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   224100篇
  免费   3433篇
  国内免费   391篇
耳鼻咽喉   1685篇
儿科学   7573篇
妇产科学   4419篇
基础医学   22392篇
口腔科学   3559篇
临床医学   16056篇
内科学   44397篇
皮肤病学   1723篇
神经病学   20558篇
特种医学   10209篇
外科学   35906篇
综合类   2498篇
一般理论   12篇
预防医学   20934篇
眼科学   3497篇
药学   12251篇
中国医学   776篇
肿瘤学   19479篇
  2023年   363篇
  2022年   903篇
  2021年   1671篇
  2020年   920篇
  2019年   1333篇
  2018年   23443篇
  2017年   18322篇
  2016年   20569篇
  2015年   2387篇
  2014年   2816篇
  2013年   3370篇
  2012年   10867篇
  2011年   24733篇
  2010年   20863篇
  2009年   13314篇
  2008年   22618篇
  2007年   24883篇
  2006年   3669篇
  2005年   5243篇
  2004年   6141篇
  2003年   6820篇
  2002年   4668篇
  2001年   713篇
  2000年   747篇
  1999年   607篇
  1998年   623篇
  1997年   574篇
  1996年   400篇
  1995年   411篇
  1994年   340篇
  1993年   245篇
  1992年   251篇
  1991年   263篇
  1990年   297篇
  1989年   197篇
  1988年   152篇
  1987年   151篇
  1986年   140篇
  1985年   154篇
  1984年   130篇
  1983年   117篇
  1982年   132篇
  1981年   84篇
  1980年   127篇
  1979年   79篇
  1978年   75篇
  1975年   58篇
  1974年   75篇
  1938年   63篇
  1932年   59篇
排序方式: 共有10000条查询结果,搜索用时 12 毫秒
991.
Competition in the health care market has intensified in recent years. Health care providers are increasingly adopting innovative marketing techniques to secure their positions in the marketplace. This paper examines an innovative marketing technique, situational segmentation, and assesses its applicability to the health care market. Situational segmentation has proven useful in many consumer goods markets but has received little attention in the context of health care marketing. A two-stage research process is used to develop a taxonomy of situational factors pertinent to health care choice. In stage one, focus group interviews are used to gather information which is instrumental to questionnaire development. In stage two, the responses of 151 subjects to a 51 item questionnaire are factor analyzed. The results demonstrate that situational segmentation is a viable strategy in the health care market.  相似文献   
992.
The European Laryngological Society is proposing a classification of different laryngeal endoscopic cordectomies in order to ensure better definitions of postoperative results. We chose to keep the word “cordectomy” even for partial resections because it is the term most often used in the surgical literature. The classification comprises eight types of cordectomies: a subepithelial cordectomy (type I), which is resection of the epithelium; a subligamental cordectomy (type II), which is a resection of the epithelium, Reinke’s space and vocal ligament; transmuscular cordectomy (type III), which proceeds through the vocalis muscle; total cordectomy (type IV); extended cordectomy, which encompasses the contralateral vocal fold and the anterior commissure (type Va); extended cordectomy, which includes the arytenoid (type Vb); extended cordectomy, which encompasses the subglottis (type Vc); and extended cordectomy, which includes the ventricle (type Vd). Indications for performing those cordectomies may vary from surgeon to surgeon. The operations are classified according to the surgical approach used and the degree of resection in order to facilitate use of the classification in daily practice. Each surgical procedure ensures that a specimen is available for histopathological examination. Received: 29 December 1998 / Accepted: 2 July 1999  相似文献   
993.
