全文获取类型
收费全文 | 4909篇 |
免费 | 457篇 |
国内免费 | 1篇 |
专业分类
5367篇 |
出版年
2023年 | 20篇 |
2022年 | 34篇 |
2021年 | 86篇 |
2020年 | 62篇 |
2019年 | 70篇 |
2018年 | 54篇 |
2017年 | 68篇 |
2016年 | 97篇 |
2015年 | 242篇 |
2014年 | 235篇 |
2013年 | 288篇 |
2012年 | 385篇 |
2011年 | 395篇 |
2010年 | 251篇 |
2009年 | 203篇 |
2008年 | 277篇 |
2007年 | 301篇 |
2006年 | 283篇 |
2005年 | 293篇 |
2004年 | 256篇 |
2003年 | 245篇 |
2002年 | 244篇 |
2001年 | 44篇 |
2000年 | 29篇 |
1999年 | 46篇 |
1998年 | 80篇 |
1997年 | 41篇 |
1996年 | 37篇 |
1995年 | 38篇 |
1994年 | 35篇 |
1993年 | 47篇 |
1992年 | 34篇 |
1991年 | 26篇 |
1990年 | 31篇 |
1989年 | 20篇 |
1988年 | 12篇 |
1987年 | 30篇 |
1986年 | 19篇 |
1985年 | 22篇 |
1984年 | 35篇 |
1983年 | 38篇 |
1982年 | 35篇 |
1981年 | 24篇 |
1980年 | 36篇 |
1979年 | 17篇 |
1978年 | 29篇 |
1977年 | 19篇 |
1976年 | 15篇 |
1974年 | 16篇 |
1973年 | 18篇 |
排序方式: 共有5367条查询结果,搜索用时 15 毫秒
71.
72.
73.
Campylobacter jejuni is an important cause of human diarrheal disease throughout the world and likeSalmonella enteritidis, has a large animal reservoir which includes most of man's domestic animals. Until recently, it has been difficult to trace the chain of transmission from animals to man because of inadequate environmental sampling techniques and means to distinguish strains. Recent improvements in these techniques have made environmental studies more feasible in 2 water-related out-breaks.In 1 study,C. jejuni was found to be an important cause of sporadic, summertime diarrheal disease among hikers in national wilderness areas of Wyoming. In this setting, illness was significantly associated with drinking untreated surface water. SubsequentlyC. jejuni was isolated from surface water, including mountian streams, and from animals in the area. Some of the environmental isolates were serotypically identical to strains isolated from humans.A second study occurred as a result of an outbreak of Campylobacter enteritis in a community in northern Illinois which was epidemiologically associated with the community water system.Campylobacter jejuni was isolated from several surface water sources and from the implicated water system. These studies demonstrate that environmental isolation ofC. jejuni is now possible and may add to our understanding of disease transmission. 相似文献
74.
This study was carried out to determine differences between old and young rhesus males in levels and diurnal patterns of testosterone, dihydrotestosterone, cortisol, and estradiol, and to determine correlations between these hormones and sexual behavior of the old males. Blood was drawn from old (n = 9) and young (n = 9) rhesus males over 5 consecutive days at 0900, 1300, and 2100 hr. The two groups of males did not differ in mean serum levels of testosterone, dihydrotestosterone, or estradiol at any time. Although the old and young did not differ in cortisol levels at 0900 and 1300 hr, the cortisol levels at 2100 hr were lower in the old males. Diurnal variations in testosterone, dihydrotestosterone, and cortisol were comparable in old and young males. Mean serum levels of estradiol were significantly higher at 0900 hr than at 1300 hr in the old males, whereas in the young males estradiol levels did not differ with time of day. There was a significant positive correlation between testosterone and yawning rate, and cortisol levels were correlated positively with rate of contacting, rate of mounting, and percentage of tests with erections. The decline in sexual performance of old rhesus males cannot be attributed to changes in the levels or diurnal patterns of testosterone, dihydrotestosterone, or estradiol, but lower cortisol levels in old males may contribute to the decline in sexual behavior. 相似文献
75.
