全文获取类型
收费全文 | 2253篇 |
免费 | 21篇 |
国内免费 | 4篇 |
专业分类
系统科学 | 51篇 |
丛书文集 | 2篇 |
理论与方法论 | 25篇 |
现状及发展 | 1401篇 |
研究方法 | 95篇 |
综合类 | 623篇 |
自然研究 | 81篇 |
出版年
2018年 | 37篇 |
2017年 | 30篇 |
2016年 | 44篇 |
2015年 | 35篇 |
2014年 | 21篇 |
2013年 | 31篇 |
2012年 | 65篇 |
2011年 | 110篇 |
2010年 | 35篇 |
2009年 | 22篇 |
2008年 | 58篇 |
2007年 | 51篇 |
2006年 | 63篇 |
2005年 | 75篇 |
2004年 | 58篇 |
2003年 | 67篇 |
2002年 | 53篇 |
2001年 | 42篇 |
2000年 | 32篇 |
1999年 | 26篇 |
1992年 | 21篇 |
1991年 | 23篇 |
1990年 | 18篇 |
1985年 | 20篇 |
1984年 | 30篇 |
1983年 | 17篇 |
1982年 | 20篇 |
1981年 | 24篇 |
1980年 | 19篇 |
1979年 | 37篇 |
1978年 | 34篇 |
1977年 | 47篇 |
1976年 | 40篇 |
1975年 | 30篇 |
1974年 | 41篇 |
1973年 | 51篇 |
1972年 | 51篇 |
1971年 | 48篇 |
1970年 | 71篇 |
1969年 | 60篇 |
1968年 | 88篇 |
1967年 | 76篇 |
1966年 | 57篇 |
1965年 | 49篇 |
1964年 | 47篇 |
1963年 | 24篇 |
1962年 | 24篇 |
1961年 | 18篇 |
1959年 | 16篇 |
1957年 | 15篇 |
排序方式: 共有2278条查询结果,搜索用时 0 毫秒
181.
Summary The effects of dosage and of cadmium pretreatment on the binding of cadmium in rat bile were studied. With increasing dose a higher cumulative biliary excretion of Cd was observed and a higher percentage of the Cd was excreted in a low-molecular-weight form. On the other hand, after cadmium pretreatment, a decrease in the cumulative biliary excretion of cadmium was observed but a greater percentage of that excreted into the bile was bound to high molecular weight compounds. 相似文献
182.
Hjorth J Watson D Fynbo JP Price PA Jensen BL Jørgensen UG Kubas D Gorosabel J Jakobsson P Sollerman J Pedersen K Kouveliotou C 《Nature》2005,437(7060):859-861
It has long been known that there are two classes of gamma-ray bursts (GRBs), mainly distinguished by their durations. The breakthrough in our understanding of long-duration GRBs (those lasting more than approximately 2 s), which ultimately linked them with energetic type Ic supernovae, came from the discovery of their long-lived X-ray and optical 'afterglows', when precise and rapid localizations of the sources could finally be obtained. X-ray localizations have recently become available for short (duration <2 s) GRBs, which have evaded optical detection for more than 30 years. Here we report the first discovery of transient optical emission (R-band magnitude approximately 23) associated with a short burst: GRB 050709. The optical afterglow was localized with subarcsecond accuracy, and lies in the outskirts of a blue dwarf galaxy. The optical and X-ray afterglow properties 34 h after the GRB are reminiscent of the afterglows of long GRBs, which are attributable to synchrotron emission from ultrarelativistic ejecta. We did not, however, detect a supernova, as found in most nearby long GRB afterglows, which suggests a different origin for the short GRBs. 相似文献
183.
José M. Bravo-San Pedro Mireia Niso-Santano Rubén Gómez-Sánchez Elisa Pizarro-Estrella Ana Aiastui-Pujana Ana Gorostidi Vicente Climent Rakel López de Maturana Rosario Sanchez-Pernaute Adolfo López de Munain José M. Fuentes Rosa A. González-Polo 《Cellular and molecular life sciences : CMLS》2013,70(1):121-136
Mutations in leucine-rich repeat kinase 2 (LRRK2) are a major cause of familial Parkinsonism, and the G2019S mutation of LRRK2 is one of the most prevalent mutations. The deregulation of autophagic processes in nerve cells is thought to be a possible cause of Parkinson’s disease (PD). In this study, we observed that G2019S mutant fibroblasts exhibited higher autophagic activity levels than control fibroblasts. Elevated levels of autophagic activity can trigger cell death, and in our study, G2019S mutant cells exhibited increased apoptosis hallmarks compared to control cells. LRRK2 is able to induce the phosphorylation of MAPK/ERK kinases (MEK). The use of 1,4-diamino-2,3-dicyano-1,4-bis[2-aminophenylthio]butadiene (U0126), a highly selective inhibitor of MEK1/2, reduced the enhanced autophagy and sensibility observed in G2019S LRRK2 mutation cells. These data suggest that the G2019S mutation induces autophagy via MEK/ERK pathway and that the inhibition of this exacerbated autophagy reduces the sensitivity observed in G2019S mutant cells. 相似文献
184.
Stockert J. C. Sogo J. M. Díez J. L. Giménez-Martín G. 《Cellular and molecular life sciences : CMLS》1969,25(7):773-774
Resumen Se describe el hallazgo de un bulbo deAllium cepa en el cual los dos nucleolos de cada nÚcleo muestran una clara diferencia de tamaño. El nucleolo menor posee aproximadamente la mitad del diámetro del nucleolo normal. Esta alteración de la morfología nucleolar se correlaciona con la ausencia de satélite en uno de los dos cromosomas normalmente satelizados.
