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排序方式: 共有151条查询结果,搜索用时 15 毫秒
51.
Williamson Jennifer L. Tye Andrew Lapworth Dan J. Monteith Don Sanders Richard Mayor Daniel J. Barry Chris Bowes Mike Bowes Michael Burden Annette Callaghan Nathan Farr Gareth Felgate Stacey Fitch Alice Gibb Stuart Gilbert Pete Hargreaves Geoff Keenan Patrick Kitidis Vassilis Juergens Monika Martin Adrian Mounteney Ian Nightingale Philip D. Pereira M. Gloria Olszewska Justyna Pickard Amy Rees Andrew P. Spears Bryan Stinchcombe Mark White Debbie Williams Peter Worrall Fred Evans Chris 《Biogeochemistry》2022,160(3):423-424
52.
The effect of interfacial parameters on cup-bone relative micromotions. A finite element investigation. 总被引:6,自引:0,他引:6
Achieving stability is a prerequisite for allowing bone to grow into the porous surface of non-cemented acetabular cups. The purpose of this study is to estimate the effects of interfacial characteristics on relative cyclical micromotion between cup and bone during gait in the immediate postoperative phase. The technique used is finite element analysis. Six models with different interfacial characteristics are created in order to study the effects of fixation technique. These include representation of a 1 mm press-fit, 2 mm press-fits (with and without an initial polar gap) and exact-fit conditions (with and without additional screw fixation). Although direct validation of the model has not been performed, the calculated micromotions under a static load of 1112 N are compared with appropriate experimental data. Generally, the model tends to underestimate micromotion and this underestimate is significant in the case of relative surface-normal micromotion in polar regions for models with low- and no-interference. The most likely cause of this significant underestimate is a failure of the model to accurately represent penetration of rough contacting surfaces under compression. Other types of micromotion, although low, are within standard deviations reported by Kwong et al. (1994 Journal of Arthroplasty 9, 163-170). Quasi-static joint contact and muscle forces, representative of the stance phase of gait are then applied and maximum micromotions are found to occur consistently prior to toe off: this being the point of maximum force. With regard to the press-fit simulations, good cup-bone contact in the superior region of the interface is required for stability and the greatest micromotions occur in the models with the larger interference and larger polar gaps. In contrast to the press-fit models, muscle activity in exact-fit models influences the calculations. Specifically, the early activity of m.semimembranosus modelled causes opening of the peripheral seal. Taken together it is found that polar gaps reduce the stability of the model and lack of pre-compresssion in the periphery allows this region of the interface to be opened up. 相似文献
53.
Long-term monitoring data (1968–2008) were used to investigate internal phosphorus (P) loading following external P loading
reduction in shallow Loch Leven, Scotland. A whole-lake sediment P inventory (upper 3 cm of sediment; 2005) suggested a release-potential
of 29.7 tonnes (t) from the release sensitive sediment P pools. 18.5 t was contained within shallow water sediments (<4.5 m
water depth) with 7.6 t in deeper water sediments below the photic zone (>5 m water depth). The “observed” release (<5.1 t),
estimated using a water column P mass balance approach (1989–2008), was <5.1 t, indicating the presence of regulating mechanisms.
Observed P release declined between 1989 and 2008, with the exception of 2003–2006. Observed P release estimates were positively
correlated with annual average water column P concentration after 1989, highlighting the role of internal loading in maintaining
poor water quality conditions after management intervention. Multiple regression analysis suggested that internal loading
was driven by the wave mixed depth in spring (positive driver), summer water temperature (positive driver) and spring water
clarity transparency (negative driver). The potential importance of biological and physico-chemical feedback mechanisms in
the regulation of benthic–pelagic coupling and water quality in Loch Leven are discussed. 相似文献
54.
