首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   3957篇
  免费   1篇
  国内免费   1篇
化学工业   15篇
建筑科学   1篇
轻工业   3篇
石油天然气   4篇
无线电   5篇
一般工业技术   9篇
冶金工业   3917篇
自动化技术   5篇
  2017年   1篇
  2016年   1篇
  2015年   1篇
  2012年   1篇
  2010年   3篇
  2009年   4篇
  2008年   1篇
  2007年   3篇
  2006年   3篇
  2005年   2篇
  2004年   1篇
  2003年   7篇
  1999年   108篇
  1998年   1184篇
  1997年   653篇
  1996年   462篇
  1995年   218篇
  1994年   205篇
  1993年   223篇
  1992年   25篇
  1991年   67篇
  1990年   63篇
  1989年   57篇
  1988年   52篇
  1987年   56篇
  1986年   41篇
  1985年   44篇
  1983年   10篇
  1982年   19篇
  1981年   22篇
  1980年   31篇
  1979年   2篇
  1978年   10篇
  1977年   103篇
  1976年   259篇
  1975年   5篇
  1972年   1篇
  1966年   1篇
  1965年   1篇
  1964年   2篇
  1955年   7篇
排序方式: 共有3959条查询结果,搜索用时 15 毫秒
901.
We present a high-resolution bacterial contig map of 3.4 Mb of genomic DNA in human chromosome 21q11-q21, encompassing the region of elevated disomic homozygosity in Down Syndrome-associated abnormal myelopoiesis and leukemia, as well as the markers, which has shown a strong association with Alzheimer's Disease that has never been explained. The map contains 89 overlapping PACs, BACs, or cosmids in three contigs (850, 850, and 1500 kb) with two gaps (one of 140-210 kb and the second <5 kb). To date, eight transcribed sequences derived by cDNA selection, exon trapping, and/or global EST sequencing have been positioned onto the map, and the only two genes so far mapped to this cytogenetic region, STCH and RIP140 have been precisely localized. This work converts a further 10% of chromosome 21q into a high-resolution bacterial contig map, which will be the physical basis for the long-range sequencing of this region. The map will also enable positional derivation of new transcribed sequences, as well as new polymorphic probes, that will help in elucidation of the role the genes in this region may play in abnormal myelopoiesis and leukemia associated with trisomy 21 and Alzheimer's Disease.  相似文献   
902.
Theophylline anhydrate microcapsules with different amounts of MA/MMA copolymer (Eudragit L) were prepared by the solvent evaporation method. Qualitative as well as quantitative investigation of the drug-polymer interaction by Fourier transform infrared (FTIR) spectroscopy with a curve fitting program was undertaken. The release mechanisms of theophylline in pH 1.2 and pH 6.8 media were also studied to elucidate the effect of drug-polymer interaction on the release characteristics of microcapsules. Direct evidence for a hydrogen bonding interaction between theophylline and Eudragit L in microcapsules was obtained. Moreover, the fraction of hydrogen bonded theophylline increased with the increase of Eudragit L. The dissolution of theophylline from microcapsules exhibited an enteric-coated release property. The drug release mechanism was found to fit the Higuchi matrix model in the simulated gastric acid condition, but drug release was much more rapid in the pH 6.8 buffer solution. The drug release rate decreased as the composition of theophylline increased, and it was proportional to the fraction of hydrogen bonded theophylline. These results suggest that the increased fraction of hydrogen bonded theophylline in microencapsulation might improve the mixing and dispersibility of theophylline in the Eudragit L matrix, thus resulting in the increase of the release rate of theophylline from microcapsules.  相似文献   
903.
Cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL) is an increasingly recognized autosomal dominant disorder that leads to cerebrovascular manifestations in early adulthood. This study delineates the phenotypic spectrum and the natural history of the disease in 102 affected individuals from 29 families with biopsy-proven CADASIL. Recurrent ischemic episodes (transient ischemic attack [TIA] or stroke) were the most frequent presentation found in 71% of the cases (mean age at onset, 46.1 years; range, 30-66 years; SD, 9.0 years). Forty-eight percent of the cases had developed cognitive deficits. Dementia (28%) was frequently accompanied by gait disturbance (90%), urinary incontinence (86%), and pseudobulbar palsy (52%). Thirty-nine patients (38%) had a history of migraine (mean age at onset, 26.0 years; SD, 8.2 years), which was classified as migraine with aura in 87% of the cases. Psychiatric disturbances were present in 30% of the cases, with adjustment disorder (24%) being the most frequent diagnosis. Ten patients (10%) had a history of epileptic seizures. To delineate the functional consequences of ischemic deficits, we studied the extent of disability in different age groups. The full spectrum of disability was seen in all groups older than age 45. Fifty-five percent of the patients older than age 60 were unable to walk without assistance. However, 14% in this age group exhibited no disability at all. Kaplan-Meier analysis disclosed median survival times of 64 years (males) and 69 years (females). An investigation of the 18 multiplex families revealed marked intrafamilial variations.  相似文献   
904.
