首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   813篇
  免费   125篇
  938篇
  2018年   9篇
  2017年   16篇
  2016年   13篇
  2015年   18篇
  2014年   22篇
  2013年   27篇
  2012年   30篇
  2011年   31篇
  2010年   15篇
  2009年   20篇
  2008年   30篇
  2007年   28篇
  2006年   23篇
  2005年   23篇
  2004年   19篇
  2003年   24篇
  2002年   17篇
  2001年   24篇
  2000年   15篇
  1999年   29篇
  1998年   16篇
  1997年   11篇
  1995年   8篇
  1994年   9篇
  1993年   9篇
  1992年   20篇
  1991年   14篇
  1990年   14篇
  1989年   14篇
  1988年   16篇
  1987年   27篇
  1986年   15篇
  1985年   18篇
  1984年   17篇
  1983年   22篇
  1982年   17篇
  1981年   12篇
  1980年   8篇
  1979年   16篇
  1978年   8篇
  1977年   12篇
  1976年   9篇
  1975年   13篇
  1974年   16篇
  1973年   14篇
  1972年   9篇
  1971年   14篇
  1970年   10篇
  1968年   10篇
  1962年   8篇
排序方式: 共有938条查询结果,搜索用时 31 毫秒
71.
Abstract

Invertebrate predation on the soil-living stages of Australian soldier fly may be a significant factor in the regulation of this pest of North Island grasslands. Laboratory and field experiments on larvae of Conoderus exsul and Agrypnus variabilis (Elateridae), adults and larvae of Thyreocephalus orthodoxus (Staphylinidae), and adults of Rhytisternus miser (Carabidae) suggest that all may influence the densities of soldier fly through predation on larvae and/or eggs. The functional responses of C. exsul, A. variabilis, and T. orthodoxus to soldier fly larval density are discussed. Selective removal of predators from pasture with insecticide showed that predation may account for up to 75% loss of the early instars of soldier fly.  相似文献   
72.

1. 1.|The effect of thyroidectomy at 12 days of age on weight gain, and on heat production and thermoregulatory ability of 4- to 5-week-old chickens at temperatures within and below the thermo-neutral zone was investigated.

2. 2.|Despit the absence of thyroid tissue, as demonstrated with radioiodine, a small amount of thyroxine was found in the plasma of some thyroidectomized (TX) birds.

3. 3.|Thyroidectomy depressed weight gain; pair-fed controls grew significantly faster than TX birds.

4. 4.|Resting heat production of TX birds at thermoneutrality (30°C) was depressed by 18% (P < 0.001) and body temperature by 0.4°C (P < 0.001).

5. 5.|At 12°C heat production of TX birds was similar to that of controls but the body temperature of TX birds was 0.7°C lower (P < 0.001).

6. 6.|Thyroidectomized birds were unable to regulate body temperature at 5°C even if thyroxine was provided on the day before and at the time of cold-exposure. This inability to thermoregulate was probably due to inadequate insulation and poor nutritional status.

Author Keywords: Gallus domesticus; thyroidectomy; thyroxine; heat production; thermoregulation; body temperature  相似文献   

73.

Introduction

Erosions of the sacroiliac joints (SIJ) on pelvic radiographs of patients with ankylosing spondylitis (AS) are an important feature of the modified New York classification criteria. However, radiographic SIJ erosions are often difficult to identify. Recent studies have shown that erosions can be detected also on magnetic resonance imaging (MRI) of the SIJ early in the disease course before they can be seen on radiography. The goals of this study were to assess the reproducibility of erosion and related features, namely, extended erosion (EE) and backfill (BF) of excavated erosion, in the SIJ using a standardized MRI methodology.

Methods

Four readers independently assessed T1-weighted and short tau inversion recovery sequence (STIR) images of the SIJ from 30 AS patients and 30 controls (15 patients with non-specific back pain and 15 healthy volunteers) ≤45 years old. Erosions, EE, and BF were recorded according to standardized definitions. Reproducibility was assessed by percentage concordance among six possible reader pairs, kappa statistics (erosion as binary variable) and intraclass correlation coefficient (ICC) (erosion as sum score) for all readers jointly.

