全文获取类型
收费全文 | 9160篇 |
免费 | 472篇 |
国内免费 | 6篇 |
专业分类
9638篇 |
出版年
2022年 | 46篇 |
2021年 | 83篇 |
2020年 | 45篇 |
2019年 | 54篇 |
2018年 | 100篇 |
2017年 | 84篇 |
2016年 | 157篇 |
2015年 | 224篇 |
2014年 | 253篇 |
2013年 | 870篇 |
2012年 | 455篇 |
2011年 | 547篇 |
2010年 | 325篇 |
2009年 | 302篇 |
2008年 | 526篇 |
2007年 | 546篇 |
2006年 | 545篇 |
2005年 | 549篇 |
2004年 | 514篇 |
2003年 | 495篇 |
2002年 | 566篇 |
2001年 | 140篇 |
2000年 | 125篇 |
1999年 | 128篇 |
1998年 | 152篇 |
1997年 | 116篇 |
1996年 | 106篇 |
1995年 | 101篇 |
1994年 | 92篇 |
1993年 | 129篇 |
1992年 | 117篇 |
1991年 | 78篇 |
1990年 | 62篇 |
1989年 | 80篇 |
1988年 | 65篇 |
1987年 | 62篇 |
1986年 | 56篇 |
1985年 | 58篇 |
1984年 | 79篇 |
1983年 | 52篇 |
1982年 | 76篇 |
1981年 | 56篇 |
1980年 | 64篇 |
1979年 | 40篇 |
1978年 | 45篇 |
1977年 | 29篇 |
1976年 | 37篇 |
1975年 | 25篇 |
1973年 | 32篇 |
1972年 | 23篇 |
排序方式: 共有9638条查询结果,搜索用时 7 毫秒
101.
102.
103.
Summary Paternal age distribution for 1279 cases of Down's syndrome born in 1952–1968 was compared with the corresponding distribution for the general population, corrected for the maternal age as well as for the year of birth of the patients. Although there was no difference in the mean paternal age, the two distributions differed significantly, largely due to the excess of fathers aged 55 years and over and to the deficit of those aged 40–44 years in the patients born to mothers aged 30 years and over. The overall pattern of the relative incidence of Down's syndrome with advancing paternal age, with maternal age controlled, seems consistent with the hypothesis proposed by Stene et al. (1977). It increased from 0.8 for fathers aged 20–24 years slowly up to 1.2 for those aged 45–49 years, though with an intermediate drop to 0.8 at the age of 40–44 years, and then sharply to 2.4 for those aged 55 years and over. This rising pattern of the relative incidence with paternal age was essentially the same for the patients born in 1952–1960 and for those born in 1961–1968, although the slope was less steep in the latter than in the former group.This paper is dedicated to Professor Heinrich Schade in honor of his 70th birthday 相似文献
104.
Summary The transepithelial resistance, the cell membrane resistance and the ratio of resistances of the serosal (baso-lateral) to the mucosal (brush border) cell membrane were measured in rat duodenum, jejunum and ileum by means of microelectrode techniques. These measured values were not affected in the presence of actively transported solutes in the mucosal bathing fluid.Contribution of an electrical conductance through the extracellular shunt pathway to the total transepithelial conductance was quantitatively estimated using an electrically equivalent circuit analysis. These values estimated in respective tissues of small intestine were approx. 95% of the total transepithelial conductance, remaining unaffected by an active solute transport.From these data, the changes in emf's of the mucosal and serosal membrane induced byd-glucose or glycine were separately evaluated. 相似文献
105.
106.
Noyes' scheme of the elementary processes for the Belousov-Zhabotinsky reaction has been simplified on the basis of reaction kinetic consideration. The simplest possible analogue (model K) has been described by a set of kinetic equations, and it is solved to examine the varieties of new ordered phases, which include temporal rhythm and spatial pattern. Particular attention has been given to the onset of new phases, which is associated with an anomalous enhancement of fluctuations. A stochastic theory of fluctuations, which was developed in our previous work, has been applied to the present case. Theoretical results compare reasonably well with experimental findings, i.e. with (1) spatial periodic structure and (2) limit cycle behaviour. 相似文献
107.
