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   5233篇
  免费   252篇
  2023年   39篇
  2020年   109篇
  2019年   111篇
  2018年   188篇
  2017年   193篇
  2016年   217篇
  2015年   138篇
  2014年   138篇
  2013年   1523篇
  2012年   167篇
  2011年   196篇
  2010年   178篇
  2009年   147篇
  2008年   119篇
  2007年   113篇
  2006年   123篇
  2005年   81篇
  2004年   59篇
  2003年   65篇
  2002年   71篇
  2001年   76篇
  2000年   67篇
  1999年   81篇
  1998年   62篇
  1997年   44篇
  1996年   36篇
  1995年   49篇
  1994年   35篇
  1993年   50篇
  1992年   33篇
  1991年   43篇
  1990年   47篇
  1989年   42篇
  1988年   33篇
  1987年   44篇
  1986年   47篇
  1985年   47篇
  1984年   52篇
  1983年   47篇
  1982年   41篇
  1981年   47篇
  1980年   42篇
  1979年   39篇
  1978年   36篇
  1977年   36篇
  1975年   27篇
  1974年   27篇
  1973年   29篇
  1972年   29篇
  1970年   27篇
排序方式: 共有5485条查询结果,搜索用时 109 毫秒
991.
The pursuit of "coordination" has been a prominent theme both in local responses to homelessness and in the current incentives in federal housing policy. This article reviews some of the conditions that make coordination important yet difficult to achieve in the homeless area. A variety of approaches for achieving coordination at the local level are reviewed, ranging from point-of-service coordination to system-design. The local examples are drawn from the cities participating in the evaluation of The Robert Wood Johnson and the Department of Housing and Urban Development Homeless Families Program. The questions of effectiveness and future prospects for homeless service coordination are raised.  相似文献   
992.
The biohistory of the human peopling of the Fuegian Archipelago and the processes of biocultural adaptation and microdifferentiation of the ethnohistorical ‘canoe’ (‘sea nomad’) and ‘foot’ natives are poorly known. Here we report the oldest human remains of a ‘sea nomad’ native discovered so far in the Beagle Channel region. The specimen consists of a deciduous molar from the site of Imiwaia I, dated to 5870 ± 145 years BP. The microwear pattern indicates the consumption of hard food processed by long chewing cycles. The histomorphometric analysis shows the presence of at least three episodes of stress between 8 and 10.5 months after birth, and an unusually thin neonatal line, suggesting a mechanically non‐stressing birth event. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   
993.
Analysis of a bundle burial of a young male recovered from 22OK905, a late prehistoric/protohistoric site located near Starkville, Mississippi, is discussed. AMS dating of the burial places it between AD 1640 and AD 1814, a time when Native American and European conflicts are well documented. One interesting finding is the presence of cut marks on the frontal bone of this individual. These marks were determined to be the result of scalping rather than defleshing marks associated with secondary burial treatment. Comparisons of bone element frequency among several bundle burials suggest that this individual died away from his village and the body was later collected for burial. A second study indicates that a stone tool may have been used to scalp the victim. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   
994.
The skeletal remains of a child aged 2.5–3.5 years, recovered during archaeological excavations at the churchyard of St Martin's Church, Birmingham, UK, were examined using gross observation, radiography and scanning electron microscopy. Lesions suggestive of the presence of rickets and of secondary hyperparathyroidism were found. This appears to be a first report of secondary hyperparathyroidism in response to rickets in a palaeopathological specimen. The potential of microscopic examination of bone for interpreting disease processes is emphasised. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   
995.
Recently, palaeopathologists have begun to diagnose brucellosis in skeletal remains from minor lytic lesions located on the anterior margins of one or some few vertebral bodies, taking these lesions to represent brucellar epiphysitis. However, review of the literature indicates that these lesions have been sporadically noted for some time by palaeopathologists and various different interpretations have been placed upon them. In view of this, a study of these vertebral marginal lesions (VML) was undertaken in a large series of archaeological skeletons from England, with the aim of characterising their morphology and frequency and shedding light upon their causes. In the study material, VML were found only in the lumbar spine, where they occurred with a prevalence of approximately 4%. It was argued that VML are consistent with two principal alternative diagnoses: brucellosis and traumatic anterior disc herniation. Differentiating these possibilities for the VML in the study material was difficult, but the balance of the evidence seemed to favour traumatic anterior disc herniation as the more likely cause. It is concluded that VML should not be used to identify the presence of brucellosis in skeletal remains unless there is further evidence, in the form of additional skeletal lesions or biomolecular evidence for residual brucella bacterial DNA, to support such a diagnosis. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   
996.
