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   2265篇
  免费   259篇
  2022年   26篇
  2021年   28篇
  2020年   12篇
  2019年   27篇
  2018年   28篇
  2017年   25篇
  2016年   37篇
  2015年   72篇
  2014年   96篇
  2013年   109篇
  2012年   140篇
  2011年   128篇
  2010年   101篇
  2009年   72篇
  2008年   97篇
  2007年   98篇
  2006年   108篇
  2005年   99篇
  2004年   88篇
  2003年   86篇
  2002年   84篇
  2001年   67篇
  2000年   85篇
  1999年   52篇
  1998年   45篇
  1997年   30篇
  1996年   39篇
  1995年   30篇
  1994年   31篇
  1993年   40篇
  1992年   43篇
  1991年   43篇
  1990年   39篇
  1989年   30篇
  1988年   40篇
  1987年   23篇
  1986年   28篇
  1985年   25篇
  1984年   26篇
  1983年   16篇
  1980年   12篇
  1979年   12篇
  1978年   17篇
  1977年   13篇
  1974年   13篇
  1973年   15篇
  1972年   16篇
  1968年   12篇
  1967年   12篇
  1966年   11篇
排序方式: 共有2524条查询结果,搜索用时 203 毫秒
1.
A 14-days' training of rats designed to adapt the animals to the procedure of blood-pressure measurement caused in the myocardium a decrease, in serum an increase in noradrenaline ( NA) content. The latter remained unchanged following 3 days of treatment with 2.5 mg depot angiotensin II (AII) but decreased by more than one-half in the myocardium of untreated animals, and increased in serum. Despite the considerable difference in myocardial NA content and blood-pressure behaviour, both trained and untrained rats showed the same morphological reactions following administration of A II. These myocardial alterations which largely correspond to the so-called epinephrine myocarditis should not therefore be due solely to NA action; rather, the involvement of A II should be considered also.  相似文献   
2.
How often will elections end in landslides? What is the probability for a head-to-head race? Analyzing ballot results from several large countries rather anomalous and yet unexplained distributions have been observed. We identify tactical voting as the driving ingredient for the anomalies and introduce a model to study its effect on plurality elections, characterized by the relative strength of the feedback from polls and the pairwise interaction between individuals in the society. With this model it becomes possible to explain the polarization of votes between two candidates, understand the small margin of victories frequently observed for different elections, and analyze the polls'' impact in American, Canadian, and Brazilian ballots. Moreover, the model reproduces, quantitatively, the distribution of votes obtained in the Brazilian mayor elections with two, three, and four candidates.  相似文献   
3.
The total population affected with non-Hodgkin's lymphoma and treated by means of radiotherapy or combined radio-chemotherapy between 1960 and 1985 at the Medical Academy Dresden was analysed as to prognosis. 247 patients were classified according to the previous German scheme, 79 were subdivided on the basis of the recommendations laid down in the Kiel classification. The remission rates and survival curves achieved will stand out the comparison with international literature (remission rates of the low malignancy group amounted to 85.3 p.c. and those of the high malignancy group to 80.0 p.c.; the 5-years survival rates of the low malignancy group amounted to 61.9 p.c. and those of the high malignancy group to 41.7 p.c.). The influence of histology, clinical stage and involvement of organs is discussed on the basis of our results and informations obtained in literature. Our analysis confirms the high importance which must be attached to a common radiologic-internal outpatient-department for co-ordinating the diagnostic and therapeutical programme.  相似文献   
4.
This study seeks to explore whether neuroticism, agreeableness, and conscientiousness moderate the influence of relationship conflict experienced in groups on changes in group members'' evaluative cognitions related to teamwork quality (teamwork-related mental models). Data from 216 students, nested in 48 groups were analyzed using a multilevel modeling approach. Our results show that the experience of relationship conflict leads to a negative shift from the pre-task to the post-task teamwork-related mental models. Moreover, the results indicate that conscientiousness buffered the negative association between relationship conflict and the change in teamwork-related mental models. Our results did not support the hypothesized moderating effect of agreeableness and show that the detrimental effect of relationship conflict on the shift in teamwork-related mental models is accentuated for group members scoring low rather than high on neuroticism. These findings open new research venues for exploring the association between personality, coping styles and change in teamwork-related mental models.  相似文献   
5.