Water quality of Loch Leven: responses to enrichment, restoration and climate change 总被引:3,自引:0,他引:3
L. Carvalho C. Miller B. M. Spears I. D. M. Gunn H. Bennion A. Kirika L. May 《Hydrobiologia》2012,681(1):35-47
It is usually assumed that climate change will have negative impacts on water quality and hinder restoration efforts. The
long-term monitoring at Loch Leven shows, however, that seasonal changes in temperature and rainfall may have positive and
negative impacts on water quality. In response to reductions in external nutrient loading, there have been significant reductions
in in-lake phosphorus concentrations. Annual measures of chlorophyll a have, however, shown little response to these reductions. Warmer spring temperatures appear to be having a positive effect
on Daphnia densities and this may be the cause of reduced chlorophyll a concentrations in spring and an associated improvement in water clarity in May and June. The clearest climate impact was
the negative relationship between summer rainfall and chlorophyll a concentrations. This is highlighted in extreme weather years, with the three wettest summers having very low chlorophyll
a concentrations and the driest summers having high concentrations. To predict water quality impacts of future climate change,
there is a need for more seasonal predictions from climate models and a greater recognition that water quality is the outcome
of seasonal responses in different functional groups of phytoplankton and zooplankton to a range of environmental drivers. 相似文献
55.
In the mammalian ovary, oocytes are contained within ovarian follicles. These consist in an oocyte surrounded by supporting cells: an inner layer of granulosa cells and an outer layer of thecal cells separated by a basal lamina. At any one time, a developing cohort of follicles exists, from which only a small species-specific number are selected for continued development towards ovulation, with the remainder dying by follicular atresia. Here, we use in vitro methods to study interactions between two follicles in culture (follicle co-cultures). We show that, when two individual follicles are grown together in culture, cells and cellular processes migrate from the outer thecal layer of one follicle to the thecal layer of the other co-cultured follicle. These cells are identified as a mixed population containing primarily endothelial but also neuronal cells. Both are able to migrate through the ovarian interstitum, making contact with the basal lamina of other follicles and with similar cells from these other follicles. Networks of such cells might be involved in interfollicular communication and in the coordination of follicle selection for ovulation. 相似文献
56.
Studies on the role of ribosomal proteins L 7 and L 12 in the in vitro synthesis of -galactosidase 总被引:10,自引:0,他引:10
H F Kung J E Fox C Spears N Brot H Weissbach 《The Journal of biological chemistry》1973,248(14):5012-5015
57.
The effect of selenium (Se), supplemented by subcutaneous injections, upon in vitro phytohemagglutinin (PHA)-stimulated T lymphocyte blastogenesis was evaluated in crossbred calves at 45, 124, and 270 d of age. Peripheral blood lymphocytes (5×105) obtained from control (n=4) and Se-treated (n=4) calves were cultured with 0, 0.4, and 0.8 μg PHA and 0.5 μCi [3H]-thymidine in a complete culture medium. At 60h, cultures were harvested for the quantitation of thymidine incorporation. The degree of lymphocyte blastogenesis or stimulation to PHA, expressed as the stimulation index (SI), was assessed for treatment and age effects. At the onset and end of the experimental period, plasma Se and blood glutathione peroxidase concentrations revealed that control calves [0.045 ppm and 48.4 units/g hemoglobin (Hb), respectively] were marginally deficient and treated calves (0.075 ppm and 143.0 units/g Hb, respectively) were adequate in Se status. The SI values were not influenced (P>0.01) by Se treatment, but were affected (P<0.003) by age. For combined treatments and PHA dosages, mean SI values were greater (P<0.05), when lymphocytes were tested at 45 (SI=25.9) than at 124 and 270 (SI=13.7 and 15.5, respectively) d of age. Lymphocyte viability and number were not affected by Se treatment, but lymphocyte number tended (P<0.1) to increase with age. In conclusion, T lymphocyte responsiveness to mitogen stimulation was normal in beef calves residing in a geographical area considered deficient in Se. 相似文献
58.
