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   7176篇
  免费   624篇
  国内免费   119篇
耳鼻咽喉   17篇
儿科学   157篇
妇产科学   51篇
基础医学   895篇
口腔科学   257篇
临床医学   1097篇
内科学   1730篇
皮肤病学   41篇
神经病学   607篇
特种医学   302篇
外科学   485篇
综合类   569篇
预防医学   899篇
眼科学   55篇
药学   505篇
中国医学   139篇
肿瘤学   113篇
  2023年   185篇
  2022年   219篇
  2021年   324篇
  2020年   398篇
  2019年   400篇
  2018年   366篇
  2017年   271篇
  2016年   224篇
  2015年   255篇
  2014年   518篇
  2013年   587篇
  2012年   353篇
  2011年   434篇
  2010年   314篇
  2009年   347篇
  2008年   325篇
  2007年   298篇
  2006年   268篇
  2005年   186篇
  2004年   192篇
  2003年   165篇
  2002年   125篇
  2001年   127篇
  2000年   104篇
  1999年   76篇
  1998年   75篇
  1997年   64篇
  1996年   66篇
  1995年   54篇
  1994年   53篇
  1993年   54篇
  1992年   45篇
  1991年   47篇
  1990年   43篇
  1989年   33篇
  1988年   34篇
  1987年   43篇
  1986年   28篇
  1985年   41篇
  1984年   34篇
  1983年   18篇
  1982年   21篇
  1981年   21篇
  1980年   18篇
  1979年   13篇
  1978年   8篇
  1977年   15篇
  1976年   8篇
  1975年   9篇
  1973年   5篇
排序方式: 共有7919条查询结果,搜索用时 62 毫秒
1.
2.
3.
ObjectiveTo determine the long-term cardiovascular disease risk of astronauts with spaceflight exposure compared with a well-matched cohort.MethodsNational Aeronautics and Space Administration (NASA) astronauts are selected into their profession based upon education, unique skills, and health and are exposed to cardiovascular disease risk factors during spaceflight. The Cooper Center Longitudinal Study (CCLS) is a generally healthy cohort from a preventive medicine clinic in Dallas, Texas. Using a matched cohort design, astronauts who were selected beginning April 1, 1959, (and each subsequent selection class through 2009) and exposed to spaceflight were matched to CCLS participants who met astronaut selection criteria; 1514 CCLS participants matched to 303 astronauts in a 5-to-1 ratio on sex, date of birth, and age. The outcome of cardiovascular mortality through December 31, 2016, was determined by death certificate or National Death Index.ResultsThere were 11 deaths caused by cardiovascular disease (CVD) among astronauts and 46 among CCLS participants. There was no evidence of increased mortality risk in astronauts (hazard ratio [HR]=1.10; 95% confidence interval [CI], 0.50 to 2.45) with adjustment for baseline cardiovascular covariates. However, the secondary outcome of CVD events showed an increased adjusted risk in astronauts (HR=2.41; 95% CI, 1.26 to 4.63).ConclusionNo increased risk of CVD mortality was observed in astronauts with spaceflight exposure compared with a well-matched cohort, but there was evidence of increased total CVD events. Given that the duration of spaceflight will increase, particularly on missions to Mars, continued surveillance and mitigation of CVD risk is needed to ensure the safety of those who venture into space.  相似文献   
4.
摘要:目的 通过对新硫肽类抗生素166A产生菌小单孢菌TMD166进行诱变选育研究,以期获得产166A的高产菌株。方 法 使用多功能等离子体诱变系统(multifunctional plasma mutagenesis system, MPMS)对出发菌株TMD166的孢子进行等离子体-紫 外(MPMS-UV)复合诱变,单孢子悬液经照射处理、涂布培养后获得单菌落,并以牛津杯固体发酵高通量筛选方法对菌株进行初 筛,之后对高产菌株进行摇瓶复筛,利用突变株摇瓶发酵的化学效价筛选出正突变菌株。结果 对比MPMS-UV不同诱变剂量发 现,MPMS105s-UV60s复合诱变剂量获得的正突变率最高,达到41.43%,相应致死率为99.88%。最终筛选出一株166A高产突 变株TMD166-MU1,在培养温度为28℃、210 r/min的条件下,经过96~120 h培养后,166A产量相比出发菌株提高了7.47倍。结 论 采用MPMS-UV复合诱变方式,再结合牛津杯固体发酵高通量筛选方法和摇瓶复筛,可高效筛选获得166A高产菌株。  相似文献   
5.
