全文获取类型
收费全文 | 300篇 |
免费 | 27篇 |
国内免费 | 8篇 |
专业分类
耳鼻咽喉 | 1篇 |
儿科学 | 17篇 |
妇产科学 | 1篇 |
基础医学 | 36篇 |
口腔科学 | 2篇 |
临床医学 | 22篇 |
内科学 | 57篇 |
皮肤病学 | 4篇 |
神经病学 | 11篇 |
特种医学 | 75篇 |
外科学 | 27篇 |
综合类 | 7篇 |
预防医学 | 31篇 |
眼科学 | 5篇 |
药学 | 17篇 |
中国医学 | 1篇 |
肿瘤学 | 21篇 |
出版年
2022年 | 2篇 |
2021年 | 6篇 |
2020年 | 3篇 |
2019年 | 5篇 |
2018年 | 8篇 |
2017年 | 7篇 |
2016年 | 4篇 |
2015年 | 10篇 |
2014年 | 7篇 |
2013年 | 11篇 |
2012年 | 6篇 |
2011年 | 7篇 |
2010年 | 15篇 |
2009年 | 16篇 |
2008年 | 8篇 |
2007年 | 17篇 |
2006年 | 11篇 |
2005年 | 11篇 |
2004年 | 3篇 |
2003年 | 9篇 |
2002年 | 5篇 |
2001年 | 6篇 |
2000年 | 5篇 |
1999年 | 6篇 |
1998年 | 19篇 |
1997年 | 15篇 |
1996年 | 9篇 |
1995年 | 7篇 |
1994年 | 10篇 |
1993年 | 8篇 |
1992年 | 2篇 |
1991年 | 2篇 |
1990年 | 3篇 |
1989年 | 4篇 |
1988年 | 6篇 |
1987年 | 4篇 |
1986年 | 8篇 |
1985年 | 5篇 |
1981年 | 2篇 |
1980年 | 7篇 |
1978年 | 2篇 |
1976年 | 3篇 |
1975年 | 2篇 |
1971年 | 2篇 |
1970年 | 2篇 |
1968年 | 2篇 |
1940年 | 2篇 |
1939年 | 4篇 |
1931年 | 4篇 |
1930年 | 3篇 |
排序方式: 共有335条查询结果,搜索用时 0 毫秒
31.
32.
We report our clinical experience with phototherapy in 3802 infants; 3629 were exposed to "standard" daylight phototherapy and 173 to "high-intensity" blue-light phototherapy. High-intensity blue-light phototherapy was twice as effective as standard daylight phototherapy in decreasing bilirubin concentrations. No failures occurred with high-intensity phototherapy compared with an overall failure rate of 1.84/1000 with daylight lamps; these cases were transferred to high-intensity phototherapy with prompt response. Rebound after cessation of phototherapy was greater in those exposed to high-intensity blue light with a significantly greater number requiring a second exposure. However, the incidence was still low. No third exposure was required in any infant. Nursing of infants under high-intensity blue light was more difficult and inconvenient as was clinical monitoring. The light also caused more stress on the nursing and medical personnel. However, the infants tolerated both types of phototherapy equally well. High-intensity blue-light phototherapy would seem to be the treatment of choice for infants with rapidly increasing or very high bilirubin levels, as well as in those not responding adequately to daylight phototherapy. 相似文献
33.
34.
目的:评价ThromCat血栓切除导管系统(ThromCat Thrombectomy Catheter System,TTC)在急性冠状动脉综合征(ACS)介入治疗应用中的安全性、工作性能及有效性.方法:入选14例ACS患者进行经皮冠状动脉介入治疗(PCI),分析TTC操作的成功率、TTC应用的临床成功率、术中并发症和术后随访30 d心脏不良事件.结果:TTC操作成功11例,TTC临床成功10例,PCI成功13例,并不伴有心脏不良事件.在PCI中,心肌梗死溶栓治疗(TIMI)血流由使用TIC前的0.5(2.0)升高至使用后的2.0(1.0),心肌灌注分级(MBG)由使用前的1.0(2.0)升高至使用后的2.0(1.0),差异均有统计学意义(均P<0.01).结论:在ACS患者中行PCI,TTC能安全有效地去除非严重钙化的冠状动脉内的血栓.并发症少,操作简单易行,减少PCI手术引起的并发症. 相似文献
35.