Results

SIJ erosions were detected in all AS patients and six controls by ≥2 readers. The median number of SIJ quadrants affected by erosion recorded by four readers in 30 AS patients was 8.6 in the iliac and 2.1 in the sacral joint portion (P < 0.0001). For all 60 subjects and for all four readers, the kappa value for erosion was 0.72, 0.73 for EE, and 0.63 for BF. ICC for erosion was 0.79, 0.72 for EE, and 0.55 for BF, respectively. For comparison, the kappa and ICC values for bone marrow edema were 0.61 and 0.93, respectively.

Conclusions

Erosions can be detected on MRI to a comparable degree of reliability as bone marrow edema despite the significant heterogeneity of their appearance on MRI.  相似文献   
74.

Introduction

Inflammation associated with synovial expression of TNFα is a recognised feature of osteoarthritis (OA), although no studies have yet reported beneficial effects of anti-TNFα therapy on clinical manifestations of inflammation in OA.

Methods

We conducted an open-label evaluation of adalimumab over 12 weeks in 20 patients with OA of the knee and evidence of effusion clinically. Inclusion criteria included daily knee pain for the month preceding study enrolment and a summed pain score of 125 to 400 mm visual analogue scale on the Western Ontario and McMaster University Osteoarthritis Index (WOMAC) pain subscale. The primary outcome was the Osteoarthritis Research Society International/Outcome Measures in Rheumatology Clinical Trials (OARSI/OMERACT) response criterion at week 12. Secondary outcomes included the WOMAC pain score 20% and 50% improvement, WOMAC stiffness and function scores, patient and physician global visual analogue scale, as well as target joint swelling.

Results

Treatment was well tolerated and completed by 17 patients with withdrawals unrelated to lack of efficacy or adverse events. By intention to treat, an OARSI/OMERACT response was recorded in 14 (70%) patients. WOMAC pain 20% and 50% responses were recorded in 14 (70%) patients and eight (40%) patients, respectively. Significant improvement was observed in mean WOMAC pain, stiffness, function, physician and patient global, as well as target joint swelling at 12 weeks (P < 0.0001 for all). After treatment discontinuation, 16 patients were available for assessment at 22 weeks and OARSI/OMERACT response compared with baseline was still evident in 10 (50%) patients.

Conclusion

Targeting TNFα may be of therapeutic benefit in OA and requires further evaluation in controlled trials.

