全文获取类型
收费全文 | 6438篇 |
免费 | 493篇 |
国内免费 | 3篇 |
专业分类
6934篇 |
出版年
2023年 | 23篇 |
2022年 | 48篇 |
2021年 | 74篇 |
2020年 | 45篇 |
2019年 | 59篇 |
2018年 | 83篇 |
2017年 | 78篇 |
2016年 | 139篇 |
2015年 | 251篇 |
2014年 | 285篇 |
2013年 | 336篇 |
2012年 | 456篇 |
2011年 | 451篇 |
2010年 | 277篇 |
2009年 | 249篇 |
2008年 | 329篇 |
2007年 | 383篇 |
2006年 | 399篇 |
2005年 | 376篇 |
2004年 | 319篇 |
2003年 | 326篇 |
2002年 | 285篇 |
2001年 | 81篇 |
2000年 | 86篇 |
1999年 | 92篇 |
1998年 | 117篇 |
1997年 | 81篇 |
1996年 | 45篇 |
1995年 | 59篇 |
1994年 | 54篇 |
1993年 | 55篇 |
1992年 | 60篇 |
1991年 | 51篇 |
1990年 | 37篇 |
1989年 | 34篇 |
1988年 | 46篇 |
1987年 | 34篇 |
1986年 | 33篇 |
1985年 | 34篇 |
1984年 | 32篇 |
1983年 | 32篇 |
1982年 | 35篇 |
1981年 | 29篇 |
1980年 | 16篇 |
1979年 | 27篇 |
1977年 | 24篇 |
1976年 | 15篇 |
1974年 | 30篇 |
1971年 | 15篇 |
1969年 | 17篇 |
排序方式: 共有6934条查询结果,搜索用时 15 毫秒
21.
Leonardo Dapporto Thomas Schmitt Roger Vila Stefano Scalercio Heinrich Biermann Vlad Dincă Severiano F. Gayubo José A. González Pietro Lo Cascio Roger L. H. Dennis 《Journal of Biogeography》2011,38(5):854-867
Aim Our aims were to verify the existence of phylogenetic disequilibrium between butterfly lineages at the subcontinental scale for islands and the nearest mainland and to test the capacity of islands for hosting ancestral populations of butterflies and the significance of such relict populations. Location The western Mediterranean continental area of Europe and North Africa together with several large and small islands (Balearics, Tuscan Archipelago, Aeolian Archipelago, Capri, Sardinia, Sicily, Corsica). Methods Using geometric morphometrics, the shape of male genitalia was analysed in two common butterflies (Pyronia cecilia and Pyronia tithonus), whose spatial heterogeneity in the Mediterranean region has recently been described. Observed patterns in genital shapes were compared with shapes predicted for islands and fossil islands to assess the contribution of historical and current events in accounting for the transition from a refugial model to an equilibrium model. Measurements were taken for 473 specimens in 90 insular and mainland sites. Results The shape of the genitalia of populations of most islands differed substantially from that predicted by the equilibrium hypothesis while closely fitting the refugial hypothesis. The comparison between different models strongly suggests that islands maintain ancestral lineages similar to those living in Spain (P. cecilia) and France (P. tithonus). A high correlation between observed and predicted patterns on islands and fossil islands occurs during the first steps of modelled introgressive hybridization while the following steps exposed a successively lower fit, suggesting that the process from a refugial to an equilibrium situation is highly skewed towards an earlier non‐equilibrium. Main conclusions The observed non‐equilibrium pattern supports the refugial hypothesis, suggesting that an ancestral lineage was originally distributed from Spain to Italy, and also occupied offshore islands. This lineage, replaced in Italy, has persisted on the islands owing to their isolation. A comparison of the distribution patterns for genetic and morphometric markers in several species indicates that the situation highlighted for Pyronia may represent a common biogeographic feature for many Mediterranean butterflies. 相似文献
22.
23.
24.
Prostate carcinomas are among the most frequently diagnosed and death causing cancers affecting males in the developed world. It has become clear that the molecular mechanisms that drive the differentiation of normal prostate cells towards neoplasia involve multiple signal transduction cascades that often overlap and interact. A critical mediator of cellular proliferation and differentiation in various cells (and cancers) is the cAMP-dependent protein kinase, also known as protein kinase A (PKA), and its activating secondary messenger, cAMP. PKA and cAMP have been shown to play critical roles in prostate carcinogenesis and are the subject of this review. In particular we will focus on the cross-talk between PKA/cAMP signaling and that of the androgen receptor. 相似文献
25.
We investigate multilayered helical packaging of double-stranded DNA, or of a general polymer chain with persistence length lb, into an ideal, inert cylindrical container, reaching densities slightly below close packaging. We calculate the free energy as a function of the packaged length, based on the energies for bending, twisting, the suffered entropy loss, and the electrostatic energy in a Debye–Hückel model. In the absence of charges on the packaged polymer, a critical packaging force can be determined, similar to the mechanism involved in DNA unzipping models. When charges are taken into consideration, in the final packaging state the charges which are chemically distant become geometrically close, and therefore a steep rise is seen in the free energy. We argue that due to the extremely ordered and almost closely packaged final state the actual packaging geometry does not influence the behaviour of the free energy, pointing towards a certain universality of this state of the polymer. Our findings are compared to a recent simulations study, showing that the model is sensitive to the screening length. 相似文献
26.
