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   2790篇
  免费   6篇
综合类   1篇
化学工业   25篇
机械仪表   3篇
建筑科学   6篇
轻工业   26篇
石油天然气   2篇
无线电   5篇
一般工业技术   16篇
冶金工业   2706篇
自动化技术   6篇
  2022年   3篇
  2021年   3篇
  2020年   1篇
  2018年   2篇
  2017年   1篇
  2016年   2篇
  2015年   1篇
  2014年   3篇
  2013年   17篇
  2012年   3篇
  2008年   1篇
  2006年   1篇
  2005年   1篇
  2004年   2篇
  2003年   5篇
  2001年   2篇
  2000年   3篇
  1999年   81篇
  1998年   826篇
  1997年   479篇
  1996年   307篇
  1995年   172篇
  1994年   135篇
  1993年   175篇
  1992年   33篇
  1991年   40篇
  1990年   27篇
  1989年   34篇
  1988年   49篇
  1987年   34篇
  1986年   30篇
  1985年   27篇
  1984年   3篇
  1983年   3篇
  1982年   13篇
  1981年   18篇
  1980年   23篇
  1979年   4篇
  1978年   6篇
  1977年   50篇
  1976年   155篇
  1975年   10篇
  1974年   1篇
  1971年   1篇
  1968年   1篇
  1967年   2篇
  1966年   1篇
  1957年   2篇
  1955年   3篇
排序方式: 共有2796条查询结果,搜索用时 31 毫秒
991.
Spatial contrast sensitivity was measured in kittens aged 6, 9, and 12 months and in adult cats. Cats had to open one of two small windows, which had a photograph of a grid, in order to obtain food reinforcement. The nonreinforced stimulus was a photograph of a uniform field of the same mean luminance. Visual acuity was constant in kittens aged 6 to 12 months. However, six-month-old kittens had low contrast sensitivity at low spatial frequencies (< 0.6 cycles/degree). At the age of nine months, contrast sensitivity over this range increased, though the level seen in adult cats was reached only at the age of 12 months. It is suggested that the increase in contrast sensitivity occurring after the critical developmental period in kittens reflects maturation of higher-order cortical fields involved in the process of recognition.  相似文献   
992.
Fusobacterium nucleatum is a Gram-negative anaerobic rod implicated in the pathogenesis of periodontal disease. However, this organism has also been frequently identified in high numbers in healthy adults. These observations suggest that the species may comprise different clonal types, some of which may participate in disease. The purpose of the present investigation was to use restriction endonuclease analysis (REA) and ribotyping to characterize F. nucleatum clonal types isolated from healthy young adults with optimal plaque control and investigate the stability of some of these clonal types. A group comprising 11 dental students and 11 dental outpatients with optimal plaque control was sampled. Clonal stability was investigated by sampling the dental student group at baseline and at 16 months. One hundred and thirty-two clinical isolates of F. nucleatum were successfully recovered from 15/22 individuals. For the positive subjects, 29 different clonal types were identified by REA and ribotyping, each subject and site being colonized by 1-4 clonal types. For the dental students, 9 and 15 different clonal types were identified at baseline and 16 months, respectively. None of the students harboured identical clonal types at both sampling times. Our results show that ribotyping is a useful technique for monitoring the distributions of F. nucleatum clonal types and indicate that healthy individuals with optimal plaque control can be colonized by more than one F. nucleatum clonal type and that these clonal types appear to be unstable.  相似文献   
993.
In sea urchin eggs, fertilisation is followed by a calcium wave, cortical granule exocytosis and fertilisation envelope elevation. Both the calcium wave and cortical granule exocytosis sweep across the egg in a wave initiated at the point of sperm entry. Using differential interference contrast (DIC) microscopy combined with laser scanning confocal microscopy, populations of cortical granules undergoing calcium-induced exocytosis were observed in living urchin eggs. Calcium imaging using the indicator Calcium Green-dextran was combined with an image subtraction technique for visual isolation of individual exocytotic events. Relative fluorescence levels of the calcium indicator during the fertilisation wave were compared with cortical fusion events. In localised regions of the egg, there is a 6s delay between the detection of calcium release and fusion of cortical granules. The rate of calcium accumulation was altered experimentally to ask whether this delay was necessary to achieve a threshold concentration of calcium to trigger fusion, or was a time-dependent activation of the cortical granule fusion apparatus after the 'triggering' event. Calcium release rate was attenuated by blocking inositol 1,4,5-triphospate (InsP3)-gated channels with heparin. Heparin extended the time necessary to achieve a minimum concentration of calcium at the sites of cortical granule exocytosis. The data are consistent with the conclusion that much of the delay observed normally is necessary to reach threshold concentration of calcium. Cortical granules then fuse with the plasma membrane. Further, once the minimum threshold calcium concentration is reached, cortical granule fusion with the plasma membrane occurs in a pattern suggesting that cortical granules are non-uniform in their calcium sensitivity threshold.  相似文献   
994.
BACKGROUND: Recognizing the disparities in cancer morbidity and mortality that exist between African Americans and whites, an urban university and its neighborhood community undertook the development of an education program to transfer state-of-the-art cancer prevention, detection, and treatment information from an academic medical center to community residents, including school-age children. METHODS: An intergenerational, multilevel intervention was developed to: 1) assess the health beliefs of the community, 2) identify, develop, and train an intergenerational group of community residents who would serve as health educators, and 3) promote behavioral change among the target population. RESULTS: Ten community residents were trained as educators. Over the course of two years they conducted cancer education programs that reached 775 adults. During the same period, the school-based educational intervention reached 264 seventh-grade students. CONCLUSIONS: Implications for the design and implementation of community-based cancer education programs in this African American community are identified.  相似文献   
995.
