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   7162篇
  免费   776篇
  国内免费   15篇
耳鼻咽喉   59篇
儿科学   229篇
妇产科学   128篇
基础医学   1211篇
口腔科学   170篇
临床医学   723篇
内科学   1330篇
皮肤病学   91篇
神经病学   691篇
特种医学   539篇
外科学   687篇
综合类   197篇
一般理论   5篇
预防医学   628篇
眼科学   216篇
药学   621篇
中国医学   10篇
肿瘤学   418篇
  2021年   85篇
  2019年   81篇
  2018年   91篇
  2017年   81篇
  2016年   75篇
  2015年   100篇
  2014年   142篇
  2013年   276篇
  2012年   278篇
  2011年   264篇
  2010年   188篇
  2009年   204篇
  2008年   265篇
  2007年   293篇
  2006年   282篇
  2005年   345篇
  2004年   305篇
  2003年   282篇
  2002年   260篇
  2001年   222篇
  2000年   245篇
  1999年   220篇
  1998年   156篇
  1997年   121篇
  1996年   148篇
  1995年   112篇
  1994年   111篇
  1993年   109篇
  1992年   175篇
  1991年   143篇
  1990年   140篇
  1989年   161篇
  1988年   144篇
  1987年   146篇
  1986年   122篇
  1985年   132篇
  1984年   112篇
  1983年   107篇
  1982年   59篇
  1981年   67篇
  1980年   71篇
  1979年   99篇
  1978年   60篇
  1977年   70篇
  1976年   54篇
  1975年   58篇
  1974年   56篇
  1973年   67篇
  1971年   54篇
  1968年   55篇
排序方式: 共有7953条查询结果,搜索用时 15 毫秒
121.
Human herpesviruses are associated with morbidity and mortality in persons with compromised immune systems, including patients infected with human immunodeficiency virus (HIV). To investigate the basis for this association, the levels of all 8 human herpesviruses (herpes simplex virus, types 1 and 2, varicella-zoster virus, cytomegalovirus, Epstein-Barr virus, and human herpesvirus 6, human herpesvirus 7, and human herpesvirus 8) were measured with the quantitative polymerase chain reaction (PCR). Viral DNA was measured in the whole blood of 20 HIV-infected patients and compared with levels in 20 healthy blood donors. There was no significant difference in the frequency of virus detection of the 8 human herpesviruses between HIV-infected patients and healthy adults. These results indicate that HIV infection is not associated with a general increase in the circulating levels of human herpesviruses, and suggest that quantitative PCR analysis is superior to qualitative PCR analysis for detection of clinically relevant disease in HIV-infected patients.  相似文献   
122.
J L White  W O Dawson 《Virology》1978,88(1):33-43
Extracts from Cowpea chlorotic mottle virus (CCMV)-infected Cowpea leaves contained membrane-bound (31,000 g pellet) and soluble (31,000 g supernatant) RNA-dependent RNA polymerase activities. The membrane-bound RNA-dependent RNA polymerase (CCMV RNA replicase) increased 12-fold 4 days after inoculation. The viral RNA synthesis in vitro proceeded linearly for 20 min and required the four nucleoside triphosphates and Mg2+ ions for activity. Manganese ion was a poor substitute for Mg2+. Optimal enzymatic activity in vitro was unaffected by exogenous RNA or KCI. The CCMV RNA replicase product was predominantly heterodisperse single-stranded RNAs, some of which comigrated with CCMV virion RNA. Small amounts of large double-stranded RNAs were also products of the replicase reaction. The soluble RNA-dependent RNA polymerase from CCMV-infected or healthy Cowpea leaves required the four nucleoside triphosphates and Mg2+ ions for activity. Its activity in the in vitro assay was stimulated by adding exogenous RNAs but was inhibited by KCI. The product of the soluble RNA-dependent RNA polymerase was predominantly double-stranded RNA of approximately 4 to 6 S. RNA-dependent RNA polymerase activity, similar to that detected in the soluble fraction, was detected in the membrane pellet. This activity, which complicates the analysis of viral replicase assay, was removed without affecting CCMV RNA replicase activity by washing the 31,000 g pellets with buffer containing 0.5 M KCI. The KCI treatment aids in preparation of membrane-bound fractions devoid of host RNA-dependent RNA polymerase activity and high in viral replicase activity.  相似文献   
123.