Two 42-d experiments were conducted with weanling male rats to study interactions between nickel and copper. In Experiment 1, a low-copper basal diet was supplemented with copper at 0 or 30 ppm and nickel at 0 or 30 ppm. Copper was added in Experiment 2 to a basal copper-deficient diet at a level of 0 or 15 ppm and nickel was supplemented at 0, 15, or 225 ppm. Responses to dietary nickel were dependent upon copper nutriture and experimental duration. Nickel had little effect on growth during the first 21 d of either study when added at low levels (15 or 30 ppm) to copper-deficient diets. Nickel supplementation depressed gains between 21 and 42 d in rats fed copper-deficient, but not copper-adequate, diets. Hematocrits and hemoglobin concentrations were not significantly affected by dietary nickel at 21 d. Nickel supplementation decreased hematocrits and hemoglobin values in copper deficient rats at 42 d in Experiment 1, but not in Experiment 2. Absorption of copper apparently was not reduced by nickel, since tissue copper concentrations were generally not decreased by increasing dietary nickel. Nickel supplementation increased lung and heart copper concentrations in Experiment 2. Liver iron was not affected by nickel, but spleen iron concentrations were reduced by nickel supplementation in copper-deficient rats in Experiment 2. The present studies suggest that nickel acts antagonistically to copper in certain biological processes. 相似文献
59.
60.
Detrital Controls on Soil Solution N and Dissolved Organic Matter in Soils: A Field Experiment 总被引:1,自引:1,他引:0
K.?LajthaEmail author S.?E.?Crow Y.?Yano S.?S.?Kaushal E.?Sulzman P.?Sollins J.?D.?H.?Spears 《Biogeochemistry》2005,76(2):261-281
We established a long-term field study in an old growth coniferous forest at the H.J. Andrews Experimental Forest, OR, USA,
to address how detrital quality and quantity control soil organic matter accumulation and stabilization. The Detritus Input
and Removal Treatments (DIRT) plots consist of treatments that double leaf litter, double woody debris inputs, exclude litter
inputs, or remove root inputs via trenching. We measured changes in soil solution chemistry with depth, and conducted long-term
incubations of bulk soils from different treatments in order to elucidate effects of detrital inputs on the relative amounts
and lability of different soil C pools. In the field, the addition of woody debris increased dissolved organic carbon (DOC)
concentrations in O-horizon leachate and at 30 cm, but not at 100 cm, compared to control plots, suggesting increased rates
of DOC retention with added woody debris. DOC concentrations decreased through the soil profile in all plots to a greater
degree than did dissolved organic nitrogen (DON), most likely due to preferential sorption of high C:N hydrophobic dissolved
organic matter (DOM) in upper horizons; percent hydrophobic DOM decreased significantly with depth, and hydrophilic DOM had
a much lower and less variable C:N ratio. Although laboratory extracts of different litter types showed differences in DOM
chemistry, percent hydrophobic DOM did not differ among soil solutions from different detrital treatments in the field, suggesting
that microbial processing of DOM leachate in the field consumed easily degradable components, thus equalizing leachate chemistry
among treatments. Total dissolved N leaching from plots with intact roots was very low (0.17 g m−2 year−1), slightly less than measured deposition to this very unpolluted forest (~s 0.2 g m−2 year−1). Total dissolved N losses showed significant increases in the two treatments without roots whereas concentrations of DOC
decreased. In these plots, N losses were less than half of estimated plant uptake, suggesting that other mechanisms, such
as increased microbial immobilization of N, accounted for retention of N in deep soils. In long-term laboratory incubations,
soils from plots that had both above- and below-ground litter inputs excluded for 5 years showed a trend towards lower DOC
loss rates, but not lower respiration rates. Soils from plots with added wood had similar respiration and DOC loss rates as
control soils, suggesting that the additional DOC sorption observed in the field in these soils was stabilized in the soil
and not readily lost upon incubation. 相似文献