Gwen S. Antell Isabel S. Fenton Paul J. Valdes Erin E. Saupe 《Proceedings of the National Academy of Sciences of the United States of America》2021,118(18)
Abiotic niche lability reduces extinction risk by allowing species to adapt to changing environmental conditions in situ. In contrast, species with static niches must keep pace with the velocity of climate change as they track suitable habitat. The rate and frequency of niche lability have been studied on human timescales (months to decades) and geological timescales (millions of years), but lability on intermediate timescales (millennia) remains largely uninvestigated. Here, we quantified abiotic niche lability at 8-ka resolution across the last 700 ka of glacial–interglacial climate fluctuations, using the exceptionally well-known fossil record of planktonic foraminifera coupled with Atmosphere–Ocean Global Climate Model reconstructions of paleoclimate. We tracked foraminiferal niches through time along the univariate axis of mean annual temperature, measured both at the sea surface and at species’ depth habitats. Species’ temperature preferences were uncoupled from the global temperature regime, undermining a hypothesis of local adaptation to changing environmental conditions. Furthermore, intraspecific niches were equally similar through time, regardless of climate change magnitude on short timescales (8 ka) and across contrasts of glacial and interglacial extremes. Evolutionary trait models fitted to time series of occupied temperature values supported widespread niche stasis above randomly wandering or directional change. Ecotype explained little variation in species-level differences in niche lability after accounting for evolutionary relatedness. Together, these results suggest that warming and ocean acidification over the next hundreds to thousands of years could redistribute and reduce populations of foraminifera and other calcifying plankton, which are primary components of marine food webs and biogeochemical cycles.Abiotic niche dynamics determine patterns of community composition over space and regulate trajectories of diversity over time (1). Both niche lability (2, 3) and conservatism (1, 4) have been proposed to spur speciation, and abiotic niche lability has been associated with ecological invasions (5–7) and with reduced risk of extinction during times of climate change (8). Thus, a deeper understanding of species’ propensity for niche stasis versus lability could improve predictions of biodiversity restructuring in response to anthropogenic climate change (9).Stasis in species’ abiotic niches through time has been documented in empirical research, but most such studies have been limited to ecological niche modeling on decadal scales (reviewed in ref. 10) or paleoecological examination on 106 to 107 y scales (5, 11, 12). Since empirical rates of niche change are scarce and difficult to acquire, many studies merely assume that niche evolution occurs at a constant rate along branches of a phylogeny (2, 3, 6, 7). Niche dynamics at intermediate timescales of centuries to millennia are particularly poorly documented (10), and studies at this meso scale have been restricted to terrestrial systems (e.g., refs. 13–15) or to comparisons between the present day and the single historical time step of the Last Glacial Maximum, ∼21 ka (16–20). Quantifying the rate and relative frequency of niche change in marine species over timescales of 102 to 105 years is important, however, because species will adapt or go extinct in response to anthropogenic ocean changes over this timescale (21).Here, we investigated climatic niche lability from the rich sedimentary archive of global planktonic foraminifera across the last 700 ka of glacial–interglacial cycles at 8-ka resolution. Planktonic foraminifera (Protista) construct “shells” (tests) of calcite, thereby sequestering carbon and recording an isotopic signature of past ocean conditions. Tests readily accumulate over large expanses of the seafloor. Consequently, the fossil record of foraminifera—arguably “the best fossil record on Earth” (22)—affords an exceptionally high-resolution view into past species distributions. This detailed record fuels studies of biostratigraphy, paleoclimatology, and paleoecology (20, 22–25). Moreover, the complete species diversity of planktonic foraminifera has been described for the Plio–Pleistocene, with good agreement between morphological and molecular phylogenies (22, 25–27). Although some have speculated that foraminifera competitively exclude each other (24), recent work found that planktonic foraminifera species seldom restrict each other’s distributions (28). Presumably, therefore, species occupy the full envelope of existing environmental conditions within their tolerance limits, and geographic distributions are determined almost entirely by physical ocean conditions.We developed five analyses to investigate the degree of abiotic niche lability in foraminifera. All methods examined the univariate niche axis of temperature, which is the single most important explanatory variable in regard to geographic distributions of foraminifera (20, 29–32) and is a climate-related stressor and extinction driver for diverse marine fauna across timescales (33, 34). The adaptive potential of thermal niches has been taken as a key determinant of global community structure and genetic connectance in plankton (35). Primary productivity and other environmental variables, however, may also structure abiotic niches of plankton (36). Our suite of analyses quantified whether and by how much planktonic foraminiferal niches shifted along a temperature axis. First, we correlated time series of species’ thermal optima with global temperature to determine whether species tracked suitable habitat or experienced environmental fluctuations in situ. We then quantified species’ niche dissimilarity between pairs of time bins—either tracking niches across bin boundaries or contrasting niches at climatic extremes of glacial maxima and interglacial thermal peaks. To characterize niche change we applied trait evolution models to time series of temperatures at occupied sites. Lastly, we explored variation in intraspecific niche lability among ecotypes while accounting for phylogenetic relatedness. SI Appendix, Table S1 lists the response variable and sample size for each analysis. 相似文献
36.
