全文获取类型
收费全文 | 6049篇 |
免费 | 417篇 |
出版年
2023年 | 28篇 |
2022年 | 38篇 |
2021年 | 82篇 |
2020年 | 64篇 |
2019年 | 64篇 |
2018年 | 77篇 |
2017年 | 91篇 |
2016年 | 130篇 |
2015年 | 267篇 |
2014年 | 303篇 |
2013年 | 352篇 |
2012年 | 506篇 |
2011年 | 430篇 |
2010年 | 272篇 |
2009年 | 219篇 |
2008年 | 354篇 |
2007年 | 369篇 |
2006年 | 363篇 |
2005年 | 278篇 |
2004年 | 282篇 |
2003年 | 258篇 |
2002年 | 274篇 |
2001年 | 77篇 |
2000年 | 78篇 |
1999年 | 74篇 |
1998年 | 57篇 |
1997年 | 34篇 |
1996年 | 43篇 |
1995年 | 56篇 |
1994年 | 29篇 |
1993年 | 44篇 |
1992年 | 53篇 |
1991年 | 49篇 |
1990年 | 56篇 |
1989年 | 42篇 |
1988年 | 42篇 |
1987年 | 36篇 |
1986年 | 53篇 |
1985年 | 45篇 |
1984年 | 44篇 |
1983年 | 43篇 |
1982年 | 38篇 |
1979年 | 31篇 |
1978年 | 44篇 |
1977年 | 33篇 |
1975年 | 27篇 |
1974年 | 35篇 |
1973年 | 27篇 |
1971年 | 24篇 |
1968年 | 21篇 |
排序方式: 共有6466条查询结果,搜索用时 15 毫秒
991.
The strength of the direct effect by scraping cladocerans and the indirect effect of nutrient regeneration by filtering herbivorous cladocerans on periphyton growth was investigated in a littoral food web. Ten enclosures were erected in a lake in an area with artificial vegetation. Five enclosures were stocked with juvenile perch ( Perca fluviatilis ) and five lacked fish. In addition, a reference area in the artificial vegetation was sampled. The mesh size of the net surrounding the cages was chosen to allow an inflow of phytoplankton into the cages from the surrounding water. The periphyton and filtering herbivorous cladoceran biomasses were highest in the fish-free treatment. There was no difference in phytoplankton biomass between treatments despite the large difference in filtering herbivorous cladoceran biomass, suggesting that the inflow of phytoplankton into enclosures completely compensated for the loss due to filtering. The reference area and the enclosure with fish showed the same patterns in developments with respect to filtering herbivorous cladocerans and periphyton. Scraping cladoceran biomass was higher in the fish-free treatment resulting in a positive correlation between scraping cladoceran and periphyton biomass. Our results suggest that the positive indirect effect of filtering herbivorous cladoceran nutrient regeneration on periphyton was stronger than the negative direct effect of grazing by scraping cladocerans on periphyton in this semi-open system, and that pelagic production by phytoplankton may foster periphyton growth in the littoral habitat via filtering herbivorous cladocerans. Furthermore, heterogeneity within trophic levels involving primary producers of different growth forms such as phytoplankton and periphyton may enhance the potential for compensatory responses via nutrient recycling. 相似文献
992.
The metathoracic dorso-longitudinal muscles of adult males of Locusta migratoria were denervated. Twenty days after denervation wet weight, protein content and the specific activities of enzymes, representative of aerobic glycolytic and β-oxidative pathways, and citric acid and glycerophosphate cycles are reduced in a statistically significant way. In contrast, the specific activity of lactate dehydrogenase is increased. It is demonstrated that locusts, in which the metathoracic dorso-longitudinal muscles are denervated on one side only, constitute an experimental system with proper experimental and control muscles. 相似文献
993.
Pharmaceutical companies try to develop new drugs that have a high success rate of reaching the market. However, current disease models lack a strong correlation to clinical reality, because of the underestimation of the complexity and variability of clinical disease processes. This leads to high attrition rates late in drug development and soaring costs. Improvement of disease models is an important issue to reduce the high attrition rates in drug development. Using cell-based disease models, which should take into account the molecular diversity of the human cytome, will improve the predictive value of drug discovery. 相似文献
994.
Jan R Leemreis Amanda M G Versteilen Pieter Sipkema A B Johan Groeneveld René J P Musters 《Cytometry. Part A》2006,69(9):973-978
BACKGROUND: Damaged and/or dysfunctional microvascular endothelium has been implicated in the pathogenesis of ischemic acute renal failure (ARF). Rapidly occurring changes in the endothelial F-actin cytoskeleton as observed in vitro might be responsible, but have been proven difficult to measure accurately in situ. Therefore, the purpose of this study was to examine several methods of digital image analysis in order to quantify the alterations of endothelial F-actin after renal ischemia and reperfusion (I/R), and to relate these to deterioration of renal function. METHODS: Frozen sections of Sham and I/R rat kidneys were fixed in 4% formaldehyde and stained with rhodamine-phallo?din. Microvascular structures were captured using a 3i Marianastrade mark digital imaging fluorescence microscope workstation. Images were analyzed using 3i SlideBooktrade mark software, employing several masking techniques and line-scans. RESULTS: Digital image analysis demonstrated a decrease in the mean intensity of rhodamine-phallo?din fluorescence after I/R from 1030 +/- 187 to 735 +/- 121 a.u. (arbitrary units, mean +/- SD, n = 7). The number of F-actin fragments per pixel increased from (15.8 +/- 4.9) x 10(-5) to (20.7 +/- 3.5) x 10(-5) (n = 7), indicating cytoskeletal fragmentation. In addition, line-scan analysis demonstrated a disturbed spatial orientation of the F-actin cytoskeleton after I/R. Finally, the loss of F-actin correlated with a rise in plasma creatinine. CONCLUSIONS: The methods of digital image analysis described in the present study demonstrate that renal I/R induces profound changes in the F-actin cytoskeletal structure of microvascular endothelial cells, implicating an injured and dysfunctional microvascular endothelium, which may contribute to acute renal failure (ARF). 相似文献
995.
