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   85865篇
  免费   7588篇
  国内免费   46篇
  93499篇
  2023年   281篇
  2022年   707篇
  2021年   1548篇
  2020年   879篇
  2019年   1108篇
  2018年   1439篇
  2017年   1241篇
  2016年   2136篇
  2015年   3630篇
  2014年   4047篇
  2013年   4811篇
  2012年   6536篇
  2011年   6481篇
  2010年   4171篇
  2009年   3798篇
  2008年   5428篇
  2007年   5431篇
  2006年   5234篇
  2005年   5031篇
  2004年   4892篇
  2003年   4668篇
  2002年   4401篇
  2001年   859篇
  2000年   628篇
  1999年   961篇
  1998年   1230篇
  1997年   821篇
  1996年   742篇
  1995年   653篇
  1994年   616篇
  1993年   666篇
  1992年   553篇
  1991年   520篇
  1990年   461篇
  1989年   397篇
  1988年   420篇
  1987年   342篇
  1986年   317篇
  1985年   400篇
  1984年   518篇
  1983年   401篇
  1982年   501篇
  1981年   486篇
  1980年   414篇
  1979年   295篇
  1978年   323篇
  1977年   281篇
  1976年   263篇
  1975年   204篇
  1974年   241篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
CLASEY, JODY L, CLAUDE BOUCHARD, LAURIE WIDEMAN, JILL KANALEY, C DAVID TEATES, MICHAEL O THORNER, MARK L HARTMAN, ARTHUR WELTMAN. The influence of anatomical boundaries, age, and sex on the assessment of abdominal visceral fat. Single-slice abdominal computed tomography (CT) scanning has been used extensively for the measurement of abdominal visceral fat (AYF). Optimal anatomical scan location and pixel density ranges have been proposed and are specifically reported to allow for the replication and standardization of AVF measurements. Standardization of the anatomical boundaries for CT measurement of AVF and the influence of age and gender on results obtained with different boundary locations have received much less attention. To determine the influence of three boundary analysis methods (AVF-1, AVF-2, and AVF-3) on the measurement of AVF by CT, 54 older (60 years to 79 years) and 37 younger (20 years to 29 years) healthy men and women were examined. The measurement boundary for AVF-1 was the internal most aspect of the abdominal and oblique muscle walls, and the posterior aspect of the vertebral body. AVF-2 used fat measurements enclosed in a boundary formed by the midpoint of the abdominal and oblique muscle walls, and the most posterior aspect of the spinous process. AVF-3 used fat measurements enclosed in a boundary formed by the external border of the abdominal and oblique muscle walls, and the external border of the erector spinae. Greater AVF measures were obtained with AVF-2 and AVF-3 compared with AVF-1 (p<0.0001). These differences were greater in older compared with younger subjects (p<0.0001) and greater in women compared with men (p<0.02). The significantly greater AVF measurements obtained with AVF-2 and AVF-3 resulted from the inclusion of larger amounts of fat that are not drained by the portal circulation. This included retroperitoneal, intermuscular, and intramuscular lipid droplets, which increase with aging. On the basis of these results, we recommend the AVF-1 anatomical boundaries for the measurement of AVF in clinical investigations, particularly with older subjects. These data demonstrate the importance of precise and reproducible anatomical boundaries for the measurement of AVF, particularly in longitudinal studies.  相似文献   
942.
Zusammenfassung Die Brutverbreitung der Vögel im Großraum Bonn wurde 1975 und 1985 in jeweils 221 Minutengitterfeldern von je 2,2 km2 Größe kartiert. Statistisch signifikante Zunahmen besetzter Gitterfelder wurden für 9 Arten, Abnahmen für 22 Arten registriert, die z. T. auch andernorts festgestellt wurden. Unter Verwendung von Kartierungsdaten 1974–1978 konnte bei 14 Arten eine signifikante Änderung (Zunahmen: Sperber, Habicht, Singdrossel; Abnahmen: Steinschmätzer, Rebhuhn, Schafstelze, Grauammer, Mehlschwalbe, Feldsperling, Fasan, Dorngrasmücke, Hänfling, Feldlerche und Zaunkönig) nachgewiesen werden. Die Untersuchung zeigt, daß die Rasterkartierung gut geeignet ist, Aussagen über großflächige und langfristige Populationsveränderungen zu machen.