Differentiation of the genetic and the acquired anaemias, particularly in areas of the world where they may co‐exist, has been a challenge for palaeopathologists for over 100 years. In this paper we present macroscopic and radiographic skeletal lesions that are associated with the thalassemias in a 14‐year‐old girl from a modern reference collection of the University of Athens. This individual is of known sex, age, cause of death, place and dates of birth and death. The case is examined in terms of epidemiology, growth, distribution and severity of lesions and differential diagnosis. The entire skeleton is affected by marrow hyperplasia: lesions of the axial skeleton are extreme, and the appendicular skeleton is severely affected as well. The odontofacial manifestations that are diagnostic of thalassemia and differentiate it from other anaemias are present and include: maxillary and mandibular hyperplasia, reduced sinuses, displacement of maxillary dental structures, overbite, and generalised osteopenia. The development of extreme bone lesions and the ‘advanced’ age‐at‐death of this individual is explained as either the result of thalassemia major under a low transfusion regimen that was the norm during her lifetime, or to a form of thalassemia intermedia that allows survival to later life at the expense of gross skeletal alterations. The present status of skeletal studies in Greece does not support the identification of a genetic anaemia in past populations. The potential contribution of the current analysis in differentiating the anaemias in antiquity is evaluated. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   
997.
998.
999.
Mineral deposits in the Cupp‐Coutunn/Promeszutochnaya cave system (Turkmenia, central Asia) record a phase of hydrothermal activity within a pre‐existing karstic groundwater conduit system. Hydrothermal fluids entered the caves through fault zones and deposited sulphate, sulphide and carbonate minerals under phreatic conditions. Locally, intense alteration of limestone wall rocks also occurred at this stage. Elsewhere in the region, similar faults contain economic quantities of galena and elemental sulphur mineralization. Comparisons between the Pb and S isotope compositions of minerals found in cave and ore deposits confirm the link between economic mineralization and hydrothermal activity at Cupp‐Coutunn. The predominance of sulphate mineralization in Cupp‐Coutunn implies that the fluids were more oxidized in the higher permeability zone associated with the karst aquifer. A slight increase in the δ34S of sulphate minerals and a corresponding δ34S decrease in sulphides suggest that partial isotopic equilibration occurred during oxidation. Carbonate minerals indicate that the hydrothermal fluid was enriched in 18O (δ18OSMOW ~ + 10‰) relative to meteoric groundwater and seawater. Estimated values for δ13CDIC (δ13CPDB ~ ? 13‰) are consistent with compositions expected for dissolved inorganic carbon (DIC) derived from the products of thermal decomposition of organic matter and dissolution of marine carbonate. Values derived for δ13CDIC and δ18Owater indicate that the hydrothermal fluid was of basinal brine origin, generated by extensive water–rock interaction. Following the hydrothermal phase, speleothemic minerals were precipitated under vadose conditions. Speleothemic sulphates show a bimodal sulphur isotope distribution. One group has compositions similar to the hydrothermal sulphates, whilst the second group is characterized by higher δ34S values. This latter group may either record the effects of microbial sulphate reduction, or reflect the introduction of sulphate‐rich groundwater generated by the dissolution of overlying evaporites. Oxygen isotope compositions show that calcite speleothems were precipitated from nonthermal groundwater of meteoric origin. Carbonate speleothems are relatively enriched in 13C compared to most cave deposits, but can be explained by normal speleothem‐forming processes under thin, arid‐zone soils dominated by C4 vegetation. However, the presence of sulphate speleothems, with isotopic compositions indicative of the oxidation of hydrothermal sulphide, implies that CO2 derived by reaction of limestone with sulphuric acid (‘condensation corrosion’) contributed to the formation of 13C‐enriched speleothem deposits.  相似文献   
1000.
A recent advancement in petroleum geochemistry is to model fossil oil composition using microthermometric and volumetric data acquired from individual fluid inclusion analysis. Fourier transform infrared (FT‐IR) microspectroscopy can record compositional information related to gas (CH4 and CO2) and alkane contents of petroleum inclusions. In this study, a quantitative procedure for FT‐IR microspectrometry has been developed to obtain, from individual fluid inclusions, mol percentage concentrations of methane, alkanes and carbon dioxide as constraints to thermodynamic modelling. A petroleum inclusion in a sample from the Québec City Promontory nappe area was used as standard to record a reference spectrum of methane. The analytical procedure is based on the measurement of CH4/alkane and CH4/CO2 band area ratios. CH4/alkane infrared band area ratio is obtained after spectral subtraction of the reference methane spectrum. This area ratio, affected by absolute absorption intensities of methane, methyl and methylene, provides a molar CH4/alkane ratio. Methyl/methylene ratio (CH2/CH3) ratio is obtained following procedures established in previous work. CO2/CH4 concentration ratio is estimated from relative absolute absorption intensities. Application to natural inclusions from different environments shows good correlation between FT‐IR quantification and PIT (petroleum inclusion thermodynamic) modelling.  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