Different nest predator faunas and nest predation risk on ground and shrub nests at forest ecotones: an experiment and a review 总被引:1,自引:0,他引:1
This study examined predator faunas of artificial ground and shrub nests and whether nest predation risk was influenced by
nest site, proximity to forest edge, and habitat structure in 38 grassland plots in south-central Sweden. There was a clear
separation of predator faunas between shrub and ground nests as identified from marks in plasticine eggs. Corvids accounted
for almost all predation on shrub nests whereas mammals mainly depredated ground nests. Nest predation risk was significantly
greater for shrub than for ground nests at all distances (i.e. 0, 15 and 30 m) from the forest edge. However, nest predation
risk was not significantly related to distance to forest edge, but significantly increased with decreasing distance to the
nearest tree. Different corvid species robbed nests at different distances from the forest edge, with jays robbing nests closest
to edges. We conclude that the relationship between the predation risk of grassland bird nests and distance to the forest
edge mainly depends on the relative importance of different nest predator species and on the structure of the forest edge
zone. A review of published articles on artificial shrub and ground nest predation in the temperate zone corroborated the
results of our own study, namely that shrub nests experienced higher rates of depredation in open habitats close to the forest
edge and that avian predators predominantly robbed shrub nests. Furthermore, the review results showed that predation rates
on nests in general are highest <50 m inside the forest and lower in open as well as forest interior habitats (≥50 m from
the edge).
Received: 16 March 1998 / Accepted: 30 July 1998 相似文献
996.
K. S?gaard H. Christensen N. Fallentin M. Mizuno B. Quistorff G. Sj?gaard 《European journal of applied physiology and occupational physiology》1998,78(5):411-416
Muscle activity was recorded from the flexor carpi radialis muscle during static and dynamic-concentric wrist flexion in
six subjects, who had exhibited large differences in histochemically identified muscle fibre composition. Motor unit recruitment
patterns were identified by sampling 310 motor units and counting firing rates in pulses per second (pps). During concentric
wrist flexion at 30% of maximal exercise intensity the mean firing rate was 27 (SD 13) pps. This was around twice the value
of 12 (SD 5) pps recorded during sustained static contraction at 30% of maximal voluntary contraction, despite a larger absolute
force level during the static contraction. A similar pattern of higher firing rates during dynamic exercise was seen when
concentric wrist flexion at 60% of maximal exercise intensity [30 (SD 14) pps] was compared with sustained static contraction
at 60% of maximal voluntary contraction [19 (SD 8) pps]. The increase in dynamic exercise intensity was accomplished by recruitment
of additional motor units rather than by increasing the firing rate as during static contractions. No difference in mean firing
rates was found among subjects with different muscle fibre composition, who had previously exhibited marked differences in
metabolic response during corresponding dynamic contractions. It was concluded that during submaximal dynamic contractions
motor unit firing rate cannot be deduced from observations during static contractions and that muscle fibre composition may
play a minor role.
Accepted: 5 May 1998 相似文献
997.
998.
Selective induction of DNA synthesis in T and B lymphocytes 总被引:78,自引:0,他引:78
999.
The frequency of epileptic seizures was observed in a controlled therapeutic trial on 23 epileptic inpatients before and after treatment with vitamin D2 or placebo in addition to anticonvulsant drugs. The number of seizures was reduced during treatment with vitamin D2 but not with placebo. The effect was unrelated to changes in serum calcium or magnesium. The results may support the concept that epileptics should be treated prophylactically with vitamin D. 相似文献
1000.
SURFACE STRUCTURE OF ISOLATED NEURONS : Detachment of Nerve Terminals during Axon Regeneration
下载免费PDF全文
![点击此处可从《The Journal of cell biology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Freehand, isolated neuronal perikarya from the hypoglossal nucleus of the rabbit have been examined with light-and electron-microscopy (transmission and scanning). The surface of the cell bodies was largely covered with spherical particles which were 0.5–2 µ in diameter. Transmission electron microscopy proved that the spherical particles were synaptic nerve terminals. Crush of the hypoglossal nerve which leads to chromatolysis and swelling of the neuronal cell bodies results in a conspicuous reduction in the number of terminals attached to the surface of hypoglossal neurons. This effect was observed both for isolated neurons and in tissue sections. The effect is considered in relation to earlier reported variations in the adherence of neuropil to isolated neuronal perikarya. The functional importance of nerve ending detachment in connection with nerve injury is discussed. 相似文献