首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   3117篇
  免费   198篇
  国内免费   26篇
耳鼻咽喉   40篇
儿科学   73篇
妇产科学   52篇
基础医学   491篇
口腔科学   54篇
临床医学   314篇
内科学   713篇
皮肤病学   35篇
神经病学   349篇
特种医学   104篇
外科学   301篇
综合类   59篇
一般理论   7篇
预防医学   271篇
眼科学   50篇
药学   207篇
中国医学   2篇
肿瘤学   219篇
  2021年   51篇
  2020年   35篇
  2019年   51篇
  2018年   58篇
  2017年   39篇
  2016年   35篇
  2015年   58篇
  2014年   59篇
  2013年   111篇
  2012年   177篇
  2011年   173篇
  2010年   114篇
  2009年   87篇
  2008年   189篇
  2007年   191篇
  2006年   183篇
  2005年   211篇
  2004年   206篇
  2003年   219篇
  2002年   197篇
  2001年   54篇
  2000年   38篇
  1999年   59篇
  1998年   39篇
  1997年   29篇
  1996年   21篇
  1995年   26篇
  1994年   23篇
  1993年   29篇
  1992年   31篇
  1991年   32篇
  1990年   23篇
  1989年   24篇
  1988年   16篇
  1987年   25篇
  1986年   26篇
  1985年   32篇
  1984年   34篇
  1983年   36篇
  1982年   31篇
  1981年   43篇
  1980年   22篇
  1979年   16篇
  1978年   13篇
  1977年   12篇
  1976年   21篇
  1975年   11篇
  1974年   14篇
  1973年   19篇
  1971年   15篇
排序方式: 共有3341条查询结果,搜索用时 62 毫秒
1.
One in five children in the UK are affected by domestic violence and abuse. However, primary care clinicians (GPs and nurses) struggle to effectively identify and support children and young people living in homes where it is present. The IRIS+ (Enhanced Identification and Referral to Improve Safety) training and advocacy support intervention aimed to improve how clinicians respond to children and young people affected by domestic violence and abuse. IRIS+ training was delivered as part of a feasibility study to four general practices in an urban area in England (UK). Our mixed method design included interviews and questionnaires about the IRIS+ intervention with general practice patients, including children and young people as well as with clinicians and advocacy service providers. We collected the number of identifications and referrals by clinicians of children experiencing domestic violence and abuse through a retrospective search of medical and agency records 10 months after the intervention. Forty-nine children exposed to domestic violence and abuse were recorded in medical records. Thirty-five children were referred to a specialist domestic violence and abuse support service over a period of 10 months. Of these, 22 received direct or indirect support. The qualitative findings indicated that children benefitted from being referred by clinicians to the service. However, several barriers at the patient and professional level prevented children and young people from being identified and supported. Some of these barriers can be addressed through modifications to professional training and guidance, but others require systematic and structural changes to the way health and social care services work with children affected by domestic violence and abuse.  相似文献   
2.
Nurse-led delivery care models have the potential to address the significant burden of heart failure in sub-Saharan Africa. Starting in 2006, the Rwandan Ministry of Health, supported by Inshuti Mu Buzima (Partners In Health–Rwanda), decentralized heart failure diagnosis and care delivery in the context of advanced nurse-led integrated noncommunicable clinics at rural district hospitals. Here, the authors describe the first medium-term survival outcomes from the district level in rural sub-Saharan Africa based on their 10-year experience providing care in rural Rwanda. Kaplan-Meier methods were used to determine median time to event for: 1) composite event of known death from any cause, lost to follow-up, or transfer to estimate worst-case mortality; and 2) known death only. Five-year event-free rates were 41.7% for the composite outcome and 64.3% for known death. While death rates are encouraging, efforts to reduce loss to follow-up are needed.  相似文献   
3.
Maternal and Child Health Journal - Food insecurity (FI) has serious academic, social, and physical health consequences for children. A recent clinical recommendation suggests FI screening during...  相似文献   
4.
5.
