in both the 2- and 24-h groups. These and other data suggest that changes during severe ischaemia may arise from both neurotransmitter and metabolic pools. It is concluded that is- chaemic damage in the penumbra may not be related to extracellular neuroactive amino acid changes generated within this region. 相似文献