全文获取类型
收费全文 | 4784篇 |
免费 | 622篇 |
国内免费 | 109篇 |
专业分类
耳鼻咽喉 | 19篇 |
儿科学 | 155篇 |
妇产科学 | 25篇 |
基础医学 | 542篇 |
口腔科学 | 55篇 |
临床医学 | 312篇 |
内科学 | 426篇 |
皮肤病学 | 18篇 |
神经病学 | 1997篇 |
特种医学 | 460篇 |
外科学 | 104篇 |
综合类 | 294篇 |
预防医学 | 518篇 |
眼科学 | 17篇 |
药学 | 447篇 |
1篇 | |
中国医学 | 74篇 |
肿瘤学 | 51篇 |
出版年
2024年 | 14篇 |
2023年 | 114篇 |
2022年 | 151篇 |
2021年 | 326篇 |
2020年 | 246篇 |
2019年 | 254篇 |
2018年 | 240篇 |
2017年 | 263篇 |
2016年 | 274篇 |
2015年 | 267篇 |
2014年 | 324篇 |
2013年 | 372篇 |
2012年 | 322篇 |
2011年 | 362篇 |
2010年 | 228篇 |
2009年 | 237篇 |
2008年 | 272篇 |
2007年 | 194篇 |
2006年 | 180篇 |
2005年 | 140篇 |
2004年 | 95篇 |
2003年 | 91篇 |
2002年 | 83篇 |
2001年 | 49篇 |
2000年 | 26篇 |
1999年 | 33篇 |
1998年 | 30篇 |
1997年 | 38篇 |
1996年 | 26篇 |
1995年 | 19篇 |
1994年 | 20篇 |
1993年 | 17篇 |
1992年 | 28篇 |
1991年 | 24篇 |
1990年 | 10篇 |
1989年 | 18篇 |
1988年 | 7篇 |
1987年 | 9篇 |
1986年 | 10篇 |
1985年 | 14篇 |
1984年 | 18篇 |
1983年 | 11篇 |
1982年 | 22篇 |
1981年 | 8篇 |
1980年 | 9篇 |
1979年 | 3篇 |
1978年 | 5篇 |
1977年 | 3篇 |
1976年 | 5篇 |
1973年 | 2篇 |
排序方式: 共有5515条查询结果,搜索用时 234 毫秒
81.
82.
Edward T. Tipper Emily I. Stevenson Victoria Alcock Alasdair C. G. Knight J. Jotautas Baronas Robert G. Hilton Mike J. Bickle Christina S. Larkin Linshu Feng Katy E. Relph Genevieve Hughes 《Proceedings of the National Academy of Sciences of the United States of America》2021,118(1)
Rivers carry the dissolved and solid products of silicate mineral weathering, a process that removes from the atmosphere and provides a key negative climate feedback over geological timescales. Here we show that, in some river systems, a reactive exchange pool on river suspended particulate matter, bonded weakly to mineral surfaces, increases the mobile cation flux by 50%. The chemistry of both river waters and the exchange pool demonstrates exchange equilibrium, confirmed by Sr isotopes. Global silicate weathering fluxes are calculated based on riverine dissolved sodium (Na+) from silicate minerals. The large exchange pool supplies Na+ of nonsilicate origin to the dissolved load, especially in catchments with widespread marine sediments, or where rocks have equilibrated with saline basement fluids. We quantify this by comparing the riverine sediment exchange pool and river water chemistry. In some basins, cation exchange could account for the majority of sodium in the river water, significantly reducing estimates of silicate weathering. At a global scale, we demonstrate that silicate weathering fluxes are overestimated by 12 to 28%. This overestimation is greatest in regions of high erosion and high sediment loads where the negative climate feedback has a maximum sensitivity to chemical weathering reactions. In the context of other recent findings that reduce the net consumption through chemical weathering, the magnitude of the continental silicate weathering fluxes and its implications for solid Earth degassing fluxes need to be further investigated.For decades, silicate weathering has been postulated to provide the negative climate feedback on Earth that prevents a runaway greenhouse climate like on Venus (1). Silicate mineral dissolution with carbonic acid converts atmospheric into carbonate, and releases essential nutrients to the terrestrial and marine biosphere (2). There have been many attempts to quantify the silicate weathering flux (3), mostly assuming that riverine dissolved sodium () is derived only from silicate minerals and rock salt. Here we show that there is a major addition of nonsilicate to the critical zone from ancient seawater, weakly bonded to sedimentary rocks and supplied to waters via the cation exchange process. The implication is not only that the silicate weathering flux is overestimated at a global scale, but that this nonsilicate is most important in regions previously thought to have the highest silicate weathering fluxes (so called weathering-limited regions) and greatest climate sensitivity.Cation exchange is a rapid chemical reaction between cations in the dissolved phase and mineral surfaces, particularly clays (4). Major and trace cations such as calcium (), magnesium (), sodium (), potassium (), and strontium () form the cation exchange pool, which balances negative charges on river-borne clay particle surfaces. This exchange takes place on interlayer sites, between the tetrahedral and octahedral layers, or on exposed surfaces (4). The importance of the cation exchange pool is well recognized in soils and aquifers (4, 5), has significant implications for enhanced weathering (6), and has been proposed as an important mechanism for buffering the composition of river waters (7–9). However, data on the riverine exchange pool are only available for two large river systems [Amazon and Ganges-Brahmaputra (10, 11)], despite its significance in providing a source of elements that are immediately bioavailable (12), and their potential for biasing the quantification of silicate weathering (9).It is increasingly recognized that rapidly reactive phases have a strong influence on the chemistry of river waters (13, 14). Cation exchange is a rapid reaction occurring continuously in soils, as riverine freshwaters evolve downstream interacting with particulate matter, and when they mix with seawater (15, 16). Important examples of cation exchange are the “swapping” of divalent cations and with , in particular when there is a major change in water composition such as when fluvial clays reach the ocean,[1]As a result, marine sediments have an exchange pool that is dominated by (17). Subsequently, these marine sediments are uplifted and emplaced on the continents where in the exchange pool is released by cation exchange with Ca-rich fresh waters (9). This has major implications for estimates of silicate weathering fluxes and associated consumption, because they are calculated using the content of rivers (3). Cerling et al. (9) proposed that the -rich exchange pool exerts an important control on natural waters, based on charge balance arguments from river water chemistry, but this hypothesis has never been rigorously tested (18) by determining the flux and composition of the exchange pool of rivers around the world.In this contribution, we present a large dataset of fluvial sediment cation exchange capacity (CEC) and composition in several of the world’s largest river basins. By comparing with the concomitant dissolved load chemistry, we demonstrate that 1) the exchange pool in river sediments is in equilibrium with the river water; 2) the fraction of mobile elements in the exchange pool relative to the dissolved pool can be significant, particularly in rapidly eroding, weathering-limited catchments; and 3) given reasonable inferences on the composition of old marine sedimentary rocks, modern-day silicate weathering has been overestimated and carbonate weathering has been underestimated. The results reduce the estimated magnitude of the silicate weathering flux, but increase the supply of base cations (e.g., , which can be a limiting nutrient) to the biosphere, suggesting a greater role of organic carbon burial compared with silicate weathering for the long-term atmospheric sink. 相似文献
83.
Maria Fatima Falangola Xingju Nie Ralph Ward Emilie T. McKinnon Siddhartha Dhiman Paul J. Nietert Joseph A. Helpern Jens H. Jensen 《NMR in biomedicine》2020,33(9)
The 3×Tg‐AD mouse is one of the most studied animal models of Alzheimer's disease (AD), and develops both amyloid beta deposits and neurofibrillary tangles in a temporal and spatial pattern that is similar to human AD pathology. Additionally, abnormal myelination patterns with changes in oligodendrocyte and myelin marker expression are reported to be an early pathological feature in this model. Only few diffusion MRI (dMRI) studies have investigated white matter abnormalities in 3×Tg‐AD mice, with inconsistent results. Thus, the goal of this study was to investigate the sensitivity of dMRI to capture brain microstructural alterations in 2‐month‐old 3×Tg‐AD mice. In the fimbria, the fractional anisotropy (FA), kurtosis fractional anisotropy (KFA), and radial kurtosis (K┴) were found to be significantly lower in 3×Tg‐AD mice than in controls, while the mean diffusivity (MD) and radial diffusivity (D┴) were found to be elevated. In the fornix, K┴ was lower for 3×Tg‐AD mice; in the dorsal hippocampus MD and D┴ were elevated, as were FA, MD, and D┴ in the ventral hippocampus. These results indicate, for the first time, dMRI changes associated with myelin abnormalities in young 3×Tg‐AD mice, before they develop AD pathology. Morphological quantification of myelin basic protein immunoreactivity in the fimbria was significantly lower in the 3×Tg‐AD mice compared with the age‐matched controls. Our results demonstrate that dMRI is able to detect widespread, significant early brain morphological abnormalities in 2‐month‐old 3×Tg‐AD mice. 相似文献
84.