Herbert M. Kagan Kathleen A. Sullivan Theodore A. Olsson III Anne L. Cronlund 《The Biochemical journal》1979,177(1):203-214
Lysyl oxidase of bovine aorta was resolved into four enzymically active species by elution from DEAE-cellulose with a salt gradient in 6m-urea, consistent with purification results obtained with enzyme of other tissues [Stassen (1976) Biochim. Biophys. Acta438, 49-60]. In the present study, each of the four peaks of activity was purified to apparent homogeneity by subsequent chromatography on gel-filtration media in 6m-urea. Each enzyme is eluted as a species with mol.wt. approx. 30000 under these conditions, although lysyl oxidase polymerizes to a series of multimers with molecular weights ranging up to 1000000 in the absence of urea. The apparent subunit molecular weight of each enzyme species determined by electrophoresis in sodium dodecyl sulphate and 8m-urea is approx. 32000-33000. The amino acid compositions of the purified forms of lysyl oxidase are similar to each other, although sufficient differences exist to conclude that each is a unique molecular species. Incorporation of alpha-toluenesulphonyl fluoride into the purification scheme does not alter the resolution of enzyme into four species, suggesting that proteolysis during isolation is not the basis of the heterogeneity. The similar sensitivities of each form of enzyme to chelating agents and to semicarbazide and isoniazid indicate that each requires the participation of a metal ion, presumably Cu(2+), and of a carbonyl compound for enzyme function. The present study describes a method for the purification of multiple species of lysyl oxidase and reveals that significant chemical differences exist between the different enzyme forms. 相似文献
76.
77.
Linda T. Tran Kathleen M. MacLeod John H. McNeill 《Molecular and cellular biochemistry》2014,392(1-2):205-211
The purpose of this study was to investigate the effect of chronic treatment with prazosin, a selective α1-adrenoceptor antagonist, on the development of hypertension in fructose-fed rats (FFR). High-fructose feeding and treatment with prazosin (1 mg/kg/day via drinking water) were initiated simultaneously in male Wistar rats. Systolic blood pressure, fasted plasma parameters, insulin sensitivity, plasma norepinephrine (NE), uric acid, and angiotensin II (Ang II) were determined following 9 weeks of treatment. FFR exhibited insulin resistance, hyperinsulinemia, hypertriglyceridemia, and hypertension, as well as elevations in plasma NE and Ang II levels. Treatment with prazosin prevented the rise in blood pressure without affecting insulin levels, insulin sensitivity, uric acid, or Ang II levels, while normalizing plasma NE levels in FFR. These data suggest that over-activation of the sympathetic nervous system, specifically α1-adrenoceptors, contributes to the development of fructose-induced hypertension, however, this over-activation does not appear to an initial, precipitating event in FFR. 相似文献
78.
Species–area curves from islands and other isolates often differ in shape from sample‐area curves generated from mainlands or sections of isolates (or islands), especially at finer scales. We examine two explanations for this difference: (1) the small‐island effect (SIE), which assumes the species–area curve is composed of two distinctly different curve patterns; and (2) a sigmoid or depressed isolate species–area curve with no break‐points (in arithmetic space). We argue that the application of Ockham’s razor – the principle that the simplest, most economical explanation for a hypothesis should be accepted over less parsimonious alternatives – leads to the conclusion that the latter explanation is preferable. We hold that there is no reason to assume the ecological factors or patterns that affect the shapes of isolate (or island) curves cause two distinctly different patterns. This assumption is not required for the alternative, namely that these factors cause a single (though depressed) isolate species–area curve with no break‐points. We conclude that the theory of the small‐island effect, despite its present standing as an accepted general pattern in nature, should be abandoned. 相似文献
79.
Cloudwater Inputs of Nitrogen to Forest Ecosystems in Southern Chile: Forms, Fluxes, and Sources 总被引:6,自引:0,他引:6
Nitrogen (N) has been considered a limiting nutrient to many aquatic and terrestrial ecosystems. However, human activity has
resulted in increased atmospheric N deposition worldwide such that N pollution is now altering ecosystem function in many
locations. Research on atmospheric deposition has focused primarily on inorganic nitrogen (DIN; NH4
+-N + NO3
−-N) via rainwater and dry deposition as the main N input to ecosystems. Recently, organic N (ON) has been shown to be an important
constituent in rainwater or dry deposition. Here we show that ON dominated (66%) total N in cloudwater from a remote site
in southern Chile. Cloudwater from more human-impacted sites in northeastern USA had lower ON concentrations and DIN was dominant.
We estimate that cloudwater delivers up to 2 kg ha−1 DIN and 9 kg ha−1 ON annually, compared to less than 1 kg ha−1 of DIN deposition via rainwater, thus it may contribute substantially to the N-economy of Chilean coastal forests. We also
suggest that the adjacent ocean, where biologic productivity is high, may be a major source of N in Chilean cloudwater. This
proposed marine-terrestrial flux via cloud deposition has not previously been identified and may be an example of the ocean
feeding the forest. We suggest that this type of cross boundary flux may be common in other upwelling zones, such as along
the west coasts of Africa, North and South America and east India, and warrants further substantiation and investigation.
Received 30 June 2000; accepted 18 October 2000 相似文献
80.