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   465313篇
  免费   57562篇
  国内免费   287篇
  2018年   4063篇
  2017年   3716篇
  2016年   5839篇
  2015年   8783篇
  2014年   9889篇
  2013年   13687篇
  2012年   15888篇
  2011年   15906篇
  2010年   10489篇
  2009年   9670篇
  2008年   13867篇
  2007年   14320篇
  2006年   12950篇
  2005年   12781篇
  2004年   12463篇
  2003年   11944篇
  2002年   11355篇
  2001年   21975篇
  2000年   22089篇
  1999年   17721篇
  1998年   6411篇
  1997年   6641篇
  1996年   6466篇
  1995年   5837篇
  1994年   5911篇
  1993年   5701篇
  1992年   13885篇
  1991年   13183篇
  1990年   12858篇
  1989年   12703篇
  1988年   11388篇
  1987年   10989篇
  1986年   10062篇
  1985年   9853篇
  1984年   8368篇
  1983年   7205篇
  1982年   5626篇
  1981年   5120篇
  1980年   4774篇
  1979年   7833篇
  1978年   6080篇
  1977年   5554篇
  1976年   5192篇
  1975年   5485篇
  1974年   5972篇
  1973年   5814篇
  1972年   5222篇
  1971年   4822篇
  1970年   4041篇
  1969年   3933篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
201.
202.
203.
204.
205.
Three hundred and thirty specimens of 7 species of passerine birds from South Bass Island, Ottawa County, Ohio, were examined for helminth parasites. The total number of helminth specimens collected was 4,333. Forty-one helminth taxa were identified. Ten species of helminths were identified as having foci of infection on the island. An index of association for these 10 species is presented. The low association revealed between helminth species utilizing common species of intermediate hosts indicated that a single intermediate host specimen is likely to harbor infective stages of only one species of parasitic helminth. An index of similarity is presented to express the species importance relationships of the helminth faunas of the 7 species of birds: red-winged blackbirds (Agelaius phoeniceus), blue jays (Cyanocitta cristata), brown-headed cowbirds (Molothrus ater), common grackles (Quiscalus quiscula), house sparrows (Passer domesticus), starlings (Sturnus vulgaris), and robins (Turdus migratorius). The results reveal that competition between these avian species for invertebrate food resources helps to maximize the transmission of those helminth populations which utilize these same invertebrates as intermediate hosts. The aggregation of birds into mixed feeding flocks insures a heavy loading of the feeding grounds with agents infective for the invertebrate populations.  相似文献   
206.
207.
208.
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   
209.
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   
210.
A series of in vitro experiments were conducted to assess three fibrolytic enzyme preparations as potential feed additives in equine diets. The three fibrolytic enzyme preparations were a concentrated cellulase (E1), an acid cellulase (E2) and a concentrated xylanase (E3). The enzymes were evaluated on their ability to modify the cell wall fraction of high-temperature dried lucerne (HTL) under various experimental conditions including differences in temperature, pH, incubation period, substrate levels and particle size to enable selection of the enzyme preparation most effective in the hydrolysis of lucerne. Results showed enzyme activities (as measured by reducing sugar assays) to be greatest at 50 °C, pH 5 and over an incubation period of greater than 20 h. E1 exhibited the greatest effect on total monosaccharide release from the HTL compared to E2 and E3. Moreover, dry matter (DM) and total non-starch polysaccharide (TNSP) losses were also greater in HTL treated with E1 compared to E2 and E3. Therefore, since the cell wall fraction of HTL contained substantial amounts of cellulose, the enzyme with the highest cellulase activity (Enzyme 1) was most effective in hydrolysing the cell walls of HTL. Consequently, it would appear that the application of exogenous fibrolytic enzyme preparations to forages requires the chemical characterisation of the target forage to enable selection of enzymes that are (a) most suitable to degrade the cell wall components of the candidate forage and (b) effective under field conditions.  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