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   25316篇
  免费   1760篇
  国内免费   207篇
耳鼻咽喉   264篇
儿科学   754篇
妇产科学   977篇
基础医学   3306篇
口腔科学   331篇
临床医学   3332篇
内科学   5086篇
皮肤病学   438篇
神经病学   1810篇
特种医学   624篇
外科学   3147篇
综合类   536篇
现状与发展   2篇
一般理论   32篇
预防医学   2064篇
眼科学   493篇
药学   1766篇
中国医学   123篇
肿瘤学   2198篇
  2023年   183篇
  2022年   368篇
  2021年   663篇
  2020年   424篇
  2019年   603篇
  2018年   699篇
  2017年   513篇
  2016年   549篇
  2015年   706篇
  2014年   924篇
  2013年   1259篇
  2012年   1916篇
  2011年   1923篇
  2010年   1146篇
  2009年   971篇
  2008年   1594篇
  2007年   1635篇
  2006年   1531篇
  2005年   1496篇
  2004年   1314篇
  2003年   1120篇
  2002年   1083篇
  2001年   540篇
  2000年   457篇
  1999年   438篇
  1998年   242篇
  1997年   187篇
  1996年   177篇
  1995年   153篇
  1994年   122篇
  1993年   102篇
  1992年   212篇
  1991年   205篇
  1990年   176篇
  1989年   142篇
  1988年   139篇
  1987年   145篇
  1986年   118篇
  1985年   106篇
  1984年   97篇
  1983年   80篇
  1982年   46篇
  1981年   58篇
  1980年   64篇
  1979年   68篇
  1978年   61篇
  1977年   49篇
  1975年   47篇
  1974年   55篇
  1972年   64篇
排序方式: 共有10000条查询结果,搜索用时 16 毫秒
991.

Introduction

“Walkable” neighborhoods offer older adults opportunities for activities that may benefit cognition-related biological mechanisms. These have not previously been examined in this context.

Methods

We objectively assessed neighborhood walkability for participants (n = 146) from the Australian Imaging, Biomarkers and Lifestyle study with apolipoprotein E (APOE) genotype and two 18-month-apart brain volumetric and/or amyloid β burden assessments. Linear mixed models estimated associations of neighborhood walkability with levels and changes in brain imaging outcomes, the moderating effect of APOE ε4 status, and the extent to which associations were explained by physical activity.

Results

Cross-sectionally, neighborhood walkability was predictive of better neuroimaging outcomes except for left hippocampal volume. These associations were to a small extent explained by physical activity. APOE ε4 carriers showed slower worsening of outcomes if living in walkable neighborhoods.

