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   2633篇
  免费   1篇
  国内免费   1篇
化学工业   4篇
轻工业   5篇
水利工程   1篇
一般工业技术   3篇
冶金工业   2619篇
自动化技术   3篇
  2021年   1篇
  2019年   2篇
  2018年   1篇
  2015年   1篇
  2008年   1篇
  2007年   1篇
  2006年   1篇
  2004年   1篇
  2003年   3篇
  1999年   75篇
  1998年   876篇
  1997年   503篇
  1996年   256篇
  1995年   166篇
  1994年   152篇
  1993年   166篇
  1992年   10篇
  1991年   19篇
  1990年   19篇
  1989年   33篇
  1988年   29篇
  1987年   21篇
  1986年   21篇
  1985年   22篇
  1983年   5篇
  1982年   12篇
  1981年   16篇
  1980年   22篇
  1978年   3篇
  1977年   55篇
  1976年   137篇
  1975年   2篇
  1955年   2篇
  1954年   1篇
排序方式: 共有2635条查询结果,搜索用时 22 毫秒
131.
Weight loss is a frequent sequela in patients with congestive heart failure and is commonly referred to as cardiac cachexia. This weight loss is unlike that seen in simple starvation because it preferentially involves the depletion of lean body mass. In addition, the presence of cardiac cachexia can have profound clinical implications for patients in terms of complications, clinical outcome, and overall cost. The mechanism for the alterations in body composition is multifactorial, but a major cause may be the cytokine-mediated host response to the underlying disease. This article reviews the syndrome of cardiac cachexia in light of recent evidence regarding the role of cytokines, as well as potential therapies.  相似文献   
132.
Mechanism of suppression of cell-mediated immunity by measles virus   总被引:2,自引:0,他引:2  
The mechanisms underlying the profound suppression of cell-mediated immunity (CMI) accompanying measles are unclear. Interleukin-12 (IL-12), derived principally from monocytes and macrophages, is critical for the generation of CMI. Measles virus (MV) infection of primary human monocytes specifically down-regulated IL-12 production. Cross-linking of CD46, a complement regulatory protein that is the cellular receptor for MV, with antibody or with the complement activation product C3b similarly inhibited monocyte IL-12 production, providing a plausible mechanism for MV-induced immunosuppression. CD46 provides a regulatory link between the complement system and cellular immune responses.  相似文献   
133.
Normal women produce small amounts of active androgens. When androgen levels are elevated, such as for example in the polycystic ovary syndrome, this is followed by the development of male physical characteristics and muscle mass, structure and function as well as android adipose tissue distribution and function. Psychological features and stress reactions also seem similar to those of men. Such women have an increased risk of developing hypertension, non-insulin-dependent diabetes mellitus and cardiovascular disease. Recent data have shown that these physical, and psychological characteristics, as well as risk of ill health, are also found in the population of women selected at random. Women in the lowest quintiles of levels of sex-hormone-binding globulin--an indicator inversely related to active androgens--are at risk of developing hypertension, non-insulin-dependent diabetes mellitus and cardiovascular mortality. The mechanism probably includes muscular insulin resistance, following a relative androgen excess. It is thus apparent that androgens, even within the highest levels of the nonselected population of women, are powerful predictors of serious disease development. The population at risk might be as large as about 20% of middle-aged women. This is an area of female disease risk which requires more attention in screening and intervention procedures.  相似文献   
134.
135.
136.
Only one molecular weight species of DNA polymerase was found in different developmental stages of the eukaryotic microorganism Dictyostelium discoideum. The molecular weight of this DNA polymerase is estimated to be about 127 000 by sucrose gradient centrifugation. The enzyme is present in all stages of growth and development, including dormant spores. All DNA polymerase activity is lost upon incubation of the crude extract with N-ethylmaleimide. The reaction properties, molecular weight and N-ethylmaleimide sensitivity of the D. discoideum DNA polymerase are similar to those of the DNA polymerase-alpha from mammalian sources.  相似文献   
137.
1. The response characteristics of visual, somatosensory, and auditory neurones in the golden hamster's superior colliculus were investigated.2. As has been noted for other mammalian species, a distinct difference between the functional organizations of the superficial and deeper layers of the superior colliculus was observed.3. Neurones in the superficial layers were exclusively visual, with small receptive-fields, and generally did not show response decrements with repeated stimulation. The sizes of the receptive-fields did not vary appreciably as a function of retinal eccentricity.4. In the deeper layers, visual receptive-fields were large, or could not be accurately delimited, and response habituation was often evident. In addition, many cells in the deeper layers of the colliculus responded only to somatosensory stimuli. Far fewer cells, which appeared to be confined to the caudal portions of the colliculus, responded to auditory stimuli. Polymodal cells were also encountered.5. Selectivity to opposing directions of movement was tested for ninety-four visual cells. Using a ;null' criterion, 27.7% of these cells were judged to be directionally selective. A distribution of the preferred directions of these cells showed a significant preference for movement with an upper-nasal component. With a statistical criterion, 60.6% of these cells were considered to show a significant asymmetry in responding to movement in opposing directions.6. Directional selectivity was also tested for ninety-two cells following acute, unilateral, lesions of the visual cortex. For the eighty cells recorded, homolateral to the ablated cortex, 27.5% were judged as directionally selective using the statistical criterion, while 12.5% were selective with the ;null' criterion. Of the twelve cells isolated in the colliculus, contralateral to the lesions, seven were judged as directionally selective with the statistical, and three with the ;null' criterion.7. The effects of visual cortical lesions upon directional selectivity appeared to be confined to cells in the superficial layers