T follicular helper (TFH) cells play an essential role in promoting B cell responses and antibody affinity maturation in germinal centers (GC). A subset of memory CD4+ T cells expressing the chemokine receptor CXCR5 has been described in human blood as phenotypically and clonally related to GC TFH cells. However, the antigen specificity and relationship of these circulating TFH (cTFH) cells with other memory CD4+ T cells remain poorly defined. Combining antigenic stimulation and T cell receptor (TCR) Vβ sequencing, we found T cells specific to tetanus toxoid (TT), influenza vaccine (Flu), or Candida albicans (C.alb) in both cTFH and non-cTFH subsets, although with different frequencies and effector functions. Interestingly, cTFH and non-cTFH cells specific for C.alb or TT had a largely overlapping TCR Vβ repertoire while the repertoire of Flu-specific cTFH and non-cTFH cells was distinct. Furthermore, Flu-specific but not C.alb-specific PD-1+ cTFH cells had a “GC TFH-like” phenotype, with overexpression of IL21, CXCL13, and BCL6. Longitudinal analysis of serial blood donations showed that Flu-specific cTFH and non-cTFH cells persisted as stable repertoires for years. Collectively, our study provides insights on the relationship of cTFH with non-cTFH cells and on the heterogeneity and persistence of antigen-specific human cTFH cells.  相似文献   
994.
Severe acute respiratorysyndrome coronavirus-2 (SARS-CoV-2) pandemic spread rapidly and this scenario is concerning worldwide, presenting more than 590 million coronavirus disease 2019 cases and 6.4 million deaths. The emergence of novel lineages carrying several mutations in the spike protein has raised additional public health concerns worldwide during the pandemic. The present study review and summarizes the temporal spreading and molecular evolution of SARS-CoV-2 clades and variants worldwide. The evaluation of these data is important for understanding the evolutionary histories of SARSCoV-2 lineages, allowing us to identify the origins of each lineage of this virus responsible for one of the biggest pandemics in history. A total of 2897 SARS-CoV-2 whole-genome sequences with available information from the country and sampling date (December 2019 to August 2022), were obtained and were evaluated by Bayesian approach. The results demonstrated that the SARS-CoV-2 the time to the most recent common ancestor (tMRCA) in Asia was 2019-12-26 (highest posterior density 95% [HPD95%]: 2019-12-18; 2019-12-29), in Oceania 2020-01-24 (HPD95%: 2020-01-15; 2020-01-30), in Africa 2020-02-27 (HPD95%: 2020-02-21; 2020-03-04), in Europe 2020-02-27 (HPD95%: 2020-02-20; 2020-03-06), in North America 2020-03-12 (HPD95%: 2020-03-05; 2020-03-18), and in South America 2020-03-15 (HPD95%: 2020-03-09; 2020-03-28). Between December 2019 and June 2020, 11 clades were detected (20I [Alpha] and 19A, 19B, 20B, 20C, 20A, 20D, 20E [EU1], 20F, 20H [Beta]). From July to December 2020, 4 clades were identified (20J [Gamma, V3], 21 C [Epsilon], 21D [Eta], and 21G [Lambda]). Between January and June 2021, 3 clades of the Delta variant were detected (21A, 21I, and 21J). Between July and December 2021, two variants were detected, Delta (21A, 21I, and 21J) and Omicron (21K, 21L, 22B, and 22C). Between January and June 2022, the Delta (21I and 21J) and Omicron (21K, 21L, and 22A) variants were detected. Finally, between July and August 2022, 3 clades of Omicron were detected (22B, 22C, and 22D). Clade 19A was first detected in the SARS-CoV-2 pandemic (Wuhan strain) with origin in 2019-12-16 (HPD95%: 2019-12-15; 2019-12-25); 20I (Alpha) in 2020-11-24 (HPD95%: 2020-11-15; 2021-12-02); 20H (Beta) in 2020-11-25 (HPD95%: 2020-11-13; 2020-11-29); 20J (Gamma) was 2020-12-21 (HPD95%: 2020-11-05; 2021-01-15); 21A (Delta) in 2020-09-20 (HPD95%: 2020-05-17; 2021-02-03); 21J (Delta) in 2021-02-26 (2020-11-02; 2021-04-24); 21M (Omicron) in 2021-01-25 (HPD95%: 2020-09-16; 2021-08-08); 21K (Omicron) in 2021-07-30 (HPD95%: 2021-05-30; 2021-10-19); 21L (Omicron) in 2021-10-03 (HPD95%: 2021-04-16; 2021-12-23); 22B (Omicron) in 2022-01-25 (HPD95%: 2022-01-10; 2022-02-05); 21L in 2021-12-20 (HPD95%: 2021-05-16; 2021-12-31). Currently, the Omicron variant predominates worldwide, with the 21L clade branching into 3 (22A, 22B, and 22C). Phylogeographic data showed that Alpha variant originated in the United Kingdom, Beta in South Africa, Gamma in Brazil, Delta in India, Omicron in South Africa, Mu in Colombia, Epsilon in the United States of America, and Lambda in Peru. The COVID-19 pandemic has had a significant impact on global health worldwide and the present study provides an overview of the molecular evolution of SARS-CoV-2 lineage clades (from the Wuhan strain to the currently circulating lineages of the Omicron).  相似文献   
995.