Trial registration

ClinicalTrials.gov: NCT00686439.  相似文献   
75.
76.
Thirteen patients with the nephrotic syndrome were treated with a high-protein diet, a 0·5 g sodium intake (equivalent to 1·3 g sodium chloride), and frusemide in increasing dosage. One became oedema-free with frusemide 240 mg daily, three became oedema-free with frusemide 500 mg daily, and two required a combination of high-dose frusemide and spironolactone. In three there was an appreciable increase in the blood urea, one patient developed hyponatraemia, and in two there was no weight loss. In these six patients infusions of human salt-poor albumin produced a prompt diuresis, loss of weight, and correction of the abnormal biochemical findings. In the seventh severely oedematous patient combined albumin and diuretic therapy led to a loss of 27 kg in 14 days.  相似文献   
77.
On September 22, 2017, “Engaging the Forgotten Parent: Conference of Experts on Fathers’ Role in Children’s Weight‐Related Behaviours and Outcomes” brought researchers, students, health professionals, community stakeholders, and knowledge users from Canada, Australia, and the United States together to Toronto, Ontario, for a 1‐day gathering focused on furthering scientific understanding of effective strategies to recruit, engage, and retain fathers in family‐based obesity research. Furthermore this conference focused on identifying key knowledge gaps and opportunities for collaborative research to further our understanding of fathers’ roles in the development of their children’s behaviors and weight outcomes. The following four themes emerged from the conference: (1) underrepresentation of fathers in family‐based obesity research, (2) fathers’ engagement in children’s weight‐related behaviors and desire to participate in research, (3) best practices in recruiting, engaging, and retaining fathers in research, and (4) key lessons learned from fatherhood research in developmental science.  相似文献   
78.
Understanding how communities of living organisms assemble has been a central question in ecology since the early days of the discipline. Disentangling the different processes involved in community assembly is not only interesting in itself but also crucial for an understanding of how communities will behave under future environmental scenarios. The traditional concept of assembly rules reflects the notion that species do not co‐occur randomly but are restricted in their co‐occurrence by interspecific competition. This concept can be redefined in a more general framework where the co‐occurrence of species is a product of chance, historical patterns of speciation and migration, dispersal, abiotic environmental factors, and biotic interactions, with none of these processes being mutually exclusive. Here we present a survey and meta‐analyses of 59 papers that compare observed patterns in plant communities with null models simulating random patterns of species assembly. According to the type of data under study and the different methods that are applied to detect community assembly, we distinguish four main types of approach in the published literature: species co‐occurrence, niche limitation, guild proportionality and limiting similarity. Results from our meta‐analyses suggest that non‐random co‐occurrence of plant species is not a widespread phenomenon. However, whether this finding reflects the individualistic nature of plant communities or is caused by methodological shortcomings associated with the studies considered cannot be discerned from the available metadata. We advocate that more thorough surveys be conducted using a set of standardized methods to test for the existence of assembly rules in data sets spanning larger biological and geographical scales than have been considered until now. We underpin this general advice with guidelines that should be considered in future assembly rules research. This will enable us to draw more accurate and general conclusions about the non‐random aspect of assembly in plant communities.  相似文献   
79.
Learning-related plasticity at excitatory synapses in the mammalian brain requires the trafficking of AMPA receptors and the growth of dendritic spines. However, the mechanisms that couple plasticity stimuli to the trafficking of postsynaptic cargo are poorly understood. Here we demonstrate that myosin Vb (MyoVb), a Ca2+-sensitive motor, conducts spine trafficking during long-term potentiation (LTP) of synaptic strength. Upon activation of NMDA receptors and corresponding Ca2+ influx, MyoVb associates with recycling endosomes (REs), triggering rapid spine recruitment of endosomes and local exocytosis in spines. Disruption of MyoVb or its interaction with the RE adaptor Rab11-FIP2 abolishes LTP-induced exocytosis from REs and prevents both AMPA receptor insertion and spine growth. Furthermore, induction of tight binding of MyoVb to actin using an acute chemical genetic strategy eradicates LTP in hippocampal slices. Thus, Ca2+-activated MyoVb captures and mobilizes REs for AMPA receptor insertion and spine growth, providing a mechanistic link between the induction and expression of postsynaptic plasticity.  相似文献   
80.
Davison A  Chiba S 《Molecular ecology》2006,15(10):2905-2919
The effect of Pleistocene climate change on the organisms of tropical and subtropical regions is rather poorly understood. We therefore studied the land snail genus Mandarina (Bradybaenidae) of oceanic Ogasawara (Bonin Islands, Japan), with the aim of using population genetic data to understand their recent history. Our analysis of a mitochondrial 16S ribosomal RNA region from more than 600 snails in five ground-living species suggests that populations on the small islands of Mukoujima, Anejima, Imotojima and Meijima, as well as on the low-lying southern and central parts of Hahajima, have probably undergone recent bottlenecks followed by subsequent expansions. Except between the main island of Hahajima and Mukouijima, there is almost no evidence for gene flow among islands even though the islands were connected repeatedly by land bridges through the Pleistocene. Within islands the population structure is severe, suggestive of a long-term, low level of gene flow (F(ST) is frequently greater than 0.5 among geographically close populations). Finally, there is a marked genetic patchiness, meaning that genetically close populations are sometimes separated by genetically distant populations. These patterns could be a consequence of expansion from bottlenecks, low active dispersal and founder effects caused by rare long-distance migrants. Unfortunately, the exact nature of the refugia and bottlenecks remains unknown because the palaeoclimate of this region is poorly understood. Dating the population size changes is also challenging because the molecular clock is uncertain. We suggest, however, that arid conditions or deforestation induced by decreased atmospheric CO(2) may have been the main factor in determining population size.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号