全文获取类型
收费全文 | 82475篇 |
免费 | 7207篇 |
国内免费 | 42篇 |
出版年
2022年 | 468篇 |
2021年 | 1009篇 |
2020年 | 615篇 |
2019年 | 768篇 |
2018年 | 979篇 |
2017年 | 909篇 |
2016年 | 1524篇 |
2015年 | 2565篇 |
2014年 | 2903篇 |
2013年 | 3982篇 |
2012年 | 4856篇 |
2011年 | 4987篇 |
2010年 | 3266篇 |
2009年 | 2944篇 |
2008年 | 4384篇 |
2007年 | 4513篇 |
2006年 | 4291篇 |
2005年 | 4256篇 |
2004年 | 4359篇 |
2003年 | 3916篇 |
2002年 | 3959篇 |
2001年 | 1022篇 |
2000年 | 773篇 |
1999年 | 1033篇 |
1998年 | 1259篇 |
1997年 | 936篇 |
1996年 | 842篇 |
1995年 | 827篇 |
1994年 | 807篇 |
1993年 | 763篇 |
1992年 | 727篇 |
1991年 | 734篇 |
1990年 | 706篇 |
1989年 | 755篇 |
1988年 | 654篇 |
1987年 | 634篇 |
1986年 | 569篇 |
1985年 | 701篇 |
1984年 | 843篇 |
1983年 | 720篇 |
1982年 | 871篇 |
1981年 | 912篇 |
1980年 | 814篇 |
1979年 | 603篇 |
1978年 | 623篇 |
1977年 | 588篇 |
1976年 | 586篇 |
1975年 | 462篇 |
1974年 | 552篇 |
1973年 | 509篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
Ferdinand Bohlmann Elena Tsankova Jasmin Jakupovic Robert M. King Harold Robinson 《Phytochemistry》1983,22(2):557-560
The investigation of Encelia canescens afforded, in addition to several known compounds, four new dimeric p-hydroxyacetophenone derivatives, two epimeric chromene dimers and two epimeric mixed dimers of euparin and encecalin. Furthermore, derivatives of tremetone and of encecalin were present. The structures were elucidated hy high field 1H NMR spectroscopy. 相似文献
62.
63.
Linda L. Laack Michael E. Tewes Aaron M. Haines John H. Rappole 《Acta theriologica》2005,50(4):505-514
The ocelotLeopardus pardalis Linnaeus, 1758 is an endangered felid in the United States currently restricted to southern Texas. The objectives of our
study were to obtain data on ocelot parturition dates, fecundity, sex ratios, den characteristics, and first year survival,
all of which are critical in development of population viability models. Sixteen parturition events were recorded ranging
from mid-April to late December for 12 wild ocelots. Cumulatively, litters consisted of 1 or 2 kittens (ˉ = 1.2 ± 0.44 SD).
Cumulative sex ratio was 1∶2.5 (male:female); however, there was no significant difference between the observed sex ratio
and a 1∶1 sex ratio. Ten den sites were in close proximity (≤ 10 m) to dense thornshrub. Adult female ocelots used 2 to 4
den sites for each litter with distance between consecutively occupied dens ranging from 110 to 280 m (ˉ = 158 m ± 93 SD).
An estimated annual survival for ocelots 0 to 1 year of age was 0.68. Evidence suggests that ocelots in the wild may breed
more frequently than had been previously hypothesized. 相似文献
64.
65.
66.
Obesity-inducing lesions of the central nervous system alter leptin uptake by the blood-brain barrier. 总被引:1,自引:0,他引:1
Leptin regulates body adiposity by decreasing feeding and increasing thermogenesis. Obese humans and some obese rodents are resistant to peripherally administered leptin, suggesting a defect in the transport of leptin across the blood-brain barrier (BBB). Defective transport of exogenous leptin occurs in some models of obesity, but in other models transport is normal. This shows that factors other than obesity are associated with impairment of leptin transport across the BBB. In order to further investigate these factors, we determined leptin transport in rats made obese by lesioning of the ventromedial hypothalamus (VMH), paraventricular nucleus (PVN), or posterodorsal amygdala (PDA). These regions all contain leptin receptors and lesions there induce obesity and hyperleptinemia and alter the levels of many feeding hormones which might participate in leptin transporter regulation. We measured the uptake of radioactively labeled leptin by the BBB by multiple-time regression analysis which divides uptake into a reversible phase (Vi, e.g., receptor/transporter binding to the brain endothelial cell) and an irreversible phase (Ki, complete transport across the BBB). Leptin uptake was not affected in rats with VMH lesions. No significant change occurred in the entry rate (Ki) for any group, although Ki declined by over 35% in rats with PVN lesions. Decreased uptake was observed in rats with PVN lesions and with PDA lesions. This was primarily due to a reduced Vi (about 21% for the PDA). This decreased uptake is most likely explained by decreased binding of leptin to the brain endothelial cell, which could be because of decreased binding by either receptors or transporters. This suggests that some of the feeding hormones controlled by the PVN and PDA may participate in regulating leptin uptake by the BBB. 相似文献
67.
68.
69.
70.
Susan Morrison Grace John-Stewart John J. Egessa Sezi Mubezi Sylvia Kusemererwa Dennis K. Bii Nulu Bulya Francis Mugume James D. Campbell Jonathan Wangisi Elizabeth A. Bukusi Connie Celum Jared M. Baeten Partners PrEP Study Team 《PloS one》2015,10(10)
During an HIV-1 prevention clinical trial in East Africa, we observed 16 cases of primary HIV-1 infection in women coincident with pregnancy or breastfeeding. Nine of eleven pregnant women initiated rapid combination antiretroviral therapy (ART), despite having CD4 counts exceeding national criteria for ART initiation; breastfeeding women initiated ART or replacement feeding. Rapid ART initiation during primary HIV-1 infection during pregnancy and breastfeeding is feasible in this setting. 相似文献