全文获取类型
收费全文 | 75953篇 |
免费 | 142627篇 |
国内免费 | 29633篇 |
专业分类
248213篇 |
出版年
2019年 | 2854篇 |
2018年 | 2482篇 |
2017年 | 2251篇 |
2016年 | 2427篇 |
2015年 | 2709篇 |
2014年 | 2837篇 |
2013年 | 2931篇 |
2012年 | 3263篇 |
2011年 | 3820篇 |
2010年 | 5074篇 |
2009年 | 10090篇 |
2008年 | 4339篇 |
2007年 | 4275篇 |
2006年 | 3107篇 |
2005年 | 3135篇 |
2004年 | 3049篇 |
2003年 | 2552篇 |
2002年 | 3323篇 |
2001年 | 4349篇 |
1999年 | 6831篇 |
1998年 | 8959篇 |
1997年 | 9141篇 |
1996年 | 8509篇 |
1995年 | 8806篇 |
1994年 | 8187篇 |
1993年 | 7830篇 |
1992年 | 7847篇 |
1991年 | 7854篇 |
1990年 | 8697篇 |
1989年 | 7936篇 |
1988年 | 7259篇 |
1987年 | 6340篇 |
1986年 | 5876篇 |
1985年 | 5315篇 |
1984年 | 4134篇 |
1983年 | 3324篇 |
1982年 | 3657篇 |
1981年 | 3299篇 |
1980年 | 3232篇 |
1979年 | 3354篇 |
1978年 | 3040篇 |
1977年 | 3014篇 |
1976年 | 2811篇 |
1975年 | 2391篇 |
1974年 | 2544篇 |
1973年 | 2523篇 |
1972年 | 2877篇 |
1971年 | 2631篇 |
1970年 | 2377篇 |
1969年 | 2435篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
92.
93.
94.
Comparisons among 16S rRNA sequences from various eubacteria reveal a natural relationship between the bacteroides (represented by the Bacteroides fragilis sequence) and a phylogenetic unit that comprises the flavobacteria, cytophagae, flexibacteria, and others (represented by the Flavobacterium heparinum sequence). Although the relationship is not a close one, it is, nevertheless, specific. rRNAs from these two organisms are not only closer to one another in overall sequence than they are to outgroup species (such as Bacillus subtilis, Escherichia coli, Desulfovibrio desulfuricans, and Agrobacterium tumefaciens), but they show common idiosyncrasies (i.e., derived characteristics) in both rRNA sequences and higher-order structures. 相似文献
95.
96.
97.
98.
Bacopa monniera Wettst. (BM, syn. Herpestis monniera L; Scrophulariaceae), is an Ayurvedic drug used as a rasayana. Its fresh juice was earlier reported to have significant antiulcerogenic activity. In continuation, methanolic extract of BM (BME) standardized to bacoside-A content (percentage-38.0 ± 0.9), when given in the dose of 10–50 mg/kg, twice daily for 5 days, showed dose-dependent anti-ulcerogenic on various gastric ulcer models induced by ethanol, aspirin, 2 h cold restraint stress and 4 h pylorus ligation. BME in the dose of 20 mg/kg, given for 10 days, twice daily showed healing effects against 50% acetic acid-induced gastric ulcers. Further work was done to investigate the possible mechanisms of its action by studying its effect on various mucosal offensive acid-pepsin secretion and defensive factors like mucin secretion, mucosal cell shedding, cell proliferation and antioxidant activity in rats. BME 20 mg/kg showed no effect on acid-pepsin secretion, increased mucin secretion, while it decreased cell shedding with no effect on cell proliferation. BME showed significant antioxidant effect per se and in stressed animals. Thus, the gastric prophylactic and curative effects of BME may be due to its predominant effect on mucosal defensive factors. 相似文献
99.
100.
Detailed flow patterns in the nasal cavity. 总被引:9,自引:0,他引:9
The human nasal cavity filters and conditions inspired air while providing olfactory function. Detailed experimental study of nasal airflow patterns has been limited because of the complex geometry of the nasal cavity. In this work, particle image velocimetry was used to determine two-dimensional instantaneous velocity vector fields in parallel planes throughout a model of the nasal cavity that was subjected to a nonoscillatory flow rate of 125 ml/s. The model, which was fabricated from 26 computed tomography scans by using rapid prototyping techniques, is a scaled replica of a human right nasal cavity. The resulting vector plots show that the flow is laminar and regions of highest velocity are in the nasal valve and in the inferior airway. The relatively low flow in the olfactory region appears to protect the olfactory bulb from particulate pollutants. Low flows were also observed in the nasal meatuses, whose primary function has been the subject of debate. Comparison of sequentially recorded data suggests a steady flow. 相似文献