In closing, with the new revelations of the Human Genome Project, notions on whether land-based race identity or ethnicity with genetic markers has been proven valid, with few exceptions. This has caused me to revisit the attempted effort to discard concepts on race, especially in medicine. Obviously there are outliers to this new work. But contrary to popular belief, and to some, the unthinkable, there may be, in fact, a biologic basis for our human distinctions. And I for one do not feel shame or seem perplexed by it. Moreover, it appears that Dr. Welsing in her earlier work was onto something, and was indeed ahead of her time. The problem African Americans, and other persons of color face, to some extent, has to do with the social political context of racism, and the biologic impact it has and is often expressed in the form of stress and injuries, simply put. Therefore, and more importantly, eliminating the nomenclature of how we classify ourselves in our intellectual interchange in science and other areas, will not correct our problems, but may in fact, if abandon, spell our doom. Because what we are murderously burdened by has to do with racism and its effects. Which are in effect, based on physical features, not mere classification. Further, the current thought on racism and why it is practiced by some is that racism serves an evolutionary drive to survive by humans, by forming alliances in among similar groups of people. Therefore, if that is the case, we had better be ready for the long haul in this battle as our history and ongoing struggles tell us. Besides, if not for racism, "we would not have had all of these problems over all these years." The National Medical Association and its publishing instruments must remain vigilant and stay focused.  相似文献   
124.
Studies on adults have suggested that a deterioration in performance (within session vigilance decrement) on a continuous performance task may be related to individual differences in baseline levels of electrodermal activity (electrodermal lability). This study investigated this relationship in 153 children, aged 9–16 years. A significant vigilance decrement was observed, as indicated by average decreases in perceptual sensitivity (d') over an 11.5-min time period. Although electrodermal labiles were overall more perceptually sensitive than electrodermal stabiles, results did not support the premise that the performance of stabiles decreases over time more than that of labiles. Performance on other cognitive tasks, involving tests of perceptual speed ability, did not appear to be highly related to vigilance performance. However labiles were not only better able to sustain their attention, but also performed better and faster on these cognitive tasks.  相似文献   
125.
Morphological studies have shown that macrophages and microglia undergo apoptosis in the central nervous system (CNS) in acute experimental autoimmune encephalomyelitis (EAE) in the Lewis rat. To assess the relative levels of macrophage and microglial apoptosis, and the molecular mechanisms involved in this process, we used three-colour flow cytometry to identify CD45lowCD11b/c+ microglial cells and CD45highCD11b/c+ macrophages in the inflammatory cells isolated from the spinal cords of Lewis rats 13 days after immunization with myelin basic protein (MBP) and complete Freund's adjuvant. Simultaneously, we analyzed the DNA content of these cell populations to assess the proportions of cells undergoing apoptosis and in different stages of the cell cycle or examined their expression of three apoptosis- regulating proteins, i.e. Fas (CD95), Fas ligand (FasL) and Bcl-2. Microglia were highly vulnerable to apoptosis and were over-represented in the apoptotic population. Macrophages were less susceptible to apoptosis than microglia and underwent mitosis more frequently than microglia. The different susceptibilities of microglia and macrophages to apoptosis did not appear to be due to variations in Fas, FasL or Bcl- 2 expression, as the proportions of microglia and macrophages expressing these proteins were similar, and were relatively high. Furthermore, in contrast to T cell apoptosis, apoptosis of microglia/macrophages did not occur more frequently in cells expressing Fas or FasL, or less frequently in cells expressing Bcl-2. These results indicate that the apoptosis of microglia and CNS macrophages in EAE is not mediated through the Fas pathway, and that Bcl-2 expression does not protect them from apoptosis. Expression of FasL by macrophages and microglia may contribute to the pathogenesis and immunoregulation of EAE through interactions with Fas+ oligodendrocytes and Fas+ T cells. The high level of microglial apoptosis in EAE indicates that microglial apoptosis may be an important homeostatic mechanism for controlling the number of microglia in the CNS following microglial activation and proliferation.   相似文献   
126.
