全文获取类型
收费全文 | 1303548篇 |
免费 | 142102篇 |
国内免费 | 728篇 |
专业分类
1446378篇 |
出版年
2018年 | 15212篇 |
2017年 | 14631篇 |
2016年 | 18041篇 |
2015年 | 21944篇 |
2014年 | 25097篇 |
2013年 | 35439篇 |
2012年 | 41599篇 |
2011年 | 44833篇 |
2010年 | 31004篇 |
2009年 | 28803篇 |
2008年 | 39598篇 |
2007年 | 41472篇 |
2006年 | 34947篇 |
2005年 | 33980篇 |
2004年 | 33350篇 |
2003年 | 32139篇 |
2002年 | 31237篇 |
2001年 | 54458篇 |
2000年 | 54997篇 |
1999年 | 43965篇 |
1998年 | 16440篇 |
1997年 | 17112篇 |
1996年 | 16248篇 |
1995年 | 15266篇 |
1994年 | 15120篇 |
1993年 | 15053篇 |
1992年 | 36951篇 |
1991年 | 36132篇 |
1990年 | 35150篇 |
1989年 | 34716篇 |
1988年 | 32074篇 |
1987年 | 31024篇 |
1986年 | 28640篇 |
1985年 | 28864篇 |
1984年 | 23854篇 |
1983年 | 20793篇 |
1982年 | 16191篇 |
1981年 | 14516篇 |
1980年 | 13808篇 |
1979年 | 23069篇 |
1978年 | 18298篇 |
1977年 | 16528篇 |
1976年 | 15572篇 |
1975年 | 17144篇 |
1974年 | 18323篇 |
1973年 | 18146篇 |
1972年 | 16638篇 |
1971年 | 15295篇 |
1970年 | 13116篇 |
1969年 | 12522篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
The probability that protected areas will deliver their potential for maintaining or enhancing biodiversity is likely to be
maximised if they are appropriately and effectively managed. As a result, governments and conservation agencies are devoting
much attention to the management of protected areas. In the U.K., the demand for performance accountability has resulted in
Public Service Agreements (PSA) that set out targets for government departments to deliver results in return for investments
being made. One such target for England is to ensure that all nationally important wildlife sites are in favourable condition
by 2010. Here, we tested the hypothesis, of potential strategic importance, that the ecological condition of these sites is
predictable from relationships with a range of physical, environmental and demographic variables. We used binary logistic
regression to investigate these relationships, using the results of English Nature’s 1997–2003 condition assessment exercise.
Generally, sites in unfavourable condition tend to be larger in area, located at higher elevations, but with higher human
population density and are more spatially isolated from units of the same habitat. However, despite the range of different
parameters included in our models, the extent to which the condition of any given site could be predicted was low. Our results
have implications for the delivery of PSA targets, funding allocation, and the location of new protected areas. 相似文献
993.
M. Gopalakrishnan P. Sureshkumar J. Thanusu V. Kanagarajan 《Journal of enzyme inhibition and medicinal chemistry》2013,28(3):347-351
Compound 26 is more potent against Escherichia coli. and 24 is more active against Staphylococcus aureus, β-Heamolytic streptococcus, Vibreo cholerae, Salmonella typhii, and Shigella flexneri than the standard drug ciprofloxacin. Moreover, of all the compounds tested, 26 is more effective against Aspergillus flavus and Mucor, than the standard drug fluconazole. 相似文献
994.
P S Simavorian D A Gevorkian I L Saakian A B Iordanian 《Biulleten' eksperimental'no? biologii i meditsiny》1987,103(3):310-312
It has been experimentally and clinically established that the determination of leucine-aminotransferase activity in the blood serum and abdominal exudate may serve as a marker for the early determination of pancreonecrosis and edematous (serous) pancreatitis. 相似文献
995.