Phenotypic and genetic variation in one species can influence the composition of interacting organisms within communities and across ecosystems. As a result, the divergence of one species may not be an isolated process, as the origin of one taxon could create new niche opportunities for other species to exploit, leading to the genesis of many new taxa in a process termed “sequential divergence.” Here, we test for such a multiplicative effect of sequential divergence in a community of host-specific parasitoid wasps, Diachasma alloeum, Utetes canaliculatus, and Diachasmimorpha mellea (Hymenoptera: Braconidae), that attack Rhagoletis pomonella fruit flies (Diptera: Tephritidae). Flies in the R. pomonella species complex radiated by sympatrically shifting and ecologically adapting to new host plants, the most recent example being the apple-infesting host race of R. pomonella formed via a host plant shift from hawthorn-infesting flies within the last 160 y. Using population genetics, field-based behavioral observations, host fruit odor discrimination assays, and analyses of life history timing, we show that the same host-related ecological selection pressures that differentially adapt and reproductively isolate Rhagoletis to their respective host plants (host-associated differences in the timing of adult eclosion, host fruit odor preference and avoidance behaviors, and mating site fidelity) cascade through the ecosystem and induce host-associated genetic divergence for each of the three members of the parasitoid community. Thus, divergent selection at lower trophic levels can potentially multiplicatively and rapidly amplify biodiversity at higher levels on an ecological time scale, which may sequentially contribute to the rich diversity of life.Population divergence is a fundamental evolutionary process contributing to the diversity of life (1). Studies of how new life forms originate typically focus on how barriers to gene flow evolve in specific lineages, resulting in their divergence into descendent daughter taxa. As a result, evolutionary biologists now have a good understanding of how variation within a population is transformed by selection into differences between taxa (13). What is less well understood is whether the divergence of one population has consequences that ripple through the trophic levels of an ecosystem and affect entire communities of interacting organisms. Studies in paleontology (46), community ecology (7, 8), systematics (8, 9), and ecosystem genetics (10, 11) suggest that evolutionary change in one lineage can influence entire communities of organisms. For example, when the genotype/phenotype of a “foundation” species influences the relative fitness of other species, evolutionary change(s) in this genotype/phenotype may affect organisms in adjacent trophic levels (10, 11). If these evolutionary changes are linked to ecological adaptation and reproductive isolation (RI), associated organisms may diverge in parallel, potentially creating entire coevolved communities distinct from one another (1215). Therefore, population divergence may not always be an isolated process, as the differentiation of one taxon could beget the divergence of many others.Such “sequential” or “cascading” divergence events may be particularly relevant to understanding why some groups of organisms, like plants, the insects that feed on them, and the parasitoids that attack the insects, are more diverse and species-rich than other groups (8, 9, 1215). Specifically, when phytophagous insects diversify by adapting to new host plants, they create a new habitat for their parasitoids to exploit (Fig. 1). If a parasitoid shifts to the new habitat, it can encounter the same divergent ecological selection pressures as its insect host, which could result in the parallel divergence of insect host and parasitoid (1215) (Fig. 1A). Moreover, sequential divergence may have multiplicative effects in generating biodiversity, as the shift of an insect to a new plant may open a new niche opportunity for not just one but the entire community of parasitoids attacking the insect host (12, 13) (Fig. 1B). However, few convincing examples of sequential divergence exist (1215), and in no study is there both genetic and ecological evidence for sequential divergence multiplicatively amplifying biodiversity.Open in a separate windowFig. 