The removal of oil from water is a worldwide challenge that must be faced to avoid irreversible marine habitat destruction. A novel fast and simple technique to obtain polydimethylsiloxane (PDMS) membranes is developed using the photopolymerization technique. The high reactivity of the acrylated PDMS formulation toward photo-induced free radical polymerization is assessed via the differential scanning photo-calorimetry (photo-DSC) technique. Two different membranes dense or porous are developed and investigated. Porous membranes, having 100–200 µm as pore size, are obtained using a low-cost environmentally friendly sodium chloride template. Thanks to the hydrophobic/oleophilic intrinsic characteristic of PDMS, the UV-cured membranes can selectively remove dodecane, selected as the target oil, from water. The dodecane sorption capability of both membranes is investigated and compared. Moreover, the membranes can be easily reused since the adsorbed oil can be recovered by simply compressing the membrane. Those PDMS sorbents show high mechanical stability after five adsorption/desorption cycles.  相似文献   
996.
This paper presents evidence that L-tyrosine oxidation products and 5,6-dihydroxyindole, an intermediate of melanin synthesis bind to and modify DNA structure, as tested by extracting cell DNA, using topoisomerase I and denaturation assays. When supercoiled plasmid pCU18 or pBR322 DNAs are treated with 5,6-dihydroxyindole the supercoiled species disappear and are converted to species less mobile in a gel retardation test with respect to relaxed DNA. 5,6-Dihydroxyindole causes an easier acid denaturation of the double helix. The results, that are dose dependent,would point to both intercalation and cross-linking of DNA by 5,6-dihydroxyindole and its oxidation product(s). 3H-L-tyrosine deriving radioactivity, bound to nuclear DNA, is higher at low pH, (5.6) if compared to pH 6.8. The highest radioactivity bound to cell DNA is found during the transition from the amelanotic to the melanotic phenotype in human melanoma cell lines. As a control, the binding of 3H-L-tyrosine radioactivity to human prostate fibroblast DNA was investigated.  相似文献   
997.
  1. SB-204269 (trans-(+)-6-acetyl-4S-(4-fluorobenzoylamino)-3,4-dihydro-2,2-dimethyl-2H-benzol[b]pyran-3R-ol, hemihydrate) shows potent anticonvulsant activity in a range of animal seizure models, with a lack of neurological or cardiovascular side-effects. The profile of the compound suggests that it may have a novel mechanism of action. This study describes the characteristics of a binding site for [3H]-SB-204269 in rat forebrain membranes.
  2. Specific [3H]-SB-204269 binding was saturable and analysis indicated binding to a homogenoeous population of non-interacting binding sites with a dissociation constant (KD) of 32±1 nM and a maximum binding capacity (Bmax) of 253±18 fmol mg−1 protein. Kinetic studies indicated monophasic association and dissociation. Binding was similar in HEPES or Tris-HCl buffers and was unaffected by Na+, K+, Ca2+ or Mg2+ ions. Specific binding was widely distributed in brain, but was minimal in a range of peripheral tissues.
  3. Specific [3H]-SB-204269 binding was highly stereoselective, with a 1000 fold difference between the affinities of SB-204269 and its enantiomer SB-204268 for the binding site. The affinities of analogues of SB-204269 for binding can be related to their activities in the mouse maximal electroshock seizure threshold (MEST) test of anticonvulsant action.
  4. None of the standard anticonvulsant drugs, phenobarbitone, phenytoin, sodium valproate, carbamazepine, diazepam and ethosuximide, or the newer anticonvulsants, lamotrigine, vigabatrin, gabapentin and levetiracetam, showed any affinity for the [3H]-SB-204269 binding site. A wide range of drugs active at amino acid receptors, Na+ or K+ channels or various other receptors did not demonstrate any affinity for the binding site.
