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   77396篇
  免费   6504篇
  国内免费   36篇
  83936篇
  2023年   387篇
  2022年   462篇
  2021年   967篇
  2020年   776篇
  2019年   835篇
  2018年   1335篇
  2017年   1193篇
  2016年   2208篇
  2015年   3603篇
  2014年   3669篇
  2013年   4865篇
  2012年   6190篇
  2011年   5788篇
  2010年   3463篇
  2009年   2841篇
  2008年   4708篇
  2007年   4646篇
  2006年   4464篇
  2005年   4187篇
  2004年   4155篇
  2003年   3884篇
  2002年   3741篇
  2001年   1199篇
  2000年   1209篇
  1999年   930篇
  1998年   837篇
  1997年   593篇
  1996年   558篇
  1995年   563篇
  1994年   490篇
  1993年   470篇
  1992年   538篇
  1991年   460篇
  1990年   389篇
  1989年   375篇
  1988年   358篇
  1987年   306篇
  1986年   299篇
  1985年   400篇
  1984年   430篇
  1983年   337篇
  1982年   439篇
  1981年   336篇
  1980年   320篇
  1979年   245篇
  1978年   269篇
  1977年   256篇
  1976年   254篇
  1974年   233篇
  1973年   239篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
131.
This article seeks to explain the disjuncture between the decline of music education in schools and the importance music has in popular youth culture and in creativity within the new knowledge economy. The data discussed in this article have been derived from analyses of major documents on curriculum reform as well as e-mail responses from music educators around the world. This analysis reveals that these music educators agree with the perception of a demise in music education around the world, as well as the significance of the disjuncture alluded to. The four major reasons found for the decline in music education are: (1) the model of curriculum supported in educational reforms; (2) an emphasis on standardized evaluation; (3) less resources available; and (4) a wrong approach on music advocacy.  相似文献   
132.
To measure fatigue in dental implants and in its components, it is necessary to use a probabilistic analysis since the randomness in the output depends on a number of parameters (such as fatigue properties of titanium and applied loads, unknown beforehand as they depend on mastication habits). The purpose is to apply a probabilistic approximation in order to predict fatigue life, taking into account the randomness of variables. More accuracy on the results has been obtained by taking into account different load blocks with different amplitudes, as happens with bite forces during the day and allowing us to know how effects have different type of bruxism on the piece analysed.  相似文献   
133.
134.
135.
A simple three-day technique is described for preparing completely cleared and high quality alizarin stained total skeletons of adult mice. Unfixed specimens are partially macerated during staining. Older specimens are heated for 15 min in 1% KOH. A heated solution of benzyl and ethyl alcohol, glycerin, and water is used for final clearing and hardening. This procedure requires about 10 min work per specimen and greatly simplifies preparation of stained and cleared skeletons of adult mice. Another technique, giving slightly better preparations, but requiring 11-14 days, is also described.  相似文献   
136.
Guaraná (Paullinia cupana) is a woody vine or sprawling shrub native to the central Amazon Basin. The seeds are commercially produced on some 6,000 ha in the state of Amazonas near Manaus. The principal article of commerce is an amber-colored, carbonated soft drink. It is also widely used as a high caffeine stimulant and in local medicines. The plant is monoecious and is damaged by a number of diseases, the most severe being anthracnose. Prospects are excellent for greatly expanded international markets.  相似文献   
137.
Following the programme started at Janssen Research Foundation searching for 5-HT(2A/2C) antagonists, we now report on the synthesis of a series of substituted 2-(Dimethylaminomethyl)-2,3,3a,8-tetrahydrodibenzo[c,f]isoxazolo[2,3-a]azepine derivatives. The 5-HT(2A), 5-HT(2C) and H(1) receptor affinities as well as the mCPP antagonistic activity of the compounds synthesised is described.  相似文献   
138.
