全文获取类型
收费全文 | 150510篇 |
免费 | 5951篇 |
国内免费 | 904篇 |
专业分类
157365篇 |
出版年
2023年 | 243篇 |
2022年 | 505篇 |
2021年 | 1005篇 |
2020年 | 632篇 |
2019年 | 808篇 |
2018年 | 12620篇 |
2017年 | 11325篇 |
2016年 | 8787篇 |
2015年 | 3009篇 |
2014年 | 2971篇 |
2013年 | 3431篇 |
2012年 | 8327篇 |
2011年 | 16621篇 |
2010年 | 14313篇 |
2009年 | 10381篇 |
2008年 | 12923篇 |
2007年 | 14492篇 |
2006年 | 3496篇 |
2005年 | 3579篇 |
2004年 | 3966篇 |
2003年 | 3768篇 |
2002年 | 3453篇 |
2001年 | 1121篇 |
2000年 | 869篇 |
1999年 | 835篇 |
1998年 | 861篇 |
1997年 | 609篇 |
1996年 | 521篇 |
1995年 | 470篇 |
1994年 | 503篇 |
1993年 | 523篇 |
1992年 | 642篇 |
1991年 | 536篇 |
1990年 | 493篇 |
1989年 | 497篇 |
1988年 | 457篇 |
1987年 | 421篇 |
1986年 | 403篇 |
1985年 | 377篇 |
1984年 | 441篇 |
1983年 | 415篇 |
1982年 | 421篇 |
1981年 | 382篇 |
1980年 | 396篇 |
1979年 | 310篇 |
1978年 | 299篇 |
1977年 | 263篇 |
1976年 | 257篇 |
1972年 | 403篇 |
1971年 | 437篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
31.
The results of recent randomized trials to test the influence of antioxidants on coronary-event rates and prognosis in patients
with coronary-artery disease were disappointing. In none of these studies did the use of vitamin E improve prognosis. In contrast,
treatment of coronary-artery disease with angiotensin-converting-enzyme (ACE) inhibitors reduced coronary-event rates and
improved prognosis. ACE inhibition prevents the formation of angiotensin II, which has been shown to be a potent stimulus
of superoxide-producing enzymes in atherosclerosis. The findings suggest that inhibition of superoxide production at enzymatic
levels, rather than symptomatic superoxide scavenging, may be the better choice of treatment. 相似文献
32.
The purpose of this review is to provide a global perspective on Oncorhynchus salmonine introductions and put-and-take fisheries based on modern stocking programs, with special emphasis on freshwater
ecosystems. We survey the global introductions of nine selected salmonines of the genus Oncorhynchus: golden trout, cutthroat trout, pink salmon, chum salmon, coho salmon, masu/cherry salmon, rainbow trout/steelhead, sockeye
salmon/kokanee, and chinook salmon. The information is organized on a geographical basis by continent, and then by species
and chronology. Two different objectives and associated definitions of ‘success’ for introductions are distinguished: (a)
seed introduction: release of individuals with the purpose of creating a wild-reproducing, self-sustaining population; and
(b) put-and-take introduction: release of individuals with the purpose of maintaining some level of wild population abundance,
regardless of wild reproduction. We identify four major phenomena regarding global salmonine introductions: (1) general inadequacy
of documentation regarding introductions; (2) a fundamental disconnect between management actions and ecological consequences
of introductions; (3) the importance of global climate change on success of previous and future introductions; and (4) the
significance of aquaculture as a key uncertainty in accidental introductions. We conclude this review with a recognition of
the need to terminate ongoing stocking programs for introduced salmonines worldwide. 相似文献
33.
34.
Bruce M. Taylor Ronald W. Sarver Gregory Fici Roger A. Poorman Barry S. Lutzke Antonio Molinari Thomas Kawabe Karl Kappenman Allen E. Buhl Dennis E. Epps 《The protein journal》2003,22(1):31-40
The time dependency of the spontaneous aggregation of the fibrillogenic β-Amyloid peptide, Aβ1–40, was measured by turbidity, circular dichroism, HPLC, and fluorescence polarization. The results by all methods were comparable and they were most consistent with a kinetic model where the peptide first slowly forms an activated monomeric derivative (AM), which is the only species able to initiate, by tetramerization, the formation of linear aggregates. The anti-Aβ antibody 6E10, raised against residues 1–17, at concentrations of 200–300 nM delayed significantly the aggregation of 50 μM amyloid peptide. The anti–Aβ antibody 4G8, raised against residues 17–24, was much less active in that respect, while the antibody A162, raised against the C-terminal residues 39–43 of the full-length Aβ was totally inactive at those concentrations. Concomitant with the aggregation experiments, we also measured the time dependency of the Aβ1–40–induced toxicity toward SH-EP1 cells and hippocampal neurons, evaluated by SYTOX Green fluorescence, lactate dehydrogenase release, and activation of caspases. The extent of cell damage measured by all methods reached a maximum at the same time and this maximum coincided with that of the concentration of AM. According to the kinetic scheme, the latter is the only transient peptide species whose concentration passes through a maximum. Thus, it appears that the toxic species of Aβ1–40 is most likely the same transient activated monomer that is responsible for the nucleation of fibril formation. These conclusions should provide a structural basis for understanding the toxicity of Aβ1–40 in vitro and possibly in vivo. 相似文献
35.
36.
37.
Lee D. Thomas 《Behavioural processes》2009,81(2):264-269
Four pigeons responded under a progressive-delay procedure. In a signaled-delay condition, a chained variable interval (VI) 30-s progressive time (PT) 4-s schedule was arranged; in an unsignaled-delay condition, a tandem VI 30-s PT 4-s schedule was arranged. Two pigeons experienced a signaled-unsignaled-signaled sequence; whereas, two pigeons experienced an unsignaled-signaled-unsignaled sequence. Effects of saline and d-amphetamine were determined under each condition. At intermediate doses (1.0 and 1.78 m/kg) delay functions were shallower, area under the curve was increased, and, when possible, break points were increased compared to saline; these effects were not systematically related to signaling conditions. These effects on control by delay often were accompanied by decreased response rates at 0 s. These results suggest that stimulus conditions associated with the delay may not play a crucial role in effects of d-amphetamine and other stimulants on behavior controlled by reinforcement delay. 相似文献
38.
Roberta L. Millstein 《Journal of the history of biology》2008,41(2):339-367
Biologists and philosophers have been extremely pessimistic about the possibility of demonstrating random drift in nature,
particularly when it comes to distinguishing random drift from natural selection. However, examination of a historical case
– Maxime Lamotte’s study of natural populations of the land snail, Cepaea nemoralis in the 1950s – shows that while some pessimism is warranted, it has been overstated. Indeed, by describing a unique signature
for drift and showing that this signature obtained in the populations under study, Lamotte was able to make a good case for
a significant role for␣drift. It may be difficult to disentangle the causes of drift and selection acting in a population,
but it is not (always) impossible. 相似文献
39.
40.