BACKGROUND: The purpose of the present study was to compare the participation of women and men in the protocols of the Modification of Diet in Renal Disease (MDRD) study, a multicenter prospective randomized clinical trial, and to assess gender differences in their renal outcomes. METHODS: Of the 840 participants in the MDRD study, 332 (39.5%) were women who were assigned randomly to the dietary protein and blood pressure groups and followed for a median of 2.2 years. A subgroup analysis of the MDRD study database was carried out to compare women and men participants in recruitment, baseline characteristics, adherence to protocol requirements, safety and outcomes, and progression of renal disease and its response to dietary and blood pressure interventions. RESULTS: Adherence by women to the requirements of the protocol including diet, record keeping, office visits, glomerular filtration rate (GFR) measurements and urine collections was equivalent to that of men. Women had different renal diagnoses, less proteinuria and lower serum creatinine levels for given GFRs than men. When participants were grouped above and below age 52, the younger women had lower mean arterial pressure than did the men. Older women compared with younger had higher mean arterial pressure, body weight and body mass index, and total low density lipoprotein cholesterol. These differences were not seen between males of the same two age groups. During follow-up, the rate of GFR fall was slower in women, especially in the younger group. However, the association between gender and the rate of fall in GFR was attenuated and became non-significant after adjusting for differences in blood pressure, proteinuria and high density lipoprotein cholesterol. In analyses of the full cohort, there were no significant differences between women and men in the effects of the low protein or low blood pressure intervention in patients with either moderate (study A) or advanced (study B) renal disease. However, in subgroup analyses of patients in study A, there was some evidence of a lesser effect in women than in men. CONCLUSIONS: This exploratory analysis of the MDRD study indicates a slower mean GFR decline in women as compared with men. The slower mean GFR decline and suggestive evidence of a lesser beneficial effect of the low protein diet and low blood pressure interventions in women suggest that gender differences should be considered in trials of the effects of these interventions on the progression of renal disease. Also, the participation of women in the MDRD study was excellent and equivalent to that of men.  相似文献   
905.
Hypoxia-induced outward currents (hyperpolarization) were examined in hippocampal CA1 neurons of rat brain slices, using the whole-cell recording technique. Hypoxic episodes were induced by perfusing slices with an artificial cerebrospinal fluid aerated with 5% CO2/95% N2 rather than 5% CO2/95% O2, for about 3 min. The hypoxic current was consistently and reproducibly induced in CA1 neurons dialysed with an ATP-free patch pipette solution. This current manifested as an outward shift in the holding current in association with increased conductance, and it reversed at -78 +/- 2.5 mV, with a linear I-V relation in the range of -100 to -40 mV. To provide extra energy resources to individual neurons recorded, agents were added to the patch pipette solution, including MgATP alone, MgATP + phosphocreatine + creatine kinase, or MgATP + creatine. In CA1 neurons dialysed with patch solutions including these agents, hypoxia produced small outward currents in comparison with those observed in CA1 neurons dialysed with the ATP-free solution. Among the above agents examined, whole-cell dialysis with MgATP + creatine was the most effective at decreasing the hypoxic outward currents. We suggest that the hypoxic hyperpolarization is closely related to energy metabolism in individual CA1 neurons, and that the energy supply provided by phosphocreatine metabolism may play a critical role during transient metabolic stress.  相似文献   
906.
The short-term results of 1,605 gastrectomies performed for stomach cancer, using different types of esophagoenterostomy, are discussed. Anastomotic leakage is the main criterion for a choice of the most optimal procedure of forming an anastomosis. The contribution of the first and second rows of sutures to leakage is evaluated. An analysis of data on anastomotic leakage incidence points to the advantages offered by application of submerged esophagus-related anastomosis. A new modification of procedure of formation of muffle-type of esophagoenterostomy is presented. Leakage was registered in 1.3% which was due to technical errors during surgery. The non-reflux properties of the anastomosis are emphasized, with particular emphasis on its reliability, good functional characteristics, simplicity and wide range of application. The clinical applications are described.  相似文献   
907.
The effects of monensin on the energy metabolism of dairy cows in early lactation were investigated in a large clinical trial that was randomized and double-blinded. A total of 1010 Holstein cows and first lactation heifers were allocated to receive a controlled-release capsule of monensin or a placebo at 3 wk prior to expected calving date. Treatments were randomized across 25 dairy farms located near Guelph, Ontario, Canada. Serum samples obtained at the time of treatment administration and at wk 1, 2, 3, 6, and 9 postcalving were analyzed for beta-hydroxybutyrate, glucose, aspartate aminotransferase, urea, total protein, calcium, and phosphorus. Cows were also assigned a body condition score at the time each sample was obtained. Monensin treatment significantly reduced serum beta-hydroxy-butyrate concentrations at wk 1, 2, and 3 postpartum and significantly raised serum glucose concentrations during wk 1 and 2 of lactation. In addition, monensin treatment significantly reduced the loss of body condition score and decreased serum activity of aspartate aminotransferase during the postpartum period. Concentrations of serum urea were significantly higher during wk 2 and 3 postpartum for the cows that were treated with monensin. Monensin treatment had no effect on the concentrations of calcium, phosphorus, or total protein.  相似文献   
908.