P Kendall-Taylor K Hall D G Johnston R W Prescott 《BMJ (Clinical research ed.)》1982,285(6340):465-467
The effect of pergolide mesylate was studied in two previously untreated men with large prolactinomas and exceptionally high prolactin concentrations. The study was designed to determine whether pergolide would be effective in alleviating symptoms, correcting hormonal abnormalities and shrinking the tumour. Starting with 50 micrograms daily the dose of pergolide was slowly increased over 10 weeks to 1 mg once daily, when repeat assessment was performed. Both patients reported complete relief of symptoms, with no side effects. Serum prolactin concentration was suppressed to normal in both subjects, and evidence to suggest tumour shrinkage was observed. Pergolide appears to be effective treatment for men with large prolactinomas. 相似文献
996.
The phenotype of a ouabain-resistant Aedes albopictus cell line has been partially characterized. Treatment of ouabain-sensitive cells with 0.005-1.0 mM ouabain resulted in an 80% reduction in the uptake of 86rubidium (86Rb+), an ion with an affinity for the K+ pump binding site; ouabain-resistant cells showed only a 40% reduction with 1.0 mM ouabain. When ouabain-sensitive cells were incubated in the presence of ouabain (0.1 mM) for one and one-half to three hours, the molar ratio of intracellular Na+/K+ rose from 0.2 to 4.2. In ouabain-resistant cells, a similar treatment had very little effect. Based on [3H] ouabain-binding studies, ouabain-resistant cells were estimated to have 60% fewer binding sites per cell than ouabain-sensitive cells. The spontaneous mutation rate from ouabain sensitivity to ouabain resistance was calculated to be 1-6 x 10(-8) mutations/cell/generation, a value similar to that reported for mammalian cells at the analogous locus. 相似文献
997.
998.
The effect of estradiol on the brain concentration of immunoreactive beta-endorphin (beta-EP) and C-terminal ACTH (CLIP) was studied in ovariectomized rats. Dopamine, a known inhibitor of pituitary intermediate lobe pro-opiomelanocortin (POMC), was examined as a possible mediator of the estradiol induced changes in brain POMC. Animals were treated for 1 or 3 weeks with either 1) saline; 2) silastic estradiol implants; or 3) estradiol implants plus haloperidol 1 mg/kg/day. After one week of treatment no significant change in hypothalamic beta-EP content was noted in any group compared to the control level of 4.13 +/- .33 (SEM) pmoles although in the neurointermediate lobe beta-EP increased from 566 +/- 72 to 942 +/- 73 pmoles after haloperidol (p less than .005). After 3 weeks, however, hypothalamic beta-EP decreased from 3.96 +/- .28 to 2.74 +/- .19 pmoles (p less than .005) and C-terminal ACTH decreased from 3.78 +/- .33 to 2.82 +/- .18 pmoles (p less than .02) in the estradiol treated rats. This estradiol induced decrease in the hypothalamic content of beta-EP and C-terminal ACTH was not blocked by haloperidol. We conclude that estradiol lowers the hypothalamic content of beta-EP and CLIP and that this effect does not appear to be mediated by dopamine. 相似文献
999.
A report was made on the outcome of a four year retrospective study in 27 064 pregnancies, of the clinical efficiency, sensitivity, and specificity of a screening programme for Down''s syndrome based on reported strategies related to the measurement of maternal serum alpha fetoprotein. This study identified 27 pregnancies affected by Down''s syndrome with a median multiple of the median maternal serum alpha fetoprotein concentration of 0.82. This figure is considerably higher than that obtained from previous reports on this subject. With an age related multiple of the median maternal serum alpha fetoprotein strategy, 30.8% of Down''s affected pregnancies were identified as well as 11.6% of unaffected pregnancies. Perhaps a United Kingdom collaborative study should begin to investigate the reasons for such wide population variance in the reports for the median multiple of the median for Down''s affected pregnancies. Until such studies are carried out, screening for Down''s syndrome based on low maternal serum alpha fetoprotein concentration is premature. 相似文献
1000.