全文获取类型
收费全文 | 5572篇 |
免费 | 534篇 |
国内免费 | 3篇 |
出版年
2021年 | 89篇 |
2020年 | 48篇 |
2019年 | 77篇 |
2018年 | 66篇 |
2017年 | 56篇 |
2016年 | 113篇 |
2015年 | 174篇 |
2014年 | 191篇 |
2013年 | 244篇 |
2012年 | 377篇 |
2011年 | 353篇 |
2010年 | 210篇 |
2009年 | 213篇 |
2008年 | 292篇 |
2007年 | 297篇 |
2006年 | 307篇 |
2005年 | 245篇 |
2004年 | 278篇 |
2003年 | 231篇 |
2002年 | 210篇 |
2001年 | 115篇 |
2000年 | 88篇 |
1999年 | 91篇 |
1998年 | 63篇 |
1997年 | 57篇 |
1996年 | 46篇 |
1995年 | 49篇 |
1994年 | 48篇 |
1993年 | 58篇 |
1992年 | 65篇 |
1991年 | 51篇 |
1990年 | 73篇 |
1989年 | 61篇 |
1988年 | 66篇 |
1987年 | 65篇 |
1986年 | 56篇 |
1985年 | 70篇 |
1984年 | 41篇 |
1983年 | 51篇 |
1982年 | 49篇 |
1980年 | 37篇 |
1979年 | 51篇 |
1977年 | 39篇 |
1976年 | 37篇 |
1975年 | 43篇 |
1974年 | 35篇 |
1973年 | 43篇 |
1972年 | 45篇 |
1971年 | 43篇 |
1970年 | 35篇 |
排序方式: 共有6109条查询结果,搜索用时 0 毫秒
1.
Investing in ecosystem health: Using rivers as a case study 总被引:2,自引:0,他引:2
Jeff Bennett 《Ecological Management & Restoration》2002,3(2):104-107
2.
3.
4.
Marianne E. Walsh Charles M. Collins Thomas F. Jenkins Alan D. Hewitt Jeff Stark Karen Myers 《Soil & Sediment Contamination》2003,12(5):631-645
Fort Greely, Alaska has an extensive complex of weapon training and testing areas located on lands withdrawn from the public domain under the Military Lands Withdrawal Act (PL106-65). The Army has pledged to implement a program to identify possible munitions contamination. Because of the large size (344,165,000 m2) of the high hazard impact areas, characterization of these constituents will be difficult. We used an authoritative sampling design to find locations most likely to contain explosives-residues on three impact areas. We focused our sampling on surface soils and collected multi-increment and discrete samples at locations of known firing events and from areas on the range that had craters, pieces of munitions, targets, or a designation as a firing point. In the two impact areas used primarily by the Army, RDX was the most frequently detected explosive. In the impact area that was also used by the Air Force, TNT was the most frequently detected explosive. Where detected, the explosives concentrations generally were low (<0.05 mg/kg) except in soils near low-order detonations, where the explosive-filler was in contact with the soil surface. These low-order detonations potentially can serve as localized sources for groundwater contamination if positioned in recharge areas. 相似文献
5.
6.
Jeff A Johnson Heather RL Lerner Pamela C Rasmussen David P Mindell 《BMC evolutionary biology》2006,6(1):65-12
Background
Populations of the Oriental White-backed Vulture (Gyps bengalensis) have declined by over 95% within the past decade. This decline is largely due to incidental consumption of the non-steroidal anti-inflammatory veterinary pharmaceutical diclofenac, commonly used to treat domestic livestock. The conservation status of other Gyps vultures in southern Asia is also of immediate concern, given the lack of knowledge regarding status of their populations and the continuing existence of taxonomic uncertainties. In this study, we assess phylogenetic relationships for all recognized species and the majority of subspecies within the genus Gyps. The continuing veterinary use of diclofenac is an unknown but potential risk to related species with similar feeding habits to Gyps bengalensis. Therefore, an accurate assessment of the phylogenetic relationships among Gyps vultures should aid in their conservation by clarifying taxonomic uncertainties, and enabling inference of their respective relatedness to susceptible G. bengalensis. 相似文献7.
Kathryn L. Butler Vijayakrishnan Ambravaneswaran Nitin Agrawal Maryelizabeth Bilodeau Mehmet Toner Ronald G. Tompkins Shawn Fagan Daniel Irimia 《PloS one》2010,5(7)
Thermal injury triggers a fulminant inflammatory cascade that heralds shock, end-organ failure, and ultimately sepsis and death. Emerging evidence points to a critical role for the innate immune system, and several studies had documented concurrent impairment in neutrophil chemotaxis with these post-burn inflammatory changes. While a few studies suggest that a link between neutrophil motility and patient mortality might exist, so far, cumbersome assays have prohibited exploration of the prognostic and diagnostic significance of chemotaxis after burn injury. To address this need, we developed a microfluidic device that is simple to operate and allows for precise and robust measurements of chemotaxis speed and persistence characteristics at single-cell resolution. Using this assay, we established a reference set of migration speed values for neutrophils from healthy subjects. Comparisons with samples from burn patients revealed impaired directional migration speed starting as early as 24 hours after burn injury, reaching a minimum at 72–120 hours, correlated to the size of the burn injury and potentially serving as an early indicator for concurrent infections. Further characterization of neutrophil chemotaxis using this new assay may have important diagnostic implications not only for burn patients but also for patients afflicted by other diseases that compromise neutrophil functions. 相似文献
8.
9.
R J Butler 《American journal of physical anthropology》1972,36(3):381-390
Investigation of two populations of 136 individuals shows several patterns of occlusal wear plane change which are positively correlated with age. For individuals up to the age of 18, there is a characteristic pattern in which the occlusal wear planes of the mandibular teeth are lingually sloped and those of the maxillary dentition buccally sloped, with the exception of the maxillary premolars, which are also lingually sloped. The long axes of the mandibular teeth give them a lingual orientation relative to the maxillary teeth, and the long axes of the maxillary molars, by contrast, are buccally oriented. In the 18-30 age range for all sexes, the mandibular M1 becomes buccally sloped on its occlusal surface while the occlusal wear plane on the maxillary M1 becomes lingually sloped. Later age changes indicate a trend for the mandibular premolars to become buccally sloped, while the wear planes of the maxillary premolars remain lingually sloped. There is a corresponding tendency for the maxillary and mandibular second molars to undergo changes in the initial orientation of the occlusal wear planes. 相似文献
10.