Discussion

These findings indicate associations between neighborhood walkability and brain imaging measures (especially in APOE ε4 carriers) minimally attributable to physical activity.  相似文献   
992.
Children of parents with a mental illness (COPMI) have a higher risk of acquiring a mental illness themselves. Feelings of guilt and shame could increase COPMI risks of acquiring mental health disorder symptoms. These feelings of guilt and shame could also impact the quality of the parent-child relationship. Data were drawn from the qualitative part of a mixed method study featuring 18 face-to-face interviews with Dutch COPMI aged 12–21. Interviewees were asked about their experiences with guilt and shame related to their parent with mental illness and the extent they felt that these feelings affected their relationships with their parents. Qualitative thematic analysis (QTA) revealed that most COPMI youth described feelings of guilt and some of them reported feelings of shame. They reported making behavioral adjustments especially using caution in parental communication. In their perception, guilt and shame did not have long-term impacts on their relationships with parents.  相似文献   
993.
The 677 C to T transition in the MTHFR gene is a genetic determinant for hyperhomocysteinemia. We investigated whether this polymorphism modulates gray matter (GM) structural covariance networks independently of white‐matter integrity in patients with Alzheimer's disease (AD). GM structural covariance networks were constructed by 3D T1‐magnetic resonance imaging and seed‐based analysis. The patients were divided into two genotype groups: C homozygotes (n = 73) and T carriers (n = 62). Using diffusion tensor imaging and white‐matter parcellation, 11 fiber bundle integrities were compared between the two genotype groups. Cognitive test scores were the major outcome factors. The T carriers had higher homocysteine levels, lower posterior cingulate cortex GM volume, and more clusters in the dorsal medial lobe subsystem showing stronger covariance strength. Both posterior cingulate cortex seed and interconnected peak cluster volumes predicted cognitive test scores, especially in the T carriers. There were no between‐group differences in fiber tract diffusion parameters. The MTHFR 677T polymorphism modulates posterior cingulate cortex‐anchored structural covariance strength independently of white matter integrities. Hum Brain Mapp 38:3039–3051, 2017. © 2017 The Authors Human Brain Mapping Published Wiley by Periodicals, Inc.  相似文献   
994.
The traditional approach for ankylosis is gap arthroplasty or interpositional arthroplasty followed by reconstruction of the condyle using, for example, costochondral grafts. As these are non-pedicled grafts, there is eventual resorption with subsequent decrease in height of the ramus, facial asymmetry and deviated mouth opening. The authors have applied the method of total and partial sliding vertical osteotomy on the posterior border of the mandibular ramus for reconstruction of the mandible condyle as a pedicled graft for the correction of temporomandibular joint (TMJ) ankylosis. From 2004 to 2008, 18 patients who were diagnosed with TMJ ankylosis underwent operations for resection of the ankylosed condyle. Two methods were performed depending on the level of osteotomy on the posterior part of the mandibular ramus. All patients were followed-up for an average of 36 months (range 24-48 months). All patients showed apparent improved joint function with no cases of re-ankylosis. The results showed that sliding vertical osteotomy on the posterior border of the mandibular ramus seems to be an alternative and promising method for condylar reconstruction in patients with TMJ bony ankylosis.  相似文献   
995.
Transesophageal Doppler color flow imaging was performed in 19 adult patients (mean age 35 years) with an atrial septal defect demonstrated by cardiac catheterization or at surgery, or both. The transesophageal study correctly identified and classified 19 of 19 shunts in contrast to 16 of 18 shunts identified by the transthoracic approach. The area of the atrial septal defect was calculated by assuming it to be circular and taking the maximal Doppler color flow jet width at the defect site as its diameter. The pulsed Doppler sample volume was placed parallel to the shunt flow direction at the defect site to obtain the mean velocity and flow duration. From these values, the shunt volume was calculated as a product of the defect area, mean velocity, flow duration and heart rate. The calculated shunt flow volume obtained by transesophageal study showed a good correlation with shunt flow volume (r = 0.91, p less than 0.001) and pulmonary to systemic blood flow ratio (r = 0.84, p less than 0.001) obtained at cardiac catheterization. The size of the defect by transesophageal Doppler color flow mapping correlated fairly well with the size estimated at surgery (r = 0.73, p = 0.004). It is concluded that transesophageal Doppler color flow imaging is useful in the detection and classification of atrial septal defects and in the assessment of shunt volumes.  相似文献   
996.
Transformed fibroblasts coinoculated with epithelial cells accelerated the growth and shortened the latency period of human epithelial tumors in athymic mice. Addition of NbF-1 fibroblasts caused epithelial tumors to grow from five marginally tumorigenic or "nontumorigenic" (nontumor-forming) human tumor cell lines or strains: PC-3 (prostate), WH (bladder), MDA-436 (breast), and cells derived from the ascites fluids of patients with metastatic renal pelvic or prostate cancers. Evidence for the human and epithelial nature of these experimental tumors was provided by histologic, immunohistochemical, Southern and dot-blot hybridization, and cytogenetic analyses. Transformed fibroblasts induced predominantly carcinosarcomas, whereas nontumorigenic fibroblasts (NIH 3T3) and lethally irradiated transformed fibroblasts induced exclusively carcinomas. The fibroblast-epithelial interaction appears to occur bidirectionally and does not result from cell fusion. Because coculture experiments in vitro did not demonstrate an increased cell proliferation, it appears that undefined host factors can influence tumor growth. This tumor model may be useful in drug-screening programs and in mechanistic studies of factors regulating human tumor growth and progression.  相似文献   
