全文获取类型
收费全文 | 7860篇 |
免费 | 826篇 |
国内免费 | 1124篇 |
出版年
2024年 | 12篇 |
2023年 | 130篇 |
2022年 | 138篇 |
2021年 | 201篇 |
2020年 | 305篇 |
2019年 | 324篇 |
2018年 | 330篇 |
2017年 | 330篇 |
2016年 | 327篇 |
2015年 | 347篇 |
2014年 | 452篇 |
2013年 | 654篇 |
2012年 | 341篇 |
2011年 | 395篇 |
2010年 | 347篇 |
2009年 | 435篇 |
2008年 | 435篇 |
2007年 | 459篇 |
2006年 | 414篇 |
2005年 | 337篇 |
2004年 | 311篇 |
2003年 | 295篇 |
2002年 | 253篇 |
2001年 | 232篇 |
2000年 | 171篇 |
1999年 | 154篇 |
1998年 | 148篇 |
1997年 | 114篇 |
1996年 | 127篇 |
1995年 | 127篇 |
1994年 | 113篇 |
1993年 | 125篇 |
1992年 | 88篇 |
1991年 | 117篇 |
1990年 | 70篇 |
1989年 | 68篇 |
1988年 | 53篇 |
1987年 | 55篇 |
1986年 | 50篇 |
1985年 | 47篇 |
1984年 | 62篇 |
1983年 | 46篇 |
1982年 | 49篇 |
1981年 | 40篇 |
1980年 | 34篇 |
1979年 | 27篇 |
1978年 | 30篇 |
1977年 | 23篇 |
1976年 | 24篇 |
1973年 | 11篇 |
排序方式: 共有9810条查询结果,搜索用时 31 毫秒
1.
2.
The copy frequency distribution of a transposable element family in a Drosophila melanogaster natural population is generally characterised by the values of the Charlesworths' model parameters α and β (Charlesworth & Charlesworth, 1983). The estimation of these parameters is made using the observed distribution of the occupied sites in a population sample. Several results have been interpreted as due either to the influence of stochastic factors or to deterministic factors (transposition, excision, selection…). The accuracy of this method was tested by estimations performed on samples from simulated populations. The results show that with the sample size usually used for natural population studies, the confidence intervals are too large to reasonably deduce either the element copy number distribution or the values of transposition and excision rate and selective coefficients. 相似文献
3.
《Saudi Journal of Biological Sciences》2020,27(10):2645-2650
Stimulation of the neonatal immune system is quite important for the proliferation and differentiation of antigen-presenting cells (APCs) and T cells. Tahneeq is a traditional method to manually rub the palatal mucosa of newborn babies with premasticated Ajwa palm dates. The present study was to investigate the tahneeq effects on IL-12 expression of dendritic cells (DCs) and blood T lymphocytes expressing CD8+ in neonatal Wistar rats. The number of 90 healthy neonatal Wistar rats have randomly divided into three groups: control group received breastmilk only, treatment group (T1) receiving breast milk + mild-scratched intensity of tahneeq, and T2 group received breastmilk + strong-scratched intensity of tahneeq on the palatal and gingival mucosa immediately after birth. Seven neonatal Wistar rats in all groups were then sacrificed in three hours after birth and days 1, 5, 7, 13, and 30 treatment. IL-12 expression in the palatal and gingival mucosa was determined using immunohistochemical staining, and blood CD8+ T-lymphocytes were quantified using a flow cytometer. One way ANOVA was used to analyze the percentage of IL-12 and CD8+ T-lymphocytes among neonatal Wistar rat groups. The T1 and T2 newborn rat groups had significantly higher IL-12 expression than the control group (p<0.001). The increased IL-12 expression in T2 groups significantly increased (p<0.001) compared to the IL-12 expression in the T1 and control groups. The percentage of CD8+ T lymphocytes in all neonatal rat groups increased on three hours after birth and day 30 treatment but remained constant on days 5 and 7 treatment and decreased on day 13 treatment. At 5, 13, and 30th days treatment, the percentage of CD8+ T lymphocytes in T1 and T2 neonatal rat groups was significantly higher (p<0.05) than that in the control group. In conclusion, the impact on systemic CD8+ T cells did not influence by the depth of the scratch. Both mild and strong tahneeq increased the systemic CD8+ T-lymphocytes in neonatal Wistar rats. The roles of anti-inflammatory cytokines and Treg cells should be further investigated to unravel those different results for the development of mucosal immunity in neonates. 相似文献
4.
