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   67846篇
  免费   4636篇
  国内免费   13篇
  2023年   373篇
  2022年   344篇
  2021年   1087篇
  2020年   884篇
  2019年   1096篇
  2018年   1763篇
  2017年   1545篇
  2016年   2188篇
  2015年   2957篇
  2014年   3134篇
  2013年   4060篇
  2012年   4711篇
  2011年   4390篇
  2010年   2877篇
  2009年   2519篇
  2008年   3454篇
  2007年   3365篇
  2006年   3249篇
  2005年   2739篇
  2004年   2729篇
  2003年   2437篇
  2002年   2297篇
  2001年   1753篇
  2000年   1649篇
  1999年   1339篇
  1998年   668篇
  1997年   491篇
  1996年   483篇
  1995年   492篇
  1994年   395篇
  1993年   370篇
  1992年   785篇
  1991年   727篇
  1990年   659篇
  1989年   653篇
  1988年   578篇
  1987年   590篇
  1986年   524篇
  1985年   499篇
  1984年   415篇
  1983年   375篇
  1982年   293篇
  1979年   396篇
  1978年   267篇
  1975年   258篇
  1974年   314篇
  1973年   315篇
  1972年   276篇
  1970年   255篇
  1969年   260篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
Animal guts and plant roots have absorption roles for nutrient uptake and converge in harboring large, complex, and dynamic groups of microbes that participate in degradation or modification of nutrients and other substances. Gut and root bacteria regulate host gene expression, provide metabolic capabilities, essential nutrients, and protection against pathogens, and seem to share evolutionary trends.  相似文献   
92.
Through Genome Wide Association Studies (GWAS) many Single Nucleotide Polymorphism (SNP)-complex disease relations can be investigated. The output of GWAS can be high in amount and high dimensional, also relations between SNPs, phenotypes and diseases are most likely to be nonlinear. In order to handle high volume-high dimensional data and to be able to find the nonlinear relations we have utilized data mining approaches and a hybrid feature selection model of support vector machine and decision tree has been designed. The designed model is tested on prostate cancer data and for the first time combined genotype and phenotype information is used to increase the diagnostic performance. We were able to select phenotypic features such as ethnicity and body mass index, and SNPs those map to specific genes such as CRR9, TERT. The performance results of the proposed hybrid model, on prostate cancer dataset, with 90.92% of sensitivity and 0.91 of area under ROC curve, shows the potential of the approach for prediction and early detection of the prostate cancer.  相似文献   
93.
Can simple enrichments enhance caged mink welfare? Pilot data from 756 sub-adults spanning three colour-types (strains) identified potentially practical enrichments, and suggested beneficial effects on temperament and fur-chewing. Our main experiment started with 2032 Black mink on three farms: from each of 508 families, one juvenile male-female pair was enriched (E) with two balls and a hanging plastic chain or length of hose, while a second pair was left as a non-enriched (NE) control. At 8 months, more than half the subjects were killed for pelts, and 302 new females were recruited (half enriched: ‘late E’). Several signs of improved welfare or productivity emerged. Access to enrichment increased play in juveniles. E mink were calmer (less aggressive in temperament tests; quieter when handled; less fearful, if male), and less likely to fur-chew, although other stereotypic behaviours were not reduced. On one farm, E females had lower cortisol (inferred from faecal metabolites). E males tended to copulate for longer. E females also weaned more offspring: about 10% more juveniles per E female, primarily caused by reduced rates of barrenness (‘late E’ females also giving birth to bigger litters on one farm), effects that our data cautiously suggest were partly mediated by reduced inactivity and changes in temperament. Pelt quality seemed unaffected, but E animals had cleaner cages. In a subsidiary side-study using 368 mink of a second colour-type (‘Demis’), similar temperament effects emerged, and while E did not reduce fur-chewing or improve reproductive success in this colour-type, E animals were judged to have better pelts. Overall, simple enrichments were thus beneficial. These findings should encourage welfare improvements on fur farms (which house 60-70 million mink p.a.) and in breeding centres where endangered mustelids (e.g. black-footed ferrets) often reproduce poorly. They should also stimulate future research into more effective practical enrichments.  相似文献   
94.
95.