Robert G. Briggs Abdul Basit Khan Arpan R. Chakraborty Carol J. Abraham Christopher D. Anderson Patrick J. Karas Phillip A. Bonney Ali H. Palejwala Andrew K. Conner Daniel L. O'Donoghue Michael E. Sughrue 《Clinical anatomy (New York, N.Y.)》2020,33(6):823-832
The superior frontal gyrus (SFG) is an important region implicated in a variety of tasks including motor movement, working memory, resting-state, and cognitive control. A detailed understanding of the subcortical white matter of the SFG could improve postoperative morbidity related to surgery around this gyrus. Through DSI-based fiber tractography validated by gross anatomical dissection, we characterized the fiber tracts of the SFG based on their relationships to other well-known neuroanatomic structures. Diffusion imaging from the Human Connectome Project from 10 healthy adult subjects was used for fiber tractography. We evaluated the SFG as a whole based on its connectivity with other regions. All tracts were mapped in both hemispheres, and a lateralization index was calculated based on resultant tract volumes. Ten cadaveric dissections were then performed using a modified Klingler technique to delineate the location of major tracts integrated within the SFG. We identified four major SFG connections: the frontal aslant tract connecting to the inferior frontal gyrus; the inferior fronto-occipital fasciculus connecting to the cuneus, lingual gyrus, and superior parietal lobule; the cingulum connecting to the precuneus and parahippocampal gyrus/uncus; and a callosal fiber bundle connecting the SFG bilaterally. The functional networks of the SFG involve a complex series of white matter tracts integrated within the gyrus, including the FAT, IFOF, cingulum, and callosal fibers. Postsurgical outcomes related to this region may be better understood in the context of the fiber-bundle anatomy highlighted in this study. Clin. Anat. 33:823–832, 2020. © 2019 Wiley Periodicals, Inc. 相似文献
85.
Grey matter brain injuries are common in Ugandan children with cerebral palsy suggesting a perinatal aetiology in full‐term infants 下载免费PDF全文
86.
Childhood trauma has been associated with long term effects on prefrontal-limbic grey matter. A literature search was conducted to identify structural magnetic resonance imaging studies of adults with a history of childhood trauma. We performed three meta-analyses. Hedges’ g effect sizes were calculated for each study providing hippocampal or amygdala volumes of trauma and non-trauma groups. Seed based differential mapping was utilised to synthesise whole brain voxel based morphometry (VBM) studies. A total of 38 articles (17 hippocampus, 13 amygdala, 19 whole brain VBM) were included in the meta-analyses. Trauma cohorts exhibited smaller hippocampus and amygdala volumes bilaterally. The most robust findings of the whole brain VBM meta-analysis were reduced grey matter in the right dorsolateral prefrontal cortex and right hippocampus amongst adults with a history of childhood trauma. Subgroup analyses and meta-regressions showed results were moderated by age, gender, the cohort’s psychiatric health and the study’s definition of childhood trauma. We provide evidence of abnormal grey matter in prefrontal-limbic brain regions of adults with a history of childhood maltreatment. 相似文献
87.