Detection of reactive oxygen species (ROS) and apoptosis in human fragmented embryos 总被引:8,自引:2,他引:8
Yang HW; Hwang KJ; Kwon HC; Kim HS; Choi KW; Oh KS 《Human reproduction (Oxford, England)》1998,13(4):998-1002
In human in-vitro fertilization (IVF)-embryo transfer, the in-vitro culture
environment differs from in-vivo conditions in that the oxygen
concentration is higher, and in such conditions the mouse embryos show a
higher concentration of reactive oxygen species (ROS) in simple culture
media. ROS are believed to cause damage to cell membranes and DNA
fragmentation in somatic cells. This study was conducted to ascertain the
level of H2O2 concentration within embryos and the morphological features
of cell damage induced by H2O2. A total of 62 human oocytes and embryos (31
fragmented, 15 non-fragmented embryos, 16 unfertilized oocytes) was
obtained from the IVF-embryo transfer programme. The relative intensity of
H2O2 concentrations within embryos was measured using
2',7'-dichlorodihydrofluorescein diacetate by Quanti cell 500 fluorescence
imaging and DNA fragmentation was observed with transmission electron
microscopy and an in-situ apoptosis detection kit. The H2O2 concentrations
were significantly higher in fragmented embryos (72.21 +/- 9.62, mean +/-
SEM) compared to non-fragmented embryos (31.30 +/- 3.50, P < 0.05) and
unfertilized oocytes (30.75 +/- 2.67, P < 0.05). Apoptosis was observed
only in fragmented embryos, and was absent in non-fragmented embryos.
Electron microscopic findings confirmed apoptotic bodies and cytoplasmic
condensation in the fragmented blastomeres. We conclude that there is a
direct relationship between increased H2O2 concentration and apoptosis, and
that further studies should be undertaken to confirm these findings.
相似文献
37.
38.
Eyal Leshem Mary Wikswo Leslie Barclay Eric Brandt William Storm Ellen Salehi Traci DeSalvo Tim Davis Amy Saupe Ginette Dobbins Hillary A. Booth Christianne Biggs Katie Garman Amy M. Woron Umesh D. Parashar Jan Vinjé Aron J. Hall 《Emerging infectious diseases》2013,19(8):1231-1238
During 2012, global detection of a new norovirus (NoV) strain, GII.4 Sydney, raised concerns about its potential effect in the United States. We analyzed data from NoV outbreaks in 5 states and emergency department visits for gastrointestinal illness in 1 state during the 2012–13 season and compared the data with those of previous seasons. During August 2012–April 2013, a total of 637 NoV outbreaks were reported compared with 536 and 432 in 2011–2012 and 2010–2011 during the same period. The proportion of outbreaks attributed to GII.4 Sydney increased from 8% in September 2012 to 82% in March 2013. The increase in emergency department visits for gastrointestinal illness during the 2012–13 season was similar to that of previous seasons. GII.4 Sydney has become the predominant US NoV outbreak strain during the 2012–13 season, but its emergence did not cause outbreak activity to substantially increase from that of previous seasons. 相似文献
39.
A reliability study was carried out to compare the short Manchester Scale (MS) to the longer Brief Psychiatric Rating Scale (BPRS), to see if similar items scored the same aspect of pathology and to find the sources of error. The raters were a psychiatrist and a psychologist cum medical student; they had recently arrived in Britain, came from different cultures and had not used the scale previously. Comparisons between the scales were made by interviewing the patients separately and together using either one or both scales. The items on the MS showed a higher interrater reliability as compared to the BPRS, both with independent and simultaneous ratings. Items based on observation only correlated poorly as compared to items based on verbal report. Lower between-scale correlations for delusions and affect were observed even where the same rater used the two scales, suggesting that the scales have different meanings for these items. The MS is a suitable alternative to the BPRS for quantifying schizophrenic symptoms. 相似文献
40.
Tartrate-resistant acid phosphatase (TRAcP) is used as a marker for osteoclasts, which are believed to be derived from phagocytic cells or phagocyte stem cell precursors. To further investigate the relationship between monocytic phagocytes and osteoclasts, acid phosphatase (AcP) activity was measured by three different techniques in human peripheral blood monocytes, monocyte-derived macrophages, and the U937 cell line. We found that cytochemistry and gel electrophoresis led to similar results, but that the colorimetric assay was inconsistent. Normal human peripheral monocytes expressed both tartrate-sensitive and -resistant AcP. In culture these cells formed polykaryons and expressed TRAcP activity that was further identified as an isoenzyme associated with bone tissue. In contrast, the U937 cells did not express TRAcP activity as measured by gel electrophoresis. Both U937 cells and monocytes possess material that interferes with interpretation of the colorimetric assay of AcP. The presence of TRAcP in monocyte-derived macrophages further supports the relationship between phagocytic cells and bone osteoclasts. 相似文献