Fluctuation of breeding populations in the area of Bonn: Analysis of grid mapping in 1975 and 1985
Summary The distribution of breeding birds in the area of Bonn was mapped in 1975 and 1985 in 221 grids of 2.2 km2 size each. These data were analyzed to see whether fluctuations in the breeding populations occurred during the 11 years. It was based on the assumption that strong population changes should be reflected in the number of grids occupied by a species. Statistically significant increases were recorded for 9 species, decreases for 22 species. Taking other grid-map data from 1974 til 1978 (Wink 1980) into account, a continuous and significant trend could be established for 14 species (increase:Accipiter nisus, A. gentilis, Turdus philomelos; decrease:Oenanthe oenanthe, Perdix perdix, Motacilla flava, Emberiza calandra, Delichon urbica, Passer montanus, Phasianus colchicus, Sylvia communis, Acanthis cannabina, Alauda arvensis, Troglodytes troglodytes). This study shows that grid-mapping may provide valuable data on population trends of breeding birds.
  相似文献   
943.
Purified enterotoxin from the bacterium Clostridium perfringens rapidly decreased the hormonally induced uptake of α-aminoisobutyric acid in primary cultures of adult rat hepatocytes. At 5 min after toxin addition the decrease in α-aminoisobutyric acid uptake appeared not due to increased passive permeation (estimated with l-glucose) or to increased α-aminoisobutyric acid efflux. When short uptake assay times were employed a depression of α-aminoisobutyric acid influx was observed in toxin-treated hepatocytes. The depression of α-aminoisobutyric acid influx was correlated with a rapid increase in intracellular Na+ (estimated using 22Na+) apparently effected by membrane damage. In contrast, the uptake of cycloleucine in the presence of unlabeled α-aminoisobutyric acid (assay for Na+-independent amino acid uptake) by hepatocytes treated with toxin for 5 min was decreased to only a small extent or not at all depending upon experimental design. At later times, C. perfringens enterotoxin increased the exodus of l-glucose, 3-O-methylglucose and α-aminoisobutyric acid from pre-loaded cells indicating that the toxin effects progressive membrane damage. When enterotoxin was removed by repeated washing after 5–20 min the decay of α-aminoisobutyric acid uptake ceased and appeared to undergo recovery towards the hormonally induced control level. The degree of recovery of α-aminoisobutyric acid uptake was inverse to the length of time of exposure to toxin. Adding at 10 min specific rabbit antiserum against C. perfringens enterotoxin without medium change also reversed the effect of toxin on increased intracellular 22Na+, and on the exodus (from preloaded cells) of α-aminoisobutyric acid, L-glucose, and 3-O-methylglucose.  相似文献   
944.
Abstract Murine monoclonal antibodies (mAbs) were selected against a cell wall glycoprotein of Saccharomyces cerevisiae . One of the mAbs (92-276/018) specifically identified S. cerevisiae and the sibling species S. paradoxus, S. pastorianus and S. bayanus in immunofluorescence studies and immunoblot analyses, while no other yeast genera except Saccharomyces were recognized. Further analysis indicated that the mAb 92-276/018 reacts with an epitope in the carbohydrate chain of the cell wall glycoproteins.  相似文献   
945.
946.
Michael Landry 《CMAJ》1977,116(12):1392-1394
Although odontogenic keratocysts are benign they are often locally destructive and tend to recur after conservative surgical treatment. They must therefore be distinguished from other cysts of the jaw. Keratocysts possess outpouchings and microscopic daughter cysts from which recurrences may arise. Histologic examination is essential for diagnosis since the appearances on roentgenograms and at operation usually do not reveal the true nature of the lesion. Since many nondental surgeons and pathologists are unaware of odontogenic keratocysts a case is presented in which surgical treatment was originally conservative and finally relatively radical.  相似文献   
947.
948.
949.
Using sensitive radio-enzymatic assays, levels of plasma total catecholamines and norepinephrine in rats change dramatically with changes in body temperature. The decrease in plasma catecholamines induced by warming the animal is reflected in an apparent arterio-venous difference when arterial blood is obtained at room temperature and tail sampling is aided by heat induced vasodilation. Combined blockade of extraneuronal and neuronal uptake reduces this arterio-venous difference. Blood samples obtained from the decapitated trunk of the rat contain similar levels of plasma catecholamines as those obtained from indwelling carotid catheters. Blood levels of dopamine-betahydroxylase were similar whether obtained by venous sampling during heat-induced vasodilation, decapitation or indwelling arterial cannula.  相似文献   
950.
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