1.Three scenarios of codivergence in a host−parasitoid system. (A) A single sequential divergence event, (B) sequential divergence with multiplicative amplification of biodiversity, and (C) cospeciation in allopatry. In A, codivergence is driven by the cascade of divergent ecological selection pressures across trophic levels in sympatry. Here, a degree of divergent ecological adaptation must accompany the host shift such that parasitoids are not merely moving between geographically separated hosts. In B, the multiplicative effects of sequential divergence can be seen as several members of the parasitoid community diverge in parallel with their host. In C, codivergence (cospeciation) occurs after host plant, fly, and parasitoid populations become jointly geographically isolated (black bar), resulting in parallel allopatric speciation. Here, little differentiation need accompany the initial host shift of fly or parasitoid. Cospeciation is not necessarily driven by the creation and adaptation to new niches but by the concordant geographic and reproductive separation of hosts and parasitoids.Here, we test for the multiplicative effects of sequential divergence in the community of parasitoid wasps (Hymenoptera: Braconidae) that attack fruit flies in the Rhagoletis pomonella sibling species complex (Diptera: Tephritidae) (Fig. 2). Several features of the biology and biogeography of the Rhagoletis−parasitoid system make it ideal for investigating the multiplicative divergence hypothesis and allow us to directly test multiple criteria supporting sympatric host race formation (16) and sequential divergence (12, 13), summarized in 13, 1719), the hypothesized initial stage of ecological speciation (16, 17). The short time frame and sympatric spatial context of R. pomonella’s shift to apple exclude passive codivergence or speciation (Fig. 1C) as an explanation for differentiation. Specifically, fly and wasp populations could not have diverged in concert, because they became jointly geographically separated in the past (Fig. 1C). Rather, if flies and wasps display concordant adaptations, it is likely due to the direct effects of divergent ecological selection resulting from host shifts cascading from host plants to flies to parasitoids (Fig. 1 A and B).Open in a separate windowFig. 2.The community of host-specific parasitoids that attack members of the (A) Rhagoletis pomonella sibling species complex: (B) D. alloeum, (C) D. mellea, and (D) U. canaliculatus that (E) emerge from the fly pupal case as adults following overwintering. (Scale bar, 1 mm.)

Table 1.

Summary of conditions (criteria) conducive to and supporting hypotheses of sympatric host race formation (modified from ref. 16) and sequential divergence (modified from refs. 12 and 13)
Criteria supporting hypotheses of sympatric host race formation and sequential divergenceRpDaDmUc
Criterion 1. Shift to new host resource and multiple host-associations occur in sympatry or close geographic proximityyesyesyesyes
Criterion 2. Host-associated populations form distinct genetic clusters (spatially replicable), but experience gene flow at appreciable ratesyesyesyesyes
Criterion 3. Females, but also potentially males, display host preferences and discriminate among alternate hostsyesyesyesyes
Criterion 4. Host choice is linked to mate choice facilitating assortative mating and resulting in prezygotic habitat isolationyesyesyesyes
Criterion 5. Host selection and fidelity are under some degree of genetic control and not due solely to maternal, learning, or environmental effectsyesyes, tested in one directionnot testednot tested
Criterion 6. Differences in insect phenologies tracks differences in the host phenologies resulting in temporal (allochronic) isolationyesyesyesyes
Criterion 7. Insect phenology under some degree of genetic control and not due solely to maternal or environmental effectsyesyesyesyes
Criterion 8. Fitness tradeoffs exist between host-associated populations resulting in migrants and hybrids having reduced fitnessyes, sometimesnot testednot testednot tested
Open in a separate windowAlso shown is whether these criteria have been empirically tested and confirmed in R. pomonella (Rp) species complex flies and three members of the parasitoid wasp community attacking the flies: D. alloeum (Da), D. mellea (Dm), and U. canaliculatus (Uc). Data for Da are from Forbes et al. (14) and the current study (criterion 5).Second, the apple and hawthorn host races of R. pomonella belong to a closely related group of sibling species, including Rhagoletis mendax (host: blueberry, Vaccinium spp.), Rhagoletis zephyria (host: snowberry, Symphoricarpos spp.), and the undescribed flowering dogwood fly (host: Cornus florida). All of these taxa purportedly radiated via sympatric host shifts (1723). In addition, other species in the genus, such as the eastern cherry fly, Rhagoletis cingulata (host: black cherry, Prunus serotina) are sympatric with R. pomonella group flies (23). Thus, the potential for sequential divergence in the Rhagoletis parasitoid community extends beyond the host races, with multiple cooccurring fly resources existing for wasps to attack, satisfying criterion 1.Third, Rhagoletis in the eastern United States are attacked by a community of host-specific endoparasitoid wasps that include the species Diachasma alloeum, Utetes canaliculatus, and Diachasmimorpha mellea (24, 25) (Fig. 2). All three species have a free-living, sexually reproducing adult life stage. This life cycle eliminates vertical