The most common sustained cardiac arrhythmias in humans are atrial tachyarrhythmias, mainly atrial fibrillation. Areas of complex fractionated atrial electrograms and high dominant frequency have been proposed as critical regions for maintaining atrial fibrillation; however, there is a paucity of data on the relationship between the characteristics of electrograms and the propagation pattern underlying them. In this study, a realistic 3D computer model of the human atria has been developed to investigate this relationship. The model includes a realistic geometry with fiber orientation, anisotropic conductivity and electrophysiological heterogeneity. We simulated different tachyarrhythmic episodes applying both transient and continuous ectopic activity. Electrograms and their dominant frequency and organization index values were calculated over the entire atrial surface. Our simulations show electrograms with simple potentials, with little or no cycle length variations, narrow frequency peaks and high organization index values during stable and regular activity as the observed in atrial flutter, atrial tachycardia (except in areas of conduction block) and in areas closer to ectopic activity during focal atrial fibrillation. By contrast, cycle length variations and polymorphic electrograms with single, double and fragmented potentials were observed in areas of irregular and unstable activity during atrial fibrillation episodes. Our results also show: 1) electrograms with potentials without negative deflection related to spiral or curved wavefronts that pass over the recording point and move away, 2) potentials with a much greater proportion of positive deflection than negative in areas of wave collisions, 3) double potentials related with wave fragmentations or blocking lines and 4) fragmented electrograms associated with pivot points. Our model is the first human atrial model with realistic fiber orientation used to investigate the relationship between different atrial arrhythmic propagation patterns and the electrograms observed at more than 43000 points on the atrial surface.  相似文献   
96.
This study investigated the in vivo effects of a commercial blend of plant extracts (carvacrol, cinnamaldehyde and capsaicin) on serum metabolic parameters closely connected with energy and protein metabolism (glucose; l-lactate; non-esterified fatty acids, NEFA; urea nitrogen, SUN; creatinine; total protein, TSP) and enzymes associated with hepatic function (aspartate-aminotransferase, AST and gamma-glutamyl transferase, GGT) in finishing-stage Belgian Blue bull calves maintained in a commercial feedlot. Monitoring was performed over 86 days in 24 animals randomly allotted to two groups: (1) a control group (CTR, no supplementation; n = 10), and (2) a group receiving dietary supplementation with a commercial blend of plant extracts (PEX, 100 mg/kg DM of concentrate; n = 14). Under the conditions of our study, supplementation with the commercial blend did not give detrimental effects, but the opposite: the decrease in serum l-lactate, NEFA and creatinine levels and the increase in SUN concentrations; suggests an improvement in the energy status and protein turnover of the supplemented animals.  相似文献   
97.
Age-related macular degeneration is the leading cause of legal blindness in people over 50 in developed countries. It is a multifactorial disease resulting from the interaction of genetic and environmental factors, and the age is the only worldwide admitted risk factor. The socioeconomic impact of the disease reaches enormous proportions, if we take into account the high cost of the available antiangiogenic therapy, the strict schedule of medical visits that it requires, and the impairment that it gives rise to. The response to treatment and the visual outcomes improve with early management of the retinal lesions, thus the early diagnosis of the disease in its initial phases, based on self-control with an Amsler grid and with regular ophthalmologic assessments, is essential.  相似文献   
98.
99.
100.
Van Doninck  Karine  Schön  Isa  Martens  Koen  Goddeeris  Boudewijn 《Hydrobiologia》2003,500(1-3):331-340
The life-cycle of the ancient asexual ostracod Darwinula stevensoni was studied during 1 year in a eutrophic pond in Belgium. The reproductive period of this species started in March and was effectively completed by September of the same year. All changes in population structure took place during the spring and summer months and a rapid turnover of the instars was observed. The life-cycle of Darwinula stevensoni appears to take one year or less in Belgium and this is considerably shorter than the 4 years which had been reported previously from subarctic populations. The difference to the present study is most likely temperature-related. Maximal densities of D. stevensoni were observed in June and July and attained 105 ind. m–2. During winter, densities were lower with a mean of 104 ind. m–2. Consequently, the calculated population size of each month was high throughout the year. Together with the low mutation rate, such a large population size could effectively counteract the stochastic loss of mutation-free genotypes as predicted by Muller's ratchet. D. stevensoni is a brooder; the maximum number of embryos and juvenile instars (up to third stage) found within a single female was 11.  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