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91.
Fatigue failure of the cement mantle has been proposed as one of the failure processes contributing to aseptic loosening of cemented joint replacements. It has also been suggested that fatigue failure is dramatically accelerated by residual stress generated during the cement polymerisation process. Previous computational models of the polymerisation process have investigated only the latter part of polymerisation by assuming both instantaneous hardening of the material (a stress locking point) and that all residual stress results from thermal shrinkage after this stress locking point. In this study, finite element models which use the local degree of polymerisation to calculate material properties and shrinkage have been used to predict residual stresses in two models of total hip replacement cement mantles. Results indicate that the final value of cement mantle stress may not be the highest stresses that the cement is subjected to during the polymerisation process. Two models are presented, a 2-dimensional model, which was adapted from a similar model in the literature (Lennon and Prendergast, 2002) and a 3-dimensional concentric-cylinders model. In both cases a chemical kinetics model was used to predict the progress of the polymerisation reaction and a second linear model used to predict cement mechanical properties and density, and so stress generation and volume change, over time. There was good agreement of the results of the 2D model with its counterpart in the literature. For the 3D model, the final residual stress magnitudes and patterns showed good agreement with similar physical and computational models in the literature. 相似文献
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New DI Chesser AM Thuraisingham RC Yaqoob MM 《American journal of physiology. Heart and circulatory physiology》2003,284(4):H1212-H1216
Impaired cerebral blood flow autoregulation is seen in uremic hypertension, whereas in nonuremic hypertension autoregulation is shifted toward higher perfusion pressure. The cerebral artery constricts in response to a rise in either lumen pressure or flow; we examined these responses in isolated middle cerebral artery segments from uremic Wistar-Kyoto rats (WKYU), normotensive control rats (WKYC), and spontaneously hypertensive rats (SHR). Pressure-induced (myogenic) constriction developed at 100 mmHg; lumen flow was then increased in steps from 0 to 98 microl/min. Some vessels were studied after endothelium ablation. Myogenic constriction was significantly lower in WKYU (28 +/- 2.9%) compared with both WKYC (39 +/- 2.5%, P = 0.035) and SHR (40 +/- 3.1%, P = 0.018). Flow caused constriction of arteries from all groups in an endothelium-independent manner. The response to flow was similar in WKYU and WKYC, whereas SHR displayed increased constriction compared with WKYU (P < 0.001) and WKYC (P < 0.001). We conclude that cerebral myogenic constriction is decreased in WKYU, whereas flow-induced constriction is enhanced in SHR. 相似文献
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95.
Urinary homovanillic acid (HVA) and vanillymandelic acid (VMA) in workers exposed to manganese dust 总被引:3,自引:0,他引:3
Ling Bao Ai Lay Ha Chua Ai Li New Bee Lan Lee Yi Mim Liu Sin Eng Chia Choon Nam Ong 《Biological trace element research》1998,64(1-3):89-99
The neurotoxicity of manganese (Mn) is well known, however, the neurochemical effect caused by this metal is less well investigated.
In this study, urinary homovanillic acid (HVA) and vanillymandelic acid (VMA), two end products of catecholamine metabolism,
were measured in 39 workers chronically exposed to Mn in a manganese smelting plant. The average duration of Mn exposure was
17.4 yr. Nineteen nonexposed workers were also studied. Concentrations of Mn in serum (MnS) and in urine (MnU) were measured
by Zeeman graphite furnace atomic absorption spectrophotometry (ZAAS), and HVA and VMA determined by high performance liquid
chromatography (HPLC). For Mn-exposed workers, the concentration of MnS was nearly 2.8 times (1.61 ± 0.16 mg/L vs 0.56 ± 0.16
mg/L) and MnU about 4.5 times higher (7.62 ± 0.17 mg/L vs 1.69 ± 0.16 mg/L) than the nonexposed. Although the geometric mean
concentration of HVA in exposed workers was similar to that of the nonexposed (3.09 ± 1.39 mg/g ere. vs 2.99 ± 1.40 mg/g cre.),
the VMA concentration was significantly higher (3.02 ± 1.43 mg/g cre. vs 2.49 ± 1.58 mg/g cre.,p = 0.033). Multiple regression analysis showed that although there were no correlations between any of these parameters with
the duration of exposure to Mn, both HVA and VMA showed significant correlations with increase in MnS and MnU. These data
provide evidence that exposure to Mn was associated with measurable increase in catecholamine metabolites. This finding is
compatible with recent observations in laboratory animals that Mn interferes with neurochemical metabolism. 相似文献
96.
DAX1 mutations map to putative structural domains in a deduced three-dimensional model. 总被引:5,自引:0,他引:5
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Y H Zhang W Guo R L Wagner B L Huang L McCabe E Vilain T P Burris K Anyane-Yeboa A H Burghes D Chitayat A E Chudley M Genel J M Gertner G J Klingensmith S N Levine J Nakamoto M I New R A Pagon J G Pappas C A Quigley I M Rosenthal J D Baxter R J Fletterick E R McCabe 《American journal of human genetics》1998,62(4):855-864
The DAX1 protein is an orphan nuclear hormone receptor based on sequence similarity in the putative ligand-binding domain (LBD). DAX1 mutations result in X-linked adrenal hypoplasia congenita (AHC). Our objective was to identify DAX1 mutations in a series of families, to determine the types of mutations resulting in AHC and to locate single-amino-acid changes in a DAX1 structural model. The 14 new mutations identified among our 17 families with AHC brought the total number of families with AHC to 48 and the number of reported mutations to 42; 1 family showed gonadal mosaicism. These mutations included 23 frameshift, 12 nonsense, and six missense mutations and one single-codon deletion. We mapped the seven single-amino-acid changes to a homology model constructed by use of the three-dimensional crystal structures of the thyroid-hormone receptor and retinoid X receptor alpha. All single-amino-acid changes mapped to the C-terminal half of the DAX1 protein, in the conserved hydrophobic core of the putative LBD, and none affected residues expected to interact directly with a ligand. We conclude that most genetic alterations in DAX1 are frameshift or nonsense mutations and speculate that the codon deletion and missense mutations give insight into the structure and function of DAX1. 相似文献
97.
98.
T. R. New 《Journal of Insect Conservation》2005,9(1):67-68
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100.