The genetic basis of the effects of ultraviolet B(UVB) radiation on the induction of contact hypersensitivity (CH) to dinitrofluorobenzene (DNFB) has been explored in genetically defined mice. It was found that acute, low-dose UVB radiation produced profound depletion of epidermal Langerhans cells (LC) at UVB-treated sites in all strains of mice tested. However, when DNFB was applied to UVB radiation sites, unresponsiveness developed in some strains of mice, but vigorous contact hypersensitivity was induced in others. The UVB-susceptible phenotype proved dominant or codominant in F1 hybrids derived from parental strains of the susceptible and UVB-resistant phenotypes. Experiments conducted in one set of F1 hybrids derived from two UVB-susceptible parental strains displayed UVB resistance, suggesting gene complementation, and showed that more than one genetic locus was involved. Segregant backcross populations, analyzed for the capacity to develop CH after UVB treatment and skin painting with DNFB, revealed that at least two, and probably three, independent genetic loci participate in determining UVB resistance. Results of experiments with H-2 congenic and recombinant mice derived from the B10 background implicated class I genes of the major histocompatibility complex as relevant genetic factors. These results indicate that there is a dissociation between the effects of UVB radiation on epidermal Langerhans cells and the capacity of a cutaneous surface to support the induction of contact hypersensitivity. The data indicate that the induction of CH to haptens is dependent on normal numbers of functional LC at the skin painting site only in some strains of mice. The data imply that in the so-called UVB-resistant strains of mice, alternative (non-Langerhans cell-dependent) mechanisms allow for the induction of CH. Several independent genetic loci, one of which appears to be H-2, govern this UVB-related effect.  相似文献   
139.
Congenic anti-Lyt-3.1 sera have recently been produced by immunizing B6-Lyt-2a mice with thymocytes from either B6-Lyt-2a, Lyt-3a or B6-Lyt-2a, Lyt-3a, H-2k mice (Boos et al. 1978). Surprisingly, mice of the congenic strain B6 failed to produce either anti-Lyt-2.1 or anti-Lyt-3.1 cytotoxic antibodies after identical immunizations. To determine the genetic basis for the difference in response to Lyt-3.1, (B6 × B6-Lyt-2a)Fa mice and progeny of the backcross, (B6 × B6-Lyt-2a)F1 × B6-Lyt-2a, were immunized with B6-Lyt-2a, Lyt-3a, H-2k thymocytes. In addition, thymic biopsies of backcross progeny were performed and thymocytes tested for the Lyt-2.2 antigenic specificity. Results indicate that gene(s) governing the immune response to Lyt-3.1 is (are) linked to theLyt-2 locus, and that the responder allele (linked toLyt-2 a ) shows very poor penetrance in Lyt-2a/Lyt-2b mice.  相似文献   
140.
We compared changes in body condition (relative weight) and mercury concentrations ([Hg]) in two species of coregonid fish (lake herring Coregonus artedi, lake whitefish C. clupeaformis) among discrete populations in Ontario between 1967 and 2006. Temporal comparisons among populations were made to determine whether 1) the establishment of Bythotrephes longimanus had affected coregonid populations, or 2) if changes in body condition or [Hg] were related to regional differences in the degree of climate change during the time period investigated. Climate data from northwestern, northeastern and southern Ontario showed a general warming trend in all regions over the period of study. However, greater temporal changes in climate were observed in the northwest where growing degree days >5°C (GDD) increased and precipitation declined over the study period compared with relatively little change in southern or northeastern Ontario. Correspondingly, northwestern Ontario coregonid populations demonstrated significantly greater declines in body condition relative to those from northeastern or southern Ontario. Declines in [Hg] of both species were also greater among northwestern populations compared with those from northeastern or southern Ontario but only significantly so for lake herring. These declines were independent of the invasion of non‐native Bythotrephes, and declines in [Hg] were opposite predictions based on the hypothesis that Bythotrephes invasion lengthened aquatic food chains. Based on our findings and further evidence from the literature, we propose that warming regional climates are capable of contributing to declines in both condition and [Hg] of fishes. Because fish condition affects both reproductive success and overwinter survival, observed condition declines of the magnitude reported here could have profound implications for the structure of future aquatic ecosystems in a warming climate.  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