A mentally retarded female child with multiple congenital abnormalities had an abnormal X chromosome and a Y chromosome; the karyotype was interpreted as 46,dup(X)(p21 leads to pter)Y. Prenatal chromosome studies in a later pregnancy indicated the same chromosomal abnormality in the fetus. The fetus and proband had normal female genitalia and ovarian tissue. H--Y antigen was virtually absent in both sibs, a finding consistent with the view that testis-determining genes of the Y chromosome may be suppressed by regulatory elements of the X. The abnormal X chromosome was present in the mother, the maternal grandmother, and a female sib: all were phenotypically normal and showed the karyotype 46,Xdup(X)(p21 leads to pter) with non-random inactivation of the abnormal X. Anomalous segregation of the Xga allele suggests that the Xg locus was involved in the inactivation process or that crossing-over at meiosis occurred.  相似文献   
127.
Chinese hamster ovary fibroblasts, as model cells, have been microencapsulated in a hydroxyethyl methacrylate-methyl methacrylate copolymer (HEMA-MMA) by interfacial precipitation. The polymer containing approximately equal to 75 mol% HEMA, dissolved in polyethylene glycol 200 (PEG 200) was coextruded with the cell suspension (4-6 X 10(5) cells/ml in the alpha-MEM with 10% foetal calf serum +/- Ficoll 400/PBS) through a concentric needle assembly. Polymer solution droplets, containing cells, were blown off the end of the needle assembly by a coaxial filtered air stream into a nonsolvent bath containing phosphate buffered saline (PBS) with 5 ppm Pluronic L101, overlaid with hexadecane. The nascent capsules hang at the hexadecane/PBS interface while the solvent is extracted into the aqueous nonsolvent, to precipitate the polymer around the cells. The resultant capsules were 500 microns-1 mm in diam. with a microporous sponge-like interior, and also very tough and flexible. The cells survived encapsulation based on subculture ability, retention of some fluorescein diacetate (FDA) activity over 5 d and direct light microscopic evidence of cell growth over 10 d after histological sectioning and staining. However, cell growth was not uniformly observed (especially in the FDA assay) and this was attributed to space limitations for growth within the microporous interior. Continued development of this process and adaptation to cells such as pancreatic islets is expected to lead to hybrid artificial organs which are capable of ameliorating metabolic disorders such as diabetes.  相似文献   
128.
Two adults with mild dementia and a history of memory loss and disequilibrium were seen in the eye clinic following complaints of acuity loss in the 20/30–20/70 (Snellen) range. Results from the fundus examination of one patient were entirely normal; the other showed minimal vascular attenuation and optic atrophy. Electrophysiology was remarkable: (1) Photopic ERG b-waves were reduced, delayed, and showed pronounced oscillations. (2) EOG light-rise potentials were absent or very small. (3) Binocular pattern-VER signals showed addition of the monocular signal. Scotopic ERG signals were normal. Brain biopsy and microscopy showed intercellular, autofluorescent ceroid deposits which provided a clear diagnosis of Kufs disease. Histology of model animal retinal cells show ceroid deposits in cell classes implicated by the human retinal signals. The cluster of electrophysiological results point toward early changes in the pigment epithelium and inner plexiform layer cells as a means of non-invasive diagnosis.  相似文献   
129.
Quality of Life Research - During the COVID-19 pandemic, widespread public health measures were implemented to control community transmission. The association between these measures and...  相似文献   
130.
Quality of Life Research - This study aimed to investigate changes over time in quality of life, perceived stress, and serious psychological distress for individuals diagnosed with COVID-19 in an...  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