Cohen DM Guthrie PH Gao X Sakai R Taegtmeyer H 《American journal of physiology. Endocrinology and metabolism》2003,285(6):E1312-E1316
To what extent does glutamine turnover keep pace with oxidative metabolism in the rat heart? To address this question, the following groups of substrates were presented to the isolated, working rat heart: 1) glucose (5 mM), insulin (40 microU/ml), and [2-13C]acetate (5 mM; high workload, n = 5); 2) pyruvate (2.5 mM) and [2-13C]acetate (5 mM; normal workload, n = 5); or 3) propionate (1 mM) and [2-13C]acetate (2.5 mM; normal workload, n = 3). In a subset of these experiments, the exchange of glutamate and glutamine was quantified by separation with ion exchange chromatography and analysis by GC-MS. There was an apparent equilibration of mass isotopomers of glutamate and glutamine after 50 min of perfusion, although the extent of equilibration was not determined. The fractional enrichment in glutamine was 31% of the enrichment of glutamate with the three different perfusates. From high-resolution nuclear magnetic resonance spectra, we found a ratio of glutamine to glutamate content of 94.1, 53.4, and 96.9%, respectively, for each experimental group. In experiments for which l-[1-13C]glutamine (5 mM) was included in the perfusate of group 2, [1-13C]glutamine was detected in the heart, but transfer of 13C from glutamine to glutamate was not detected (n = 4). We conclude that, in the perfused working heart, production of glutamine by amidation of glutamate takes place and can be detected, whereas the reverse process, generation of glutamate from glutamine, remains undetected. 相似文献
27.
Summary Deionized water as well as simulated and genuine fermentation media, which contained varying concentrations of benzaldehyde and 4-decanolide, were used to investigate the applicability of selected styrene-divinylbenzene resins for low pressure downflow adsorption of aroma compounds in a fixed bed. The effects of flow rate and matrix on the respective breakthrough curve slopes were examined using the LUB-and the MTZ-model. 相似文献
28.
Ralf W. Dittmann Peter M. Wehmeier Alexander Schacht Martin Lehmann Gerd Lehmkuhl 《Attention deficit and hyperactivity disorders》2009,1(2):187-200
To report on (1) psychometric properties of the Rosenberg Self-Esteem Scale (SES) studied in adolescents with ADHD, (2) correlations of SES with ADHD scale scores, and (3) change in patient-reported self-esteem with atomoxetine treatment. ADHD patients (12–17 years), treated in an open-label study for 24 weeks. Secondary analyses on ADHD symptoms (assessed with ADHD-RS, CGI, GIPD scales) and self-esteem (SES) were performed. One hundred and fifty-nine patients were treated. A dichotomous structure of the SES could be confirmed. Reliability and internal consistency were moderate to excellent. Highest coefficients were found for the correlation between SES and GIPD scores. Self-esteem significantly increased over time, accompanied by an improvement of ADHD symptoms and related perceived difficulties. The Rosenberg SES was shown to be internally consistent, reliable, and sensitive to treatment-related changes of self-esteem. According to these findings, self-esteem may be an important individual patient outcome beyond the core symptoms of ADHD. 相似文献
29.
Thomas Ratschiller Marcus-André Deutsch Julia Calzada-Wack Frauke Neff Christiane Roesch Ralf Guenzinger Ruediger Lange Markus Krane 《Journal of visualized experiments : JoVE》2015,(102)
The heterotopic cervical heart transplantation in mice is a valuable tool in transplant and cardiovascular research. The cuff technique greatly simplifies this model by avoiding challenging suture anastomoses of small vessels thereby reducing warm ischemia time. In comparison to abdominal graft implantation the cervical model is less invasive and the implanted graft is easily accessible for further follow-up examinations. Anastomoses are performed by pulling the ascending aorta of the graft over the cuff with the recipient’s common carotid artery and by pulling the main pulmonary artery over the cuff with the external jugular vein. Selection of appropriate cuff size and complete mobilization of the vessels are important for successful revascularization. Ischemia-reperfusion (I/R) injury can be minimized by perfusing the graft with a cardioplegic solution and by hypothermia. In this article, we provide technical details for a simplified and improved cuff technique, which should allow surgeons with basic microsurgical skills to perform the procedure with a high success rate. 相似文献
30.
The spatial and temporal organization of DNA replication was investigated in living cells with a green fluorescent protein fusion to the DNA polymerase clamp PCNA. In situ extractions and photobleaching experiments revealed that PCNA, unlike RPA34, shows little if any turnover at replication sites, suggesting that it remains associated with the replication machinery through multiple rounds of Okazaki fragment synthesis. Photobleaching analyses further showed that the transition from earlier to later replicons occurs by disassembly into a nucleoplasmic pool of rapidly diffusing subcomponents and reassembly at newly activated sites. The fact that these replication sites were de novo assembled in close proximity to earlier ones suggests that activation of neighboring origins may occur by a domino effect possibly involving local changes in chromatin structure and accessibility. 相似文献