Medial knee osteoarthritis is a debilitating disease. Surgical and conservative interventions are performed to manage its progression via reduction of load on the medial compartment or equivalently its surrogate measure, the external adduction moment. However, some studies have questioned a correlation between the medial load and adduction moment. Using a musculoskeletal model of the lower extremity driven by kinematics–kinetics of asymptomatic subjects at gait midstance, we aim here to quantify the relative effects of changes in the knee adduction angle versus changes in the adduction moment on the joint response and medial/lateral load partitioning. The reference adduction rotation of 1.6° is altered by ±1.5° to 3.1° and 0.1° or the knee reference adduction moment of 17 N m is varied by ±50% to 25.5 N m and 8.5 N m. Quadriceps, hamstrings and tibiofemoral contact forces substantially increased as adduction angle dropped and diminished as it increased. The medial/lateral ratio of contact forces slightly altered by changes in the adduction moment but a larger adduction rotation hugely increased this ratio from 8.8 to a 90 while in contrast a smaller adduction rotation yielded a more uniform distribution. If the aim in an intervention is to diminish the medial contact force and medial/lateral load ratio, a drop of 1.5° in adduction angle is much more effective (causing respectively 12% and 80% decreases) than a reduction of 50% in the adduction moment (causing respectively 4% and 13% decreases). Substantial role of changes in adduction angle is due to the associated alterations in joint nonlinear passive resistance. These findings explain the poor correlation between knee adduction moment and tibiofemoral compartment loading during gait suggesting that the internal load partitioning is dictated by the joint adduction angle. 相似文献
5.
《Journal of biomechanics》2014,47(15):3734-3743
In this study, a three-dimensional finite element model was used to investigate the changes in tissue composition and mechanical signals within human lumbar intervertebral disc during the degenerative progression. This model was developed based on the cell-activity coupled mechano-electrochemical mixture theory. The disc degeneration was simulated by lowering nutrition levels at disc boundaries, and the temporal and spatial distributions of the fixed charge density, water content, fluid pressure, Von Mises stress, and disc deformation were analyzed. Results showed that fixed charge density, fluid pressure, and water content decreased significantly in the nucleus pulposus (NP) and the inner to middle annulus fibrosus (AF) regions of the degenerative disc. It was found that, with degenerative progression, the Von Mises stress (relative to that at healthy state) increased within the disc, with a larger increase in the outer AF region. Both the disc volume and height decreased with the degenerative progression. The predicted results of fluid pressure change in the NP were consistent with experimental findings in the literature. The knowledge of the variations of temporal and spatial distributions of composition and mechanical signals within the human IVDs provide a better understanding of the progression of disc degeneration. 相似文献
6.
Sylvia Hofmann Sabine Kraus Tsering Dorge Michael Nothnagel Georg Miehe 《Journal of Biogeography》2014,41(11):2162-2172
7.
8.
Adrián Melo-Carrillo Jacob C. Dunn Liliana Cortés-Ortiz 《American journal of primatology》2020,82(8):e23160
Genetic diversity provides populations with the possibility to persist in ever-changing environments, where selective regimes change over time. Therefore, the long-term survival of a population may be affected by its level of genetic diversity. The Mexican howler monkey (Alouatta palliata mexicana) is a critically endangered primate restricted to southeast Mexico. Here, we evaluate the genetic diversity and population structure of this subspecies based on 83 individuals from 31 groups sampled across the distribution range of the subspecies, using 29 microsatellite loci. Our results revealed extremely low genetic diversity (HO = 0.21, HE = 0.29) compared to studies of other A. palliata populations and to other Alouatta species. Principal component analysis, a Bayesian clustering method, and analyses of molecular variance did not detect strong signatures of genetic differentiation among geographic populations of this subspecies. Although we detect small but significant FST values between populations, they can be explained by a pattern of isolation by distance. These results and the presence of unique alleles in different populations highlight the importance of implementing conservation efforts in multiple populations across the distribution range of A. p. mexicana to preserve its already low genetic diversity. This is especially important given current levels of population isolation due to the extreme habitat fragmentation across the distribution range of this primate. 相似文献
9.
10.
We compare the performances of local and global rules for smoothingparameter choice, in terms of asymptotic mean squared errorsof the resulting estimators. In some instances there is surprisinglylittle to choose between local and global approaches; our analysisidentifies contexts where the differences are small or large.This work motivates development of smoothing rules that forma half-way house between local and global smoothing.There, interpolation provides a basis for partial local smoothing.A key result shows that interpolation on even a coarse gridcan produce a very good approximation to full local smoothing.Our theoretical and numerical results lead us to suggest linearinterpolation of a bandwidth obtained by integral approximationson discrete intervals. 相似文献