997.
Galanin is a widely distributed regulatory peptide which modulates the pituitary secretion of PRL and GH. Estrogen administration strongly stimulates galanin gene expression in the rat anterior pituitary. In adult female Fischer 344 rats, estrogen also induces hyperplasia of lactotropes. We used immunocytochemical analysis to assess the effects of estrogen on galanin-like immunoreactivity (Gal-IR) in the rat pituitary and hypothalamus during sc diethylstilbestrol (DES) implantation and after its removal at 30 days. In the anterior pituitary, DES implantation increased the portion of Gal-IR-containing cells from less than 2% in the control rats to 18.3% after 3 days of DES and 36% after 30 days. These changes paralleled the lactotrope hyperplasia exhibited in response to DES exposure. Ten and 30 days after removal of the DES capsules, the percentage of Gal-IR-containing cells in the anterior pituitary decreased to 6.3% and 1.5%, respectively. Colocalization studies revealed that Gal-IR-containing cells were predominantly lactotropes. Immunoelectron microscopy demonstrated that Gal-IR was concentrated in the Golgi region of these hyperplastic lactotropes and suggests that little of the synthesized galanin is secreted. The distribution of Gal-IR in the hypothalamus, median eminence, and neurohypophysis was unaffected by DES treatment. These data demonstrate that galanin is synthesized by hyperplastic pituitary lactotropes of Fischer 344 rats and that peptide accumulation is dependent on the presence of circulating estrogens. In contrast, neuronal galanin synthesis in the hypothalamus does not appear to be regulated by estrogen.  相似文献   
998.
Calcifying matrix vesicles (MVs) are released from chondrocytes and osteoblasts in monolayer culture. In the present studies, we tested the ability of rachitic versus normal rat growth plate chondrocytes in micromass or monolayer primary cultures to produce MVs. Unlike earlier reports of in vitro MV biogenesis by chicken chondrocytes in which most MVs were released into the medium, we found that most of the released rat matrix vesicles were entrapped in a newly formed cartilaginous matrix enveloping the cells. These matrix-associated MVs could be isolated by mild collagenase treatment and concentrated by differential centrifugation. Vesicle production slowed in the older 2- to 4-week-old cultures and, unlike vesicle release from cultured chicken chondrocytes, active vesicle production did not show a second burst of activity at 3 to 4 weeks. Alkaline phosphatase (ALP) activity diminished with time in culture in cells and matrix vesicles, suggesting a decrease in differentiative expression. Protein profiles on SDS polyacrylamide gels of native matrix vesicles and culture-derived MVs from rachitic and normal cells were quite similar and showed a typical simplified protein pattern as compared to chondrocyte plasma membrane proteins. There were distinctive proteins migrating at 130, 80 to 95, 66, 43, 20, and 14 kd. Culture-derived MVs showed vigorous in vitro calcifying activity that was ALP related. We conclude that 1) rachitic chondrocytes are essentially normal in their matrix vesicle production; 2) matrix entrapment of MVs is a characteristic of rat chondrocyte cultures; and 3) culture-produced MVs are similar to native MVs in protein profile and calcifiability, and thus can be studied as a model for normal MV composition and calcification.  相似文献   
999.
Two Hodgkin's Reed-Sternberg cell (H-RS) lines, HDLM-1 and KM-H2, have phenotypes and functional properties very similar to those of H-RS cells in tissues. These two types of cells were induced to differentiate with a combination of phorbol ester, retinoic acid, and extracellular matrix. The induced cells displayed the morphology of histiocytes or histiocytelike cells, with a small, round or oval, eccentric nucleus and abundant cytoplasm. In ultrastructural studies, many cytoplasmic projections and rugae were observed. These induced cells exhibited abundant cytoplasmic lysosomal enzymes, such as esterase, acid phosphatase, alpha 1-antitrypsin, or lysozyme. The histiocytic nature of these induced cells was further confirmed by the increased expression of many monocyte/histiocyte markers, including CD11b, CD11c, CD13, CD14, CD15, CD33, CD68, Mac387, and 1E9. In functional tests, the induced cells were shown to produce interleukin-1, tumor necrosis factor, macrophage colony-stimulating factor, and/or prostaglandin E2. Phagocytosis was detected in less than 5% to 10% of the cells when Candida albicans was added to cultures. The results strongly suggest that H-RS cells are related to cells of histiocyte lineage.  相似文献   
1000.

Objective

Previous studies have found a temporal concordance in the increased prevalence of atopic diathesis/atopic diseases, attention-deficit hyperactivity disorder (ADHD), and autistic spectrum disorder (ASD) worldwide. But, the temporal association among these 3 distinct diseases is unknown.

Method

14,812 atopic subjects diagnosed with any atopic disease (asthma, atopic dermatitis, allergic rhinitis, or allergic conjunctivitis) before the age of 3 (atopic cohort) and 6944 non-atopic subjects with no lifetime atopic disease (non-atopic cohort), born between 1997 and 2000, were enrolled and followed to December 31, 2010 to identify the development of ADHD and ASD.

Results

The presence of any atopic disease in early childhood increased the risk of developing ADHD (hazard ratio [HR]: 1.97) and ASD (HR: 3.40) in later life. Greater numbers of atopic comorbidities (4 comorbidities: ADHD: HR: 2.53; ASD: HR: 4.29) were significantly related to a greater risk of developing ADHD and ASD.

Discussion

Atopic diathesis in early childhood elevated the risk of developing ADHD and ASD in later life, with the dose-dependent relationship of more atopic comorbidities with a greater likelihood of ADHD and ASD.  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