We carried out a longitudinal study on the associations between residential greenness and depression risk in urban areas in Finland. Residential greenness indicators were estimated within various buffer sizes around individuals' home locations (selected n = 14424) using time-series of normalized differential vegetation index (NDVI) and CORINE land cover data (CLC). We estimated individuals’ cumulative exposure to residential greenness over a 5-years and 14-years follow-up. We used doctor-diagnosed depression and Beck Depression Inventory for depression assessment. Our multi-logistic model showed an inverse association between residential greenness and depression, implying lowered depression risk for individuals with higher residential greenness. The association was particularly evident when using NDVI-based residential greenness (within a buffer of 100 m radius) and doctor-diagnosis depression data, adjusted with individual-level covariates. The odds ratio was 0.56 (95% CI 0.33 to 0.96) for the 5-years follow-up, and 0.54 (95% CI 0.30 to 0.98) for the 14-years follow-up. The associations between CLC-based total residential green space and depression varied across the different buffer sizes. In general, all the associations depended on the type of depression assessment, quality of greenness indicators, and the spatial scale of analysis. The associations also varied across the socio-demographic groups and neighborhood socioeconomic disadvantage level.  相似文献   
6.
《Dental materials》2022,38(4):e69-e82
Objectivestesting if hypothetical transverse centripetal strains due to polymerization contraction of luting materials produce differential alterations in its bonding to luted structures, depending on distances to the center of the luting mass, and if this effect is C-factor related. Two hypotheses were tested: (1) there is a statistically significant decreasing relationship between the bonding strength and the transverse distances to the center of the luting material, and (2) there is a statistically significant difference between bonding strengths among luting spaces with different configuration factors.Methods10 PMMA (15 mm Ø) pairs of cylinders were cemented (Scotchbond Universal adhesive & Relyx Universal, both chemically cured) in a compliant setup under two (20 and 70 N) luting forces forming 2 groups (5 samples each), resulting in different C-factors. Whole samples were sectioned in x and y directions obtaining non-trimmed beams from all along the luting surfaces. Their relative positions in each sample were assessed before separating and categorized (10 categories) according to their distances to the center of the sample. All beams were tested in tension and, because of their uneven bonding areas and to balance its influence, UTS results were transformed into UTSres. First hypothesis was tested trough a linear relationship between UTSres and distances to vertical centers per samples. Second hypothesis was tested using Mann-Whitney U tests to compare UTSres between groups, along all categories. Further Weibull analysis was applied.ResultsANOVA’s p of the regression UTSres – categories were statistically significant for all samples in group 70 N and for all except one in group 20 N: first hypothesis is partially maintained. Although Mann-Whitney tests p comparing UTSres of both groups for all categories but the first were statistically significant this hypothesis was maintained relying in Weibull analysis. Significance: bonded attachment of cemented materials decreases from centers to outbounds in plane, extensive surfaces, and this decrease is C-factor related.  相似文献   
7.