The p21 protein is a universal inhibitor of cyclin-dependent kinases and of cell-cycle progression and is involved in numerous growth-inhibitory pathways in cell culture systems. Recent studies suggest that p21 regulates hepatocyte cell cycle progression in models of liver regeneration. The present study was designed to investigate the possible involvement of p21 in the control of hepatocyte proliferation in human liver diseases. To examine that, the expression of p21 in clinical liver biopsy specimens was determined by immunohistochemistry. This was correlated with hepatocyte Ki-67 immunostaining (a marker of hepatocyte proliferation in vivo) as well as histologic features. Little p21 or Ki-67 expression was detected in normal human liver or in specimens of nonalcoholic steatohepatitis. In patients with alcoholic hepatitis, increased expression of p21, but not of Ki-67, was observed. In specimens with chronic hepatitis C, hepatocyte p21 expression was significantly correlated with Ki-67 immunostaining, as well as with the degree of inflammation and fibrosis. These results indicate that hepatocyte p21 expression is upregulated in response to hepatic injury and correlates with histologic markers of proliferation and disease activity. This study provides evidence that p21 plays a role in the regulation of hepatocyte proliferation in human liver diseases.  相似文献   
909.
The primary cellular receptor for mouse hepatitis virus (MHV), a murine coronavirus, is MHVR (also referred to as Bgp1a or C-CAM), a transmembrane glycoprotein with four immunoglobulin-like domains in the murine biliary glycoprotein (Bgp) subfamily of the carcinoembryonic antigen (CEA) family. Other murine glycoproteins in the Bgp subfamily, including Bgp1b and Bgp2, also can serve as MHV receptors when transfected into MHV-resistant cells. Previous studies have shown that the 108-amino-acid N-terminal domain of MHVR is essential for virus receptor activity and is the binding site for monoclonal antibody (MAb) CC1, an antireceptor MAb that blocks MHV infection in vivo and in vitro. To further elucidate the regions of MHVR required for virus receptor activity and MAb CC1 binding, we constructed chimeras between MHVR and other members of the CEA family and tested them for MHV strain A59 (MHV-A59) receptor activity and MAb CC1 binding activity. In addition, we used site-directed mutagenesis to introduce selected amino acid changes into the N-terminal domains of MHVR and these chimeras and tested the abilities of these mutant glycoproteins to bind MAb CC1 and to function as MHV receptors. Several recombinant glycoproteins exhibited virus receptor activity but did not bind MAb CC1, indicating that the virus and MAb binding sites on the N-terminal domain of MHVR are not identical. Analysis of the recombinant glycoproteins showed that a short region of MHVR, between amino acids 34 and 52, is critical for MHV-A59 receptor activity. Additional regions of the N-terminal variable domain and the constant domains, however, greatly affected receptor activity. Thus, the molecular context in which the amino acids critical for MHV-A59 receptor activity are found profoundly influences the virus receptor activity of the glycoprotein.  相似文献   
910.
PURPOSE: To compare the analgesic efficacy and safety of nonpreserved ketorolac tromethamine 0.5% with those of its vehicle in the treatment of postsurgical ocular pain following radial keratotomy. METHODS: This study employed a multicenter, double-masked, randomized, parallel-group design. Radial keratotomy patients were treated with either nonpreserved ketorolac tromethamine 0.5% or its vehicle four times daily for up to 3 days following surgery. Patients were provided with an escape medication (acetaminophen) for use only as needed for intolerable pain. RESULTS: Patients treated with ketorolac reported significantly greater pain relief (P < or =.023), less pain intensity (P < or =.047), less use of escape medication (P < or =.001), fewer symptoms of ocular discomfort (P=.024), and fewer sleep disturbances (P < or =.013) than did patients treated with vehicle. No treatment-related adverse events were reported in the ketorolac group, and only one treatment-related adverse event was reported in the vehicle group. Most other safety findings were equivalent in the two treatment groups except that there were significantly less eyelid erythema (P=.026) and eyelid edema (P < or =.001) in the ketorolac group. CONCLUSIONS: Nonpreserved ketorolac tromethamine 0.5% ophthalmic solution was significantly more effective than, and as safe as, vehicle in the treatment of postoperative pain associated with radial keratotomy. Therefore, topical ketorolac may be a valuable treatment option for the maintenance of patient comfort following refractive surgery.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号