transmission as a factor facilitating codivergence. U. canaliculatus oviposits into Rhagoletis eggs laid beneath the skin of ripe fruit, while D. alloeum and D. mellea oviposit into late instar larvae feeding within fruit (24). A degree of niche partitioning for oviposition sites therefore exists among species (25), potentially facilitating coexistence on the same fly host. As a result, multiple host associations of wasps exist in close geographic proximity, fulfilling the requirements of criterion 1 for sympatric race formation and sequential divergence for the wasp community as well.In addition, a previous study documented that one parasitoid attacking Rhagoletis, D. alloeum, is undergoing sequential divergence (14). Population genetic surveys, field observations, behavioral assays of host choice, and studies of life history timing support the existence of an ecologically derived population of D. alloeum attacking the recently formed apple-infesting host race of R. pomonella, meeting criteria 2, 3, 4, 6, and 7. We hypothesize that if U. canaliculatus and D. mellea are undergoing sequential divergence, they will show similar patterns of host-associated ecological and genetic divergence.Two dimensions of divergent ecological selection generate RI among host-associated populations of Rhagoletis and D. alloeum: host-specific mating (habitat isolation) and differences in eclosion phenology (temporal isolation). With respect to habitat isolation, Rhagoletis (26) and D. alloeum (14) court and mate on or near the fruit of their respective host plants. The most important long- to intermediate-range cues that flies use to find and discriminate among plants are the volatile compounds emitted from the surface of ripening fruit (2730). Flies display genetically based behavioral preference for natal fruit surface volatiles and avoid the volatiles of alternative fruit (29). Similarly, D. alloeum prefer natal and avoid nonnatal host fruit volatiles in behavioral assays (14), supporting criterion 3. Consequently, differences in host choice translate directly to mate choice, generating prezygotic habitat-related RI for both flies and wasps, fulfilling criterion 4. Additionally, host odor discrimination may also act as a postzygotic barrier to gene flow in R. pomonella, as they suffer behavioral host choice sterility mediated by a reduced chemosensory ability to find suitable host fruit for mating and oviposition (30). Whether or not hybrid D. alloeum display a similar behavior is unknown. Lastly, criteria 5 is partially met for Rhagoletis, as the host fruit environment has no effect on host odor discrimination behaviors for hawthorn-origin R. pomonella reared in apple fruit, indicating that host selection and fidelity are under (partial) genetic control (27, 30). Similar experiments have not yet been conducted in D. alloeum (but see Reciprocal Rearing of Diachasma).With respect to temporal isolation (criterion 6), the timing of overwinter diapause is an important host-related ecological adaptation for Rhagoletis. The host plants of Rhagoletis fruit at different times of the year (19, 31, 32). For example, apple varieties favored by R. pomonella ripen 3–4 wk before native hawthorns in sympatry. Thus, flies must eclose to coincide with the availability of ripe fruit to find mates and oviposition sites. Rhagoletis are univoltine, and their lifespan is short (1 mo). Differences in eclosion timing between races therefore results in partial allochronic mating isolation (19, 2932). The differences in eclosion timing also confer a degree of postzygotic isolation because hybrids will possess eclosion patterns asynchronous with fruit ripening (29, 31, 32). Rhagoletis attacking blueberries and flowering dogwoods display similar differences in eclosion time related to variation in host fruiting phenology (31).The life cycle of D. alloeum mirrors that of Rhagoletis, generating the same divergent ecological selection pressures. As a result, populations of D. alloeum attacking different Rhagoletis eclose to match the phenology of fly larvae feeding within host fruit (14). In addition, longevity of D. alloeum (∼2 wk) is half that of Rhagoletis, generating even more pronounced allochronic mating isolation compared to the fly (14), supporting criterion 6. Significant allele frequency differences between sympatric populations of D. alloeum attacking different fly hosts (criterion 2) were associated with differences in eclosion time (14), confirming criterion 7. The same has also been found for Rhagoletis (19, 31, 32), connecting host-related life history adaptation and RI to patterns of genetic differentiation among flies and wasps.Here, we test for the multiplicative hypothesis of sequential divergence in the Rhagoletis−parasitoid system using the criteria in 33, 34), difficulty in reciprocally transplanting wasps precludes these experiments at this time but remain an area for future study.  相似文献   
6.
