全文获取类型
收费全文 | 22848篇 |
免费 | 2912篇 |
国内免费 | 11篇 |
出版年
2022年 | 224篇 |
2021年 | 445篇 |
2020年 | 263篇 |
2019年 | 367篇 |
2018年 | 345篇 |
2017年 | 333篇 |
2016年 | 539篇 |
2015年 | 888篇 |
2014年 | 1023篇 |
2013年 | 1211篇 |
2012年 | 1450篇 |
2011年 | 1361篇 |
2010年 | 808篇 |
2009年 | 743篇 |
2008年 | 1004篇 |
2007年 | 981篇 |
2006年 | 863篇 |
2005年 | 794篇 |
2004年 | 787篇 |
2003年 | 709篇 |
2002年 | 646篇 |
2001年 | 572篇 |
2000年 | 539篇 |
1999年 | 471篇 |
1998年 | 259篇 |
1997年 | 251篇 |
1996年 | 222篇 |
1995年 | 234篇 |
1994年 | 219篇 |
1993年 | 192篇 |
1992年 | 377篇 |
1991年 | 349篇 |
1990年 | 373篇 |
1989年 | 320篇 |
1988年 | 310篇 |
1987年 | 315篇 |
1986年 | 302篇 |
1985年 | 320篇 |
1984年 | 259篇 |
1983年 | 231篇 |
1982年 | 202篇 |
1981年 | 202篇 |
1979年 | 278篇 |
1978年 | 228篇 |
1977年 | 188篇 |
1976年 | 175篇 |
1975年 | 181篇 |
1974年 | 196篇 |
1973年 | 170篇 |
1972年 | 176篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
Larisa R. G. DeSantis Jessica R. Scott Blaine W. Schubert Shelly L. Donohue Brian M. McCray Courtney A. Van Stolk Amanda A. Winburn Michael A. Greshko Mackie C. O’Hara 《PloS one》2013,8(8)
The analysis of dental microwear is commonly used by paleontologists and anthropologists to clarify the diets of extinct species, including herbivorous and carnivorous mammals. Currently, there are numerous methods employed to quantify dental microwear, varying in the types of microscopes used, magnifications, and the characterization of wear in both two dimensions and three dimensions. Results from dental microwear studies utilizing different methods are not directly comparable and human quantification of wear features (e.g., pits and scratches) introduces interobserver error, with higher error being produced by less experienced individuals. Dental microwear texture analysis (DMTA), which analyzes microwear features in three dimensions, alleviates some of the problems surrounding two-dimensional microwear methods by reducing observer bias. Here, we assess the accuracy and comparability within and between 2D and 3D dental microwear analyses in herbivorous and carnivorous mammals at the same magnification. Specifically, we compare observer-generated 2D microwear data from photosimulations of the identical scanned areas of DMTA in extant African bovids and carnivorans using a scanning white light confocal microscope at 100x magnification. Using this magnification, dental microwear features quantified in 2D were able to separate grazing and frugivorous bovids using scratch frequency; however, DMTA variables were better able to discriminate between disparate dietary niches in both carnivorous and herbivorous mammals. Further, results demonstrate significant interobserver differences in 2D microwear data, with the microwear index remaining the least variable between experienced observers, consistent with prior research. Overall, our results highlight the importance of reducing observer error and analyzing dental microwear in three dimensions in order to consistently interpret diets accurately. 相似文献
12.
13.
F Brown 《BMJ (Clinical research ed.)》1987,295(6594):347-348
14.
15.
16.
17.
Effects of phorbol dibutyrate on M currents and M current inhibition in bullfrog sympathetic neurons 总被引:8,自引:0,他引:8
1. Effects of bath-applied phorbol dibutyrate (PDBu) on M currents (IM) and on the inhibition of IM by muscarine and luteinizing hormone-releasing hormone (LHRH) were recorded in voltage-clamped bullfrog lumbar sympathetic ganglion cells. 2. PDBu (0.1-30 microM) produced a slowly developing, irreversible and partial (less than or equal to 60%) inhibition of IM. This effect was not replicated by 4-alpha-phorbol or by vehicle. 3. After treatment with PDBu, residual IM showed a reduced sensitivity to inhibition by muscarine or LHRH but not by Ba2+. The reduced response to muscarine appeared to result from a 10-fold shift in the concentration dependence for inhibition. 4. PDBu did not clearly reproduce the ability of muscarine to inhibit the slow, Ca-activated K current IAHP or to increase the leak conductance at hyperpolarized potentials. The latter effect of muscarine was enhanced, rather than inhibited, by PDBu. 5. IM and IAHP were not inhibited by 1 mM dibutyryl cyclic AMP or by 20 microM forskolin. 6. It is concluded that activation of protein kinase C, but not protein kinase A, partly replicates the effect of muscarine on frog sympathetic neurons. 相似文献
18.
A phosphotransferase-dependent aryl-β-glucoside uptake and utilisation system (abg) was isolated from the ruminal Clostridium (“C. longisporum”). The system is composed of three genes, abgG, abgF and abgA, and a number of regulatory regions, including terminator/antiterminator type stem-loop structures preceding the abgG and abgF genes. Similarity analysis of the proteins encoded by these genes indicated that they were responsible for the regulation
of the abg system through antitermination (AbgG), the uptake and phosphorylation of aryl-β-glucosides (AbgF) and the hydrolysis of the
intracellular phosphorylated glycosides (AbgA). Experimental evidence for the functions of AbgF and AbgA was obtained. Although
it was not possible to demonstrate any function for AbgG, a promoter 5′ to the abgG gene was identified which was responsible for expression of the downstream genes. The abg system is remarkably similar to operons from the gram negative Enterobacteriaceae, both in the coding and non-coding regulatory
regions.
Received: 3 April 1997 / Accepted: 8 September 1997 相似文献
19.
Simon Rosenstein Harry D. Brown 《Biochimica et Biophysica Acta (BBA)/General Subjects》1980,629(1):195-198
Comparative assays were made in a spectrophotometer and a microcalorimeter for the reaction between acetylcholinesterase (EC 3.1.1.7) and acetylthiocholine. The rate of light absorbance change and the rate of heat flow were measured from similar and simultaneous reactions in spectrophotometer and microcalorimeter, respectively. At the enzyme activity levels studied, i.e., 0.05–0.15 I.U. in calorimetry and 1–4 I.U. in spectrophotometry, the reaction rates were linear and showed first-order kinetics. A highly significant positive correlation was seen between the two methods (r = 0.997). More importantly, spectrophotometric assay with acetylthiocholine (which utilized a secondary reaction with chromagen, dithiobisnitrobenzoic acid) stood in highly significant positive correlation with calorimetric assays (which did not require a chromagen) either with the same substrate (r = 0.976) or with acetylcholine (r = 0.900). It appears that microcalorimetry can be used in preference to spectrophotometry for enzyme kinetic studies to overcome the complexity of reaction mixture and interference problems and with the advantage of using natural substrates. 相似文献
20.