《Inhalation toxicology》2013,25(9):532-543
Background: Nanoparticles are being rapidly assimilated into numerous research fields and consumer products. A concurrent increase in human exposure to such materials is expected. Magnetic nanoparticles (MNPs) possess unique and beneficial features, increasing their functionality and integrative potential. However, MNP toxicity characterization is limited, especially in regards to the human respiratory system. This study aimed to assess the in vitro effects of airborne MNPs on BEAS-2B cells. Uncoated iron oxide was compared with two amorphous silica-coated MNPs, hypothesizing the coatings reduced toxicity and increased particle stability.Method: BEAS-2B cells were cultured at an air–liquid interface and exposed to airborne MNPs using a fabricated exposure device. Indices of cytotoxicity, inflammatory response, oxidative stress, and iron homeostasis were monitored postexposure via cell viability assays and qRT-PCR. Concentrations of soluble iron-associated with different MNPs were also examined before and after contact with several aqueous organic and inorganic acids.Results: The silica-coated MNPs had reduced soluble iron concentrations. This result indicates that the silica coating provides a barrier to and prevents the mobilization of soluble iron from the particle to the cell, thereby reducing the risk of oxidative stress or alterations of iron homeostasis. Cells exposed to MagSilica50 and MagSilica50–85® showed little to no indications of cytotoxicity or induction of inflammatory response/oxidative stress at the examined delivery concentrations.Conclusion: MNPs coated with amorphous silica are protected from acidic erosion. Correspondingly, the particle stability translates into reduced cytotoxicity and cellular influence on human airway epithelial cells. 相似文献
88.
《Journal of the American Medical Directors Association》2014,15(12):904-910
ObjectivesThis study was designed to explore the appropriate blood pressure (BP) target required to reduce cognitive decline and brain white matter lesions (WMLs) in elderly hypertensive patients.MethodsElderly patients (n = 294, ≥80 years of age) being treated for hypertension were enrolled in a longitudinal study examining cognitive impairment after an initial assessment and a period of 4 years. All patients underwent neurological and cognitive assessment, laboratory examination, and magnetic resonance imaging of the brain.ResultsThe 4-year follow-up examination revealed that body mass index, alcohol consumption, systolic blood pressure (SBP), diastolic blood pressure, and Mini-Mental State Examination (MMSE) all showed a significant decline, whereas fasting plasma glucose, white matter hyperintensities (WMH) volume, and the WMH/total intracranial volume (TIV) ratio were significantly increased when compared with baseline observations. Interestingly, the decline in MMSE, as well as the increment of WMH and WMH/TIV ratio was smaller in patients with SBP ranging from 140 to 160 mm Hg than in those whose SBP was lower than 140 mm Hg or higher than 160 mm Hg (P < .05). Furthermore, we observed that a 15 to 35 mm Hg targeted lowering of SBP in the elderly patients was more beneficial to our cognitive analysis than treatments that achieved less than 15 mm Hg or greater than 35 mm Hg (P < .05).ConclusionsIn elderly hypertensive patients, there exists a beneficial target for SBP lowering beyond which treatment may not be beneficial for improving or delaying the progression of cognitive impairment and WMLs. 相似文献
89.
Testing associations between cannabis use and subcortical volumes in two large population‐based samples 下载免费PDF全文
Nathan A. Gillespie Michael C. Neale Timothy C. Bates Lisa T. Eyler Christine Fennema‐Notestine Jasmin Vassileva Michael J. Lyons Elizabeth C. Prom‐Wormley Katie L. McMahon Paul M. Thompson Greig de Zubicaray Ian B. Hickie John J. McGrath Lachlan T. Strike Miguel E. Rentería Matthew S. Panizzon Nicholas G. Martin Carol E. Franz Margaret J. Wright 《Addiction (Abingdon, England)》2018,113(9):1661-1672
90.
《Neurocirugía (Asturias, Spain)》2014,25(6):268-274
IntroductionThe insula is a highly connected area, as an intricate network of afferent and efferent projections connect it with adjacent and distant cortical regions.ObjectiveTo perform an extensive review of recent literature to analyse the anatomy of the associative tracts related to the insula.ResultsThe frontal aslant tract, arcuate fasciculus, horizontal portion of the superior longitudinal fasciculus and the middle longitudinal fasciculus are associative tracts connected to the opercula. The inferior fronto-occipital fasciculus (IFOF) and uncinate fasciculus run under the anterior and inferior portion of the insula.Conclusionsthe pars triangularis and orbicularis of the inferior frontal gyrus, as well as the middle and anterior part of the superior temporal gyrus, have few connections with the perisylvian associative network. Consequently, in the trans-opercular approach to the insula, these 2 regions represent anatomical corridors that give access to the insula. The IFOF and the uncinate fasciculus represent the deep functional margin of resection. 相似文献