Seed dispersal influences a wide range of ecological processes. However, measuring dispersal patterns, particularly long‐distance dispersal, has been a difficult task. Marking bird‐dispersed seeds with stable 15N isotopes has been shown to be a user‐friendly method to trace seed dispersal. In this study, we determined whether 15N urea solution could be used to enrich seeds of two common wind‐dispersed plants, Eupatorium glaucescens (Asteraceae) and Sericocarpus tortifolius (Asteraceae). We further tested if the water type (distilled versus tap) in 15N urea solutions influences the level and variability of enrichment of plant seeds, and if increasing spraying frequency per se increases enrichment. Because droughts may lower seed set or kill plants, we wanted to investigate if the additional use of an externally applied anti‐transpirant affects the intake of externally applied 15N into seeds. The results demonstrate that 15N enrichment of seeds can facilitate dispersal experiments with wind‐dispersed plants. The use of distilled water in 15N urea solutions did not increase 15N enrichment compared to tap water. Further, enrichment was more efficient at lower spray frequencies. Both the use of tap water and low frequencies could lower time, effort and project costs. The results suggest that species can be protected from drought using an anti‐transpirant without decreasing the incorporation of 15N into seeds.  相似文献   
6.
Dendritic cells (DCs) play an important role in the generation of anti-cancer immune responses, however there is evidence that DCs in cancer patients are dysfunctional. Lipid accumulation driven by tumor-derived factors has recently been shown to contribute to DC dysfunction in several human cancers, but has not yet been examined in mesothelioma. This study investigated if mesothelioma tumor cells and/or their secreted factors promote increases in DC lipid content and modulate DC function. Human monocyte-derived DCs (MoDCs) were exposed to human mesothelioma tumor cells and tumor-derived factors in the presence or absence of lipoproteins. The data showed that immature MoDCs exposed to mesothelioma cells or factors contained increased lipid levels relative to control DCs. Lipid accumulation was associated with reduced antigen processing ability (measured using a DQ OVA assay), upregulation of the co-stimulatory molecule, CD86, and production of the tolerogenic cytokine, IL-10. Increases in DC lipid content were further enhanced by co-exposure to mesothelioma-derived factors and triglyceride-rich lipoproteins, but not low-density lipoproteins. In vivo studies using a murine mesothelioma model showed that the lipid content of tumor-infiltrating CD4+CD8α- DCs, CD4-CD8α- DCs DCs and plasmacytoid DCs increased with tumor progression. Moreover, increasing tumor burden was associated with reduced proliferation of tumor-antigen-specific CD8+ T cells in tumor-draining lymph nodes. This study shows that mesothelioma promotes DC lipid acquisition, which is associated with altered activation status and reduced capacity to process and present antigens, which may impair the ability of DCs to generate effective anti mesothelioma T cell responses.  相似文献   
7.
8.
9.
10.
The colonies of thecate hydroids are covered with a chitinous tubelike outer skeleton, the perisarc. The perisarc shows a species-specific pattern of annuli, curvatures, and smooth parts. This pattern is exclusively formed at the growing tips at which the soft perisarc material is expelled by the underlying epithelium. Just behind the apex of the tip, this material hardens. We treated growing cultures of Laomedea flexuosa with substances we suspected would interfere with the hardening of the perisarc (L-cysteine, phenylthiourea) and those we expected would stimulate it (dopamine, N-acetyldopamine). We found that the former caused a widening of and the latter a reduction in the diameter of the perisarc tube. At the same time, the length of the structure elements changed so that the volume remained almost constant. We propose that normal development involves a spatial and temporal regulation of the hardening process. When the hardening occurs close to the apex, the diameter of the tube decreases. When it takes place farther from the apex, the innate tendency of the tip tissue to expand causes a widening of the skeleton tube. An oscillation of the position at which hardening takes place causes the formation of annuli.  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