Cavernous hemangioma is a rare tumor with infrequent cardiac involvement. Preoperative or antemortem diagnosis may be difficult. Several prior case reports have described echocardiographic findings of cavernous hemangioma. We report here a 50-year-old white female patient with this tumor. Transesophageal echocardiography detected a mass with an echocardiographic appearance not previously described for cavernous hemangioma. The tumor appeared as a large echolucent unilocular cystic mass, leading to an erroneous preoperative diagnosis of pericardial cyst. This previously unreported finding should be recognized by echocardiographers in the evaluation of cardiac masses.  相似文献   
996.
Androgen-binding protein/sex hormone-binding globulin (ABP/SHBG) is an extracellular binding protein that regulates the bioavailability of sex steroids. ABP/SHBG is closely related to the globular (G) domain of vitamin K-dependent protein S family of proteins and more distantly related to the G domains of several extracellular matrix proteins. ABP/SHBG appears to have evolved from the fusion of two ancestral G domains. Expanding evidence suggests that ABP/SHBG has other functions that are mediated through membrane binding, including signal transduction; however, the types of binding proteins (receptors) have not been identified. Sequence comparisons of ABP/SHBG with G domains of its homologs protein S, Gas6, laminin, and agrin have identified regions of ABP/SHBG that may bind receptors related to homolog receptors. These membrane receptors include beta-integrins, alpha-dystroglycan, and receptor tyrosine kinases. The G domains of laminin and related proteins have clearly evolved from a common ancestor to interact with specific receptors and binding proteins. It remains to be determined if ABP/SHBG followed this evolutionary pathway.  相似文献   
997.
Dihydropteroate synthase (H2Pte synthase) is the target of the sulfur-based antimalarial drugs, which are frequently used in synergistic combination with inhibitors of dihydrofolate reductase (H2folate reductase) to combat chloroquine-resistant malaria. We have isolated the H2Pte synthase coding sequence of the most pathogenic human parasite Plasmodium falciparum. It forms part of a longer coding sequence, located on chromosome 8, that also specifies 6-hydroxymethyl-7,8-dihydropterin pyrophosphokinase (CH2OH-H2pterinPP kinase) at its 5' proximal end. This domain is unusually large, with two long insertions relative to other CH2OH-H2pterinPP kinase molecules. To investigate a possible genetic basis for clinical resistance to sulfa drugs, we sequenced the complete H2Pte synthase domains from eleven isolates of P. falciparum with diverse geographical origins and levels of sulfadoxine resistance. Overall, point mutations in five positions were observed, affecting four codons. Parasite lines exhibiting high-level resistance were found to carry either a double mutation, altering both Ser436 and Ala613, or a single mutation affecting Ala581. The mutations at positions 436 and 581 have the same location relative to each of two degenerate repeated amino acid motifs that are conserved across all other known H2Pte synthase molecules. The amino acid alteration at residue 613 is identically positioned relative to a different conserved motif. The fourth amino acid residue (437) affected by mutation, though adjacent to the apparently crucial residue 436, shows no obvious correlation with resistance. Although these mutations have no exact counterparts in any other organism, that at position 581 falls within a region of three amino acids where H2Pte synthase is modified in various ways in a number of sulfonamide-resistant pathogenic bacteria. Copy-number analysis indicated that there was no amplification of the H2Pte synthase domain in resistant parasite lines of P. falciparum, compared to sensitive lines.  相似文献   
998.
A retrospective study was performed of 30 women who had undergone breast reduction and subsequently wished to breastfeed. Breastfeeding capabilities were assessed by a trained lactation consultant. Findings indicate that in women who have a physiological type of operation then breastfeeding is usually possible (18 patients out of 19), although complementary feeds may be required. We strongly suggest that all functioning breast tissue that remains after reduction mammaplasty be left attached to the nipple in a physiological manner to allow subsequent breastfeeding.  相似文献   
999.
OBJECTIVE: To observe the features of the changes of color pattern electroretinogram (CP-ERG) in early primary open-angle glaucoma (POAG). METHOD: The detection and analyses of CP-ERGs in 29 patients (48 eyes) with POAG, 10 patients (16 eyes) with ocular hypertension and 33 age-matched normal volunteers (48 eyes) were made. RESULTS: The changes of CP-ERG in early POAG included reduction of amplitudes of P1 and N2 waves, prolongation of the peak times of red/black N2 and blue/black P1 waves. Part of the patients with ocular hypertension had the amplitude of N2 wave reduced. Five factors including the amplitude of blue/black N2 wave, the amplitudes of red/black P1 and N2 waves, the peak time of red/black N2 wave and the white/black P1 to N2 wave amplitude ratio from CP-ERG which are significantly correlated to the diagnosis of early POAG were selected by the multiple factor stepwise discriminatory analysis, and the stepwise discriminatory function was set up. CONCLUSIONS: CP-ERG is a useful method in detecting the retinal functions in early POAG, and is helpful to its early diagnosis.  相似文献   
1000.
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