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   534881篇
  免费   57584篇
  国内免费   751篇
  2018年   6319篇
  2017年   5949篇
  2016年   8458篇
  2015年   12365篇
  2014年   13806篇
  2013年   18586篇
  2012年   22209篇
  2011年   22201篇
  2010年   14539篇
  2009年   12568篇
  2008年   19219篇
  2007年   19593篇
  2006年   18231篇
  2005年   17330篇
  2004年   17111篇
  2003年   15919篇
  2002年   15245篇
  2001年   20440篇
  2000年   20396篇
  1999年   16425篇
  1998年   6234篇
  1997年   6117篇
  1996年   5794篇
  1995年   5585篇
  1994年   5315篇
  1993年   5194篇
  1992年   13421篇
  1991年   13312篇
  1990年   12955篇
  1989年   12397篇
  1988年   11508篇
  1987年   10770篇
  1986年   10278篇
  1985年   10113篇
  1984年   8404篇
  1983年   7284篇
  1982年   5488篇
  1981年   4984篇
  1980年   4610篇
  1979年   7797篇
  1978年   6322篇
  1977年   5619篇
  1976年   5275篇
  1975年   6102篇
  1974年   6653篇
  1973年   6480篇
  1972年   5713篇
  1971年   5335篇
  1970年   4503篇
  1969年   4421篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
201.
An investigational drug (2-picoline, 6-amino-4-nitro-, 1-oxide) was evaluated to characterize the anti-coccidial spectrum of the compound. Two concentrations of the drug (125 and 250 ppm) were evaluated for bioactivity; weight gain, survival, dropping, and lesion scores were the response variables utilized to ascertain activity. The activities of the picoline derivative were compared with monensin, maduramicin, and a narasin/nicarbazin (1:1) combination. The investigational drug had significant activity against Eimeria tenella and Eimeria necatrix, and the 250-ppm level was significantly more active than 125 ppm. At 250 ppm, the E. tenella activity of the picoline derivative was comparable to both monensin (120 ppm) and the 50-ppm narasin/nicarbazin combination, significantly less effective than maduramicin (6 ppm), and significantly more efficacious than 30 ppm narasin/nicarbazin. At the same level (250 ppm), the picoline derivative had significantly less E. necatrix activity than monensin (120 ppm), maduramicin (6 ppm), and narasin/nicarbazin (50 ppm), and significantly greater activity than 30 ppm narasin/nicarbazin. At best, only extremely weak Eimeria acervulina, Eimeria brunetti, and Eimeria maxima activities were noted with the investigational drug; higher concentrations of the picoline derivative may achieve greater anti-coccidial activity against these species. The efficacy of narasin/nicarbazin compared favorably with monensin and maduramicin; the 50-ppm level of the combination appeared significantly more efficacious than 30-ppm.  相似文献   
202.
203.
The differences between the composition of volatile substances in two specimens of dry smoked sausages produced using a standard and experimental (a mixture of propionic acid bacteria and bifidobacteria) cultures were studied by capillary gas chromatography. It was found that the experimental starter culture intensified the flavor-formation processes as compared with the standard culture. The experimental specimen had richer qualitative and quantitative compositions and displayed more intensive aroma and flavor. The contents of lactones and volatile terpenoids in the experimental specimen were much higher than in the control. The organoleptic characteristics of experimental dry smoked sausage specimen were considerably better.  相似文献   
204.
ObjectiveTo evaluate the contribution of FDG-PET in the management of anal carcinoma, with special emphasis on its impact on therapeutic strategy.Materials and methodsFrom March 2005 to August 2008, 48 PET were performed on 43 patients with anal epidermoid carcinoma, in initial staging (IS: 20 exams), therapeutic response assessment (TRA: 11), and recurrence assessment (RA: 17). We compared initial therapeutic strategies defined on conventional assessment results, to final ones chosen after PET.ResultsPET revealed lesions that were undetected by conventional investigation in 23% of cases (IS: 25%; TRA: 18%; RA: 23%) and cleared suspicious lesions in 21% of cases (IS: 10%; TRA: 18%; RA: 35%). It influenced the therapeutic strategy, and sometimes even modified it radically, in 44% of cases (IS: 35%; TRA: 54%; RA: 47%). This therapeutic impact was stronger in settings with diagnostic ambiguity, in which PET allowed to specify the diagnosis and to orientate consequently the therapeutic choice.ConclusionPET is interesting in the management of anal carcinoma, especially in uncertain diagnostic settings, in which the metabolic information brought allows to influence the therapeutic choice in almost half of the cases.  相似文献   
205.