7.
固定矫治错患者牙周维护中龈下微生物的变化   总被引:1,自引:0,他引:1  
目的 探讨固定矫治中0.12%氯已定溶液漱口对龈下微生物的影响。方法 选择龈炎明显的48例10-17岁固定矫治患者,经龈上洁治后随机分为生理盐水漱口(对照组)、口腔卫生宣教加生理盐水漱口(NS组)、口腔卫生宣教加0.12%氯已定漱口(CH组)3组,镜下比较洁治前及洁治后1周、1个月和3个月时龈下微生物(球菌、杆菌、螺旋体)比例的变化。结果 对照组和NS组的球菌比例先升后降,螺旋体的比例变化相反;CH组4次的检测结果,球菌比例呈升高状态,螺旋体呈下降趋势。对照组和CH组洁治后3个月检测,球菌和杆菌比例变化的差异有显著性(P<0.05)。结论 用0.12%氯已定溶液漱口对龈下微生物的变化有良性影响,有助于正畸患者牙周健康的维护。  相似文献   
8.
Teaching of implant dentistry in the predoctoral dental curriculum has evolved dramatically over the past 20 years. In 1974, only one third of American dental schools addressed the topic of implants. Today, 48 of the 54 American dental schools have predoctoral curricula. The Creighton University experience offers some unique and instructive insights into a 10-year process of developing and implementing a predoctoral implant dentistry curriculum. All interested students may perform both the surgical placement and restoration of implant prostheses. Clinical instruction involves all restorative and surgical faculty members. Favorable 3-year (91%) and 5-year (87%) surgical success rates have been maintained. This article presents one university's program for examination and discussion.  相似文献   
9.
Tooth extraction is a common procedure in dentistry. The normal healing response to the procedure results in a significant loss of bone and collapse of the surrounding gingiva. In addition to normal healing, a substantial percentage of extraction sites suffer postoperative complications. This article presents histology that supports the concept that the first response to extraction is bone death and resorption of the socket wall. The stages of extraction socket healing also will be discussed. Additionally, the article will present a regenerative method that skips the resorptive phase, the clotting phase, the granulation of tissue phase, and the collagen-producing phase of normal extraction-socket healing, while avoiding extraction-socket complications.  相似文献   
10.
Cannula position in HeartMate II and HeartWare left ventricular assist devices (LVADs) is associated with clinical outcome. This study aimed to investigate the clinical implication of the device positioning in HeartMate 3 LVAD cohort. Consecutive patients who underwent HeartMate 3 LVAD implantation were followed for one year from index discharge. At index discharge, chest X-ray parameters were measured: (a) cannula coronal angle, (b) height of pump bottom, (c) cannula sagittal angle, and (d) cannula lumen area. The association of each measurement of cannula position with one-year clinical outcomes was investigated. Sixty-four HeartMate 3 LVAD patients (58 years old, 64% male) were enrolled. In the multivariable Cox regression model, the cannula coronal angle was a significant predictor of death or heart failure readmission (hazard ratio 1.27 [1.01-1.60], P = .045). Patients with a cannula coronal angle ≤28° had lower central venous pressure (P = .030), lower pulmonary capillary wedge pressure (P = .027), and smaller left ventricular size (P = .019) compared to those with the angle >28°. Right ventricular size and parameters of right ventricular function were also better in the narrow angle group, as was one-year cumulative incidence of death or heart failure readmission (10% vs. 50%, P = .008). Narrow cannula coronal angle in patients with HeartMate 3 LVADs was associated with improved cardiac unloading and lower incidence of death or heart failure readmission. Larger studies to confirm the implication of optimal device positioning are warranted.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号