Takamichi Ichinose Akira K. Suzuki Hirokazu Tsubone Masaru Sagai 《Life sciences》1982,31(18):1963-1972
Strain differences of mice in their susceptibility to nitrogen dioxide (NO2) were examined by measuring the activities of antioxidative protective enzymes, and the amounts of antioxidants and lipid peroxides in lungs. Four strains of mice: ICR, BALB/c, ddy and C57BL/6 were used in this study and their LC50 values after exposure to NO2 for 16 hr were: 38, 49, 51 and 64 ppm, respectively (1).Genetic strain differences were observed in the enzyme activities, the antioxidant contents and lipid peroxide contents among these four different strains. The activities of glutathione peroxidase (GPx), glutathione S-transferase, and superoxide dismutase (SOD), and the contents of non-protein sulfhydryls (NPSH), α-tocopherol (α-Toc) and total lipids in lungs of the four strains were related to their LC50, while TBA reactants in lungs of the four strains were inversely related to their LC50.After exposure to 20 ppm NO2 for 16 hr, the activities of the protective enzymes and the contents of NPSH decreased, while the level of α-Toc increased markedly. The activities of GPx, 6-phosphogluconate dehydrogenase, SOD and disulfide reductase, and the contents of NPSH, α-Toc and total lipids were also related to their LC50. On the other hand, TBA reactants increased higher than those of the control groups and were inversely related to their LC50.These results suggest that the protective enzymes and the antioxidants are important factors as defence mechanism in lungs to NO2 and that the intensity of the protective systems in pigmented strains is generally greater than that in albino strains. 相似文献
108.
Intraventricular administration of muscimol (25–100 ng) and intravenously applied aminooxyacetic acid (2.5–10 mg/kg) depressed the crossed extensor reflex response in a dose-dependent manner. The inhibitory effects of both drugs were clearly antagonized by a subconvulsive dose of bicuculline. A very small dose of bicuculline (10–40 μg/kg, i.v.) produced a dose-related enhancement of the crossed extensor reflex response without any sign of convulsion. These results suggest that the crossed extensor reflex response is very sensitive to GABAergic drugs and central GABAergic mechanisms play a role in the modulation of the crossed extensor reflex response. 相似文献
109.
When carrot explants were cultured with phytohormones, DNA synthesistook place synchronously in the explants and a satellite DNAwith a heavier density in CsCl than the bulk DNA replicatedin the earliest phase of the first replication period. The earlyreplicating carrot satellite consisted of a component havingan identical density to carrot rDNA and another component havinga density between the p-value of carrot rDNA and that of thebulk DNA. DNA-rRNA hybridization was used to explore the possibilitythat this early replication of the satellites leads to amplificationof rDNA in the explant cells, in which massive ribosome synthesisis known to occur. The results showed that there was neitheramplification nor underreplication of rRNA genes during callusformation and its growth. Experiments with explants of Jerusalem artichoke tuber, whichare well known as a synchronous replication system, showed thata component slightly heavier than the bulk DNA was synthesizedat the early phases of the first replication period. However,the density of this early replicating satellite differed fromthat of artichoke rDNA. DNA-rRNA hybridization experiments againshowed no gross changes of rDNA content during dedifferentiationof this plant system. (Received September 30, 1981; Accepted January 5, 1982) 相似文献
110.
T Yoshimura T Ozaki Y Shintani H Watanabe 《Archives of biochemistry and biophysics》1979,193(2):301-313
For the purpose of understanding the electron paramagnetic resonance (epr) spectral change of nitrosylhemoproteins under various conditions, the epr spectra for the model system have been analyzed. The model system consists of the nitrogen oxide complex of the iron(II) protoporphyrin IX dimethyl ester and various imidazole derivatives (three hindered and six unhindered imidazole derivatives). The results of the analysis indicate the existence of two molecular species in the model system, which differ in structure of the FeNO unit. These observations were compared with those for the nitrosylhemoproteins. 相似文献