We present here a new, dedicated mechanical device for monitoring quick-release movements of the wrist. The ergometer was designed to easily assess musculotendinous properties during wrist flexion. Maximal voluntary contractions (MVC) and quick-release (QR) movements during wrist flexion were performed on 14 subjects. A validation of the ergometer, using a test-retest methodology, was performed to assess its reliability and sensitivity. The device has been technically and biomechanically validated in a range of situations, including inertia measurement (mean inertia was found 0.0119+/-0.0012 N m s(2) rad(-1)) and appearance of the unloading reflex. Our results indicate that the device provides highly reliable, sensitive evaluation of wrist muscle stiffness (intraclass correlation coefficient for inertia, maximal voluntary contraction and stiffness index were 0.873, 0.994 and 0.930, respectively). Its portability facilitates measurement of the influence of repetitive, occupational activity on the musculotendinous complex of the wrist flexors.  相似文献   
206.
We report here that a previously described cell surface antigen (Brower, Smith & Wilcox, 1980) is expressed in a segmentally repeating pattern of stripes in the epidermis and nervous system of segmented Drosophila embryos. We also report that the antigenic activity is found on two closely related cell surface glycoproteins. The pattern of expression of this antigen is reminiscent of the expression of some segmentation genes and is affected by mutation of at least two of these genes, fushi tarazu and paired. Thus these glycoproteins are candidates for cell surface molecules involved in carrying out the patterning processes controlled by segmentation genes.  相似文献   
207.
A short single-stranded tail on one end of an otherwise duplex DNA molecule enables recA protein, in the presence of ATP and MgCl2, to form a complex with the DNA which extends into the duplex portion of the molecule. Nuclease protection studies at a concentration of MgCl2 which permits homologous pairing showed that cleavage by restriction endonucleases at sites throughout the duplex region was inhibited, whereas digestion by DNase I was not affected. These results indicate that recA protein binds to the duplex portion of tailed DNA allowing access by DNase I to a random sample of the many sites at which it cleaves, but providing limited protection of the relatively rare restriction sites. Electron microscopy revealed that the recA nucleoprotein complex with duplex DNA is indeed a segmented or interrupted filament that, with time, extends further from the single-stranded tail into the duplex region. recA protein binding extended into the duplex region more rapidly for duplexes with 5' tails than for those with 3' tails. These observations show that recA protein translocates from a single-stranded region into duplex DNA in the form of a segmented filament by a mechanism that is not strongly polarized.  相似文献   
208.
Escherichia coli strains MC4100 (parent) and a mutant strain derived from this (IC007) were evaluated for their ability to produce H2 and organic acids (OAs) via fermentation. Following growth, each strain was coated with Pd(0) via bioreduction of Pd(II). Dried, sintered Pd-biomaterials (‘Bio-Pd’) were tested as anodes in a proton exchange membrane (PEM) fuel cell for their ability to generate electricity from H2. Both strains produced hydrogen and OAs but ‘palladised’ cells of strain IC007 (Bio-PdIC007) produced ~threefold more power as compared to Bio-PdMC4100 (56 and 18 mW respectively). The power output used, for comparison, commercial Pd(0) powder and Bio-Pd made from Desulfovibrio desulfuricans, was ~100 mW. The implications of these findings for an integrated energy generating process are discussed.  相似文献   
209.
Columba livia is an important reservoir and carrier of Cryptococcus neoformans, Cryptococcus uniguttulatus, Cryptococcus laurentii and Cryptococcus albidus. Upper digestive tract of this species is also known as a habitat for Cryptococcus neoformans. Given the increasing clinical interest of this microorganism, 331 swabs from crop and 174 dropping samples from pigeon lofts in Grand Canary Island have been studied. The obtained results show an extensive presence samples 81 positive (24.47%) of Cryptococcus spp. in analysed crops: 32 (9.66%) for C. neoformans, 24 (7.2%) for C. uniguttulatus, 23 (6.9%) for C. albidus and 2 (0.6%) for C. laurentii. In the same way, Cryptococcus spp was also isolated in 82 (47.13%), dropping samples: C. neoformans in 59 (33.9%), C. uniguttulatus, in 9 (5.17%), C. laurentii in 8 (4.59%) and C. albidus in 6 (3.44%) of the investigated samples, respectively. The cryptococcosis produced by species of cryptococci other than C. neoformans has become more important during the last decade, supporting the study on the role of pigeon in the epidemiology of this disease.  相似文献   
210.
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