  5. These studies indicate that SB-204269 possesses a specific CNS binding site which may mediate its anticonvulsant activity. This binding site does not appear to be directly related to the sites of action of other known anticonvulsant agents, but may have an important role in regulating neuronal excitability.
  相似文献   
998.
We herein present a study conducted on 14 patients presenting cancer of the lower rectum or of the anal canal (10 adenocarcinomas and 4 squamous-cell carcinomas) and submitted to the Miles abdominal perineal resection in which a new perineal sphincter was constructed. PURPOSE: The aim of this study was to evaluate the efficacy of this new perineal sphincter constructed by transposing the gracilis muscles around an orthotopic colostomy in the attempt to avoid a permanent abdominal colostomy. METHODS: In all cases both gracilis muscles were employed. The right one was placed along the posterior wall of the pelvis and fixed to the controlateral ischiatic tuberosity, creating a sling comparable to the levator ani muscles. The left gracilis was passed around the colon and attached to the ipsilateral or controlateral tuberosity according to its length, reconstructing a muscular ring. The entire procedure was performed in one step in nine cases and in more steps in the remaining five. RESULTS: Of the 14 operated patients, 2 died of vascular disease and 1 developed necrosis of the colonic stump which required reconversion to an abdominal colostomy. Of the remaining 11 patients available for long-term evaluations, 8 showed adequate stool control. The remaining three manifested an incomplete level of continence. During the three-year follow-up period, all patients were evaluated by clinical examination, defecography, endoluminal ultrasonography, nuclear magnetic resonance, CT scan, and endoluminal manometry. CONCLUSION: This neosphincter realizes an elastic stenosis responsible for an efficient level of continence. Best results are observed in the young and educated patients submitted to surgery in two steps. Contraindications to this surgery seem to be advanced cancer, old age, and obesity.  相似文献   
999.
PURPOSE: We herein report our experience with transanal endoscopic microsurgery. The new technique combines an endoscopic view and access of the rectum under gas insufflationviaa stereoscopic telescope with all conventional surgical maneuvers such as tissue preparation, coagulation and control of bleeding, irrigation, suction, and, finally, suturing of the parietal defect. METHODS: The main indication for transanal endoscopic microsurgery is the removal of broad-based sessile polyps and excision of early rectal cancers. We performed local excision of pT2, G1-2 adenocarcinomas and excision of advanced rectal cancer in high-risk patients. The reported series includes 35 consecutive patients, who have been enrolled in a prospective clinical trial. Five patients were excluded for different reasons. The patients were submitted to 29 total wall excisions with or without perirectal fat and one mucosectomy. RESULTS: Postoperative histologic examination showed 9 adenomas and 21 adenocarcinomas. Morbidity included 2 (5.6 percent) perioperative and 2 (5.6 percent) late complications. There was no operative mortality and the mean postoperative hospital course was six days. All patients are in follow-up observation with a mean time of 10.3 months. In the group of adenomas and adenocarcinomas, we did not observe local recurrence. CONCLUSIONS: Considering our experience with the overall results reported by other authors, we believe that transanal endoscopic microsurgery is the procedure of choice for the treatment of rectal polyps and early rectal cancers provided strict patient selection criteria are met.  相似文献   
1000.
As the country strives to produce larger numbers of generalist physicians, considerable controversy has arisen over whether or not generalist applicants can be identified, recruited, and influenced to keep a generalist-oriented commitment throughout medical training. The authors present new and existing data to show that: 1) preadmission (BA/MD or post-baccalaureate) programs can help to identify generalist-oriented students; 2) characteristics determinedat admission to medical school are predictive of future generalist career choice; 3) current inpatient-oriented training programs strongly push students away from a primary care career; 4) women are more likely than men to choose generalist careers, primarily because of those careers’ interpersonal orientation; and 5) residency training programs are able to select applicants likely to become generalists. Therefore, to produce more generalists, attempts should be made to encourage generalist-oriented students to enter medical schools and to revise curricula to focus on outpatient settings in which students can establish effective and satisfying relationships with patients. These strategies are most likely to be successful if enacted within the context of governmental and medical school-based changes that allow for more reimbursement and respect for the generalist disciplines.  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