The work reported in this paper was undertaken during the tenure of a Research Training Fellowship awarded by the International Agency for Research on Cancer. 相似文献
The work reported in this paper was undertaken during the tenure of a Research Training Fellowship awarded by the International Agency for Research on Cancer. 相似文献
185.
N. -E. Andén B.-E. B. -E. Roos B. Werdinius 《Cellular and molecular life sciences : CMLS》1963,19(7):359-360
Zusammenfassung Es wird papierchromatographisch gezeigt, dass Cerebrospinalflüssigkeit gesunder Menschen sowohl Homovanillinsäure als auch 3-Methoxy-4-hydroxymandelsäure enthält. Die Konzentration der ersteren ist etwa 75 ng/ml und die der letzteren etwa 25 ng/ml.
Acknowledgment. This work has been supported by grants from Leo Ltd., Hälsingborg, and from the Medical Faculty, University of Göteborg (Sweden). For technical assistance we are indebted to Mrs.I. Olofson. 相似文献
Acknowledgment. This work has been supported by grants from Leo Ltd., Hälsingborg, and from the Medical Faculty, University of Göteborg (Sweden). For technical assistance we are indebted to Mrs.I. Olofson. 相似文献
186.
Effects of butyrate and insulin and their interaction on the DNA synthesis of rumen epithelial cells in culture 总被引:1,自引:0,他引:1
Summary Rumen epithelial cells (REC) were incubated in the presence of various concentrations of butyrate or insulin or with both of them, to obtain information on their effect on the DNA synthesis of cultured cells. The 24-h values of3H-thymidine incorporation into cellular DNA were measured in the presence of butyrate, insulin or butyrate plus insulin. While butyrate reduced DNA synthesis, insulin produced an increase over the control. Combined butyrate plus insulin treatment influenced the incorporation of label in accordance with the relative proportion of these two substances. 相似文献
187.
Determination of surface topography of biological specimens at high resolution by scanning tunnelling microscopy 总被引:1,自引:0,他引:1
A M Baró R Miranda J Alamán N García G Binnig H Rohrer C Gerber J L Carrascosa 《Nature》1985,315(6016):253-254
Although techniques are available for the determination of the three-dimensional structure of biological specimens, for example scanning electron microscopy, they all have some serious drawback, such as low resolution, the requirement for crystals or for the sample to be analysed in a high vacuum. In an attempt to develop a technique for high-resolution three-dimensional structure analysis of non-crystalline biological material, we have tested the applicability of scanning tunnelling microscopy (STM), a method that has been used successfully in the analysis of metal and semiconductor surface structures. We report here that scanning tunnelling electron microscopy can be used to determine the surface topography of biological specimens at atmospheric pressure and room temperature, giving a vertical resolution of the order of 1 A. Our results show that quantum mechanical tunnelling of electrons through biological material is possible provided that the specimen is deposited on a conducting surface. 相似文献
188.
Aging alters resynchronization of the circadian system in rats after a shift of the light-dark cycle
Four days following an 8-h advance of the light-dark cycle, the circadian rhythms in the pineal N-acetyltransferase activity and melatonin content reappeared in 7-week-old rats, but were still abolished in 24-month-old animals. 相似文献
189.
This paper analyzes the network of passes among the players of the Spanish team during the last FIFA World Cup 2010, where they emerged as the champion, with the objective of explaining the results obtained from the behavior at the complex network level. The team is considered a network with players as nodes and passes as (directed) edges. A temporal analysis of the resulting passes network is also done, looking at the number of passes, length of the chain of passes, and to network measures such as player centrality and clustering coefficient. Results of the last three matches (the decisive ones) indicate that the clustering coefficient of the pass network remains high, indicating the elaborate style of the Spanish team. The effectiveness of the opposing team in negating the Spanish game is reflected in the change of several network measures over time, most importantly in drops of the clustering coefficient and passing length/speed, as well as in their being able in removing the most talented players from the central positions of the network. Spain’s ability to restore their combinative game and move the focus of the game to offensive positions and talented players is shown to tilt the balance in favor of the Spanish team. 相似文献
190.
Samuel Sarmiento Juan Manuel García-Manso Juan Manuel Martín-González Diana Vaamonde Javier Calderón Marzo Edir Da Silva-Grigoletto 《系统科学与复杂性》2013,26(1):104-116
The aim of this paper is to describe and analyse the behaviour of heart rate variability (HRV) during constant-load, high-intensity exercise using a time frequency analysis (Wavelet Transform). Eleven elite cyclists took part in the study (age: 18.6±3.0 years; VO2max: 4.88±0.61 litres·min?1). Initially, all subjects performed an incremental cycloergometer test to determine load power in a constant load-test (379.55±36.02 W; 89.0%). HRV declined dramatically from the start of testing (p <0.05). The behaviour of power spectral density within the LF band mirrored that of total energy, recording a significant decrease from the outset LF peaks fell rapidly thereafter, remaining stable until the end of the test. HF-VHF fell sharply in the first 20 to 30 seconds. The relative weighting (%) of HF-VHF was inverted with the onset of fatigue, [1.6% at the start, 7.1 (p <0.05) at the end of the first phase, and 43.1% (p <0.05) at the end of the test]. HF-VHFpeak displayed three phases: a moderate initial increase, followed by a slight fall, thereafter increasing to the end of the test. The LF/HF-VHF ratio increased at the start, later falling progressively until the end of the first phase and remaining around minimal values until the end of the test. 相似文献