目的 探讨台式虚拟现实显示器对近距离观看视频所致数字眼疲劳的预防作用。设计 交叉实验研究。研究对象 成年健康受试者31例。方法 受试者随机分两组,第一组先使用手机、后使用台式虚拟显示器观看相同的视频10分钟,中间休息30分钟;第二组使用相反的顺序观看。使用开放式验光仪记录观看视频前、中、后的屈光度,并计算调节误差、调节微波动。选取右眼数据进行统计分析。使用Likert量表记录观看视频前、后的主观视疲劳评分。主要指标 观看视频前后屈光度、调节微波动、调节误差、视疲劳量表评分。结果 观看视频前,采用手机和采用台式虚拟现实显示器者的屈光度分别为(-0.30±0.54)D和(-0.29±0.58)D(P=0.928),调节微波动分别为(0.15±0.07)D和(0.15±0.08)D(P=0.858)。观看视频5分钟时,采用手机者的屈光度为(-3.32±0.68)D,较观看前明显降低(P=0.000);采用台式虚拟现实显示器观看者的屈光度为(-0.39±0.63)D,较观看前未见明显降低(P=0.168);采用手机观看和采用台式虚拟现实显示器观看者的调节微波动分别为(0.33±0.10)D和(0.21±0.10)D(P=0.000);采用手机观看者调节误差为滞后(0.25±0.68)D,采用台式虚拟现实显示器观看者调节误差为超前(0.09±0.63)D(P=0.044)。观看完视频后,采用手机观看者的屈光度为(-0.53±0.56D)D,较观看前明显降低(P=0.000);采用台式虚拟显示器观看者的屈光度为(-0.34±0.61)D,较观看视频前未见明显降低(P=0.335)。观看视频后,采用手机和台式虚拟显示器者的视疲劳量表评分较观看前均增加,分别增加了(8.4±8.3)和(2.5±3.8)(P=0.001)。结论 应用台式虚拟显示器可以预防近距离观看视频引起的数字眼疲劳,这可能与观看时不需要视近调节有关。(眼科,2022, 31: 225-230)  相似文献   
8.
9.
目的分析中国高龄人群高血压、糖尿病患病情况以及高血压合并糖尿病的相关因素。方法获取中国老年健康影响因素跟踪调查(CLHLS)2018年横断面数据,计算高血压、糖尿病患病率及服药治疗率,分析患病率及服药治疗率的性别、年龄差异,并采用有序多结局Logistic回归分析的比值比(OR)及其95%置信区间(95%CI)评估性别、年龄及地域差异与高龄人群患高血压合并糖尿病的相关性。结果纳入研究对象15874例,平均年龄为(85.46±11.70)岁,男性占43.62%。高血压、糖尿病患病率及高血压与糖尿病共患率分别为42.93%(95%CI为42.13%~43.74%)、10.14%(95%CI为9.64%~10.64%)和7.49%(95%CI为7.05%~7.92%)。女性高血压和糖尿病患病率均高于男性,差异有统计学意义(P<0.05)。高血压服药治疗信息缺失率大于30%,糖尿病服药治疗信息缺失率大于50%。随着年龄增长,高血压和糖尿病患病率均呈下降趋势,差异有统计学意义(P<0.05)。百岁以上老人的高血压、糖尿病患病率分别为25.91%(95%CI为23.88%~27.93%)和2.14%(95%CI为1.45%~2.82%)。有序三结局Logistic回归分析结果显示,男性、大于90岁、居住在北方是高龄人群高血压合并糖尿病的保护因素(P<0.05)。结论高龄群体高血压、糖尿病的分布存在性别差异,患病率随着年龄增长而降低,高血压合并糖尿病与性别、年龄及地域均有关联。  相似文献   
10.
Depression is one of the most prevalent psychiatric disorders and the leading cause of disability among adolescents, with sleep duration as its vital influential factor. Adolescents might be mentally sensitive to the stress caused by the COVID-19 pandemic. Yet, the alteration of adolescents' sleep duration, depression, and their associations within the context of the COVID-19 pandemic have not been well documented. We conducted a longitudinal study, recruiting 2496 adolescents from 3 junior high schools to examine the alteration of their sleep duration and depressive symptoms before and during the pandemic, and to explore their potential association(s). Data were collected before (December 2019) and during the pandemic (July 2020). Paired samples t-test revealed a significant decrease in sleep duration and a significant increase in depressive symptoms during the COVID-19 pandemic. Higher grades, COVID-19 infection history, higher CES-DC score, and the COVID-19 pandemic itself might contribute to decreased sleep duration, while longer exercise duration during the pandemic might be a protective factor. According to the cross-lagged analysis, the existence of depressive symptoms before the pandemic was significantly associated with a shorter sleep duration during the pandemic (β = −0.106, p < 0.001). Previously shortened sleep duration was significantly associated with a greater likelihood of depressive symptoms during the pandemic (β = −0.082, p < 0.001). Our findings revealed that the COVID-19 pandemic has a negative influence on adolescents’ mental health and sleep. Mental preparedness should be highlighted to mitigate the psychosocial influences of any possible public emergencies in the future. Sleep duration represents a viable home-based intervention for depressive symptoms.  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