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   131875篇
  免费   3660篇
  国内免费   829篇
  2023年   151篇
  2022年   388篇
  2021年   753篇
  2020年   493篇
  2019年   616篇
  2018年   12441篇
  2017年   11180篇
  2016年   8507篇
  2015年   2402篇
  2014年   2287篇
  2013年   2686篇
  2012年   7316篇
  2011年   15766篇
  2010年   13761篇
  2009年   9960篇
  2008年   12214篇
  2007年   13778篇
  2006年   2676篇
  2005年   2757篇
  2004年   3181篇
  2003年   3029篇
  2002年   2751篇
  2001年   744篇
  2000年   513篇
  1999年   494篇
  1998年   522篇
  1997年   385篇
  1996年   333篇
  1995年   306篇
  1994年   256篇
  1993年   296篇
  1992年   280篇
  1991年   231篇
  1990年   181篇
  1989年   193篇
  1988年   186篇
  1987年   163篇
  1986年   116篇
  1985年   155篇
  1984年   183篇
  1983年   141篇
  1982年   142篇
  1981年   119篇
  1980年   102篇
  1979年   62篇
  1978年   76篇
  1977年   70篇
  1976年   49篇
  1972年   274篇
  1971年   299篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Cholecalciferol administration to vitamin D-deficient chicks produces, 24 h after treatment, a specific increase of the phosphatidylcholine content in the intestinal mitochondrial inner membrane plus matrix fraction without changes in its proportion in the outer membrane. The ratio of unsaturated/saturated fatty acids in the outer membrane phosphatidylcholine was increased by that treatment. The inner membrane plus matrix presents a decrease of 16:1 in phosphatidylethanolamine and 18:0 in the phosphatidylcholine fraction. Cardiolipin shows the largest change in the ratio of unsaturated/saturated fatty acids predominantly by an increase in the linoleic acid. The present data suggest that phosphatidylcholine and fatty acids modifications in both mitochondrial subfractions caused by vitamin D3 might have some role in the intestinal mitochondrial Ca transport.  相似文献   
992.
Fingerprints of South American monkeys of the genera Alouatta, Ateles and Cebus were studied. Dermatoglyphics were analysed in relation to pattern intensities; the distribution of symmetric and asymmetric designs was also determined. Results have been related to some aspects of the contribution of dermatoglyphics to the study of Primate morphological evolution.  相似文献   
993.
Based on regional stakeholder preferences and planning guidelines as allocation criteria for SRC, this study aims at providing a transparent approach to evaluate multiple environmental effects and the regional significance of SRC systems. Using the example of two poplar SRC-systems (4-year rotation, 9-year rotation) the potential effects on ground water supply, wind erosion, and biodiversity aspects are evaluated in comparison to arable land for two selected municipalities in the district of Uelzen, Germany. Building on fuzzy membership functions and simple fuzzy-logic rules, the qualitative multi-criteria assessment is transparent and easily to adapt. This approach is transferable to other regions and spatial levels, since it derives from commonly available data and scientific evidence. Results show that implementation of SRC could provide multiple beneficial environmental effects, especially in areas with low landscape heterogeneity. The tools provided allow for a multi-criteria evaluation of environmental effects, and reveal the sensitivity to distinct allocation patterns. Physiographical conditions of the study area implicate a preference for mini-SRC systems. This is supported by smaller decline of annual deep percolation water compared to maxi-SRC. On average, decline in groundwater recharge of mini-SRC (92mm a−1) is comparable to irrigated arable land (80mm a−1), which is common practice in the study area. Currently, the utilization of beneficial environmental SRC effects is quite limited, since only 3 % of arable land is suitable for SRC implementation regarding farmers’ preferences for SRC allocation. Allocation preferences could however change substantially with increasing incentives for SRC, e.g., due to regional bioenergy schemes or “Greening” initiatives within the European Common Agricultural Policy, which is to be reformed by 2013.  相似文献   
994.
Beginning in the 1960s the first systematic projects dedicated to testing whether great apes could acquire some aspects of human language were conducted. The ape subjects demonstrated remarkable capacities to learn and use elements of either sign language or an artificial language. The results from research across several laboratories drew a mixture of excitement and skepticism, and critiques and debates have ensued since the earliest reports were published. This continues today. Terrace (2019, Nim: A chimpanzee who learned sign language. New York, NY: Columbia University Press) repeats many of the same points made decades earlier, and has added some additional critiques. That scientists hold different perspectives on what to conclude from ape language studies is expected. However, any conclusion one draws should be based upon available evidence, which we outline in this review. We also address the critiques offered by Terrace (2019), including the stance that apes cannot understand or use words. Focusing on symbol use by chimpanzees and bonobos we describe evidence that argues for understanding of words, including capacities for declarative communication and intersubjectivity found in these apes. We conclude that the many decades of research using a variety of symbol systems challenges the absolutist position that chimpanzees and bonobos cannot learn language or understand the concept of a word.  相似文献   
995.

Background  

Infection with severe malaria in African children is associated with not only a high mortality but also a high risk of cognitive deficits. There is evidence that interventions done a few years after the illness are effective but nothing is known about those done immediately after the illness. We designed a study in which children who had suffered from severe malaria three months earlier were enrolled into a cognitive intervention program and assessed for the immediate benefit in cognitive, academic and behavioral outcomes.  相似文献   
996.
997.
A series of derivatives of the amine of 9-dihydro-9-O-ethylamino-N-desmethyl-N-isopropyl erythromycin A derivatives were synthesized as motilin agonists. The compounds were developed for potency without showing antibacterial activity and inhibition of the hERG potassium channel. The formamide of the amide series was found to show the optimal combination of properties relative to carbamates, ureas, thioureas, and amines. This prompted an investigation of heterocyclic isosteres for the amide. In this series the triazole had the optimal combination of properties. From the study, two compounds met the criteria for detailed pharmacokinetic studies.  相似文献   
998.
999.
Biomechanics and Modeling in Mechanobiology - One of only a few approved and available anabolic treatments for severe osteoporosis is daily injections of PTH (1-34). This drug has a specific dual...  相似文献   
1000.
Genetic analysis for germline mutations of RET proto‐oncogene has provided a basis for individual management of medullary thyroid carcinoma (MTC) and pheochromocytoma. Most of compound mutations have more aggressive phenotypes than single point mutations, but the compound C634Y/V292M variant in MTC has never been reported. Thus, we retrospectively investigated synergistic effect of C634Y and V292M RET germline mutations in family members with multiple endocrine neoplasia type 2A. Nine of 14 family members in a northern Chinese family underwent RET mutation screening using next‐generation sequencing and PCR followed by direct bidirectional DNA sequencing. Clinical features of nine individuals were retrospectively carefully reviewed. In vitro, the scratch‐wound assay was used to investigate the difference between the cells carrying different mutations. We find no patients died of MTC. All 3 carriers of the V292M variant were asymptomatic and did not have biochemical or structural evidence of disease (age: 82, 62 and 58). Among 4 C634Y mutation carriers, 2 patients had elevated calcitonin with the highest (156 pg/mL) in an 87‐year‐old male. Two carriers of compound C634Y/V292M trans variant had bilateral MTC with pheochromocytoma or lymph node metastasis (age: 54 and 41 years, respectively). Further, the compound C634Y/V292M variant had a faster migration rate than either single point mutation in vitro (P < .05). In conclusion, the V292M RET variant could be classified as ‘likely benign’ according to ACMG (2015). The compound variant V292M/C634Y was associated with both more aggressive clinical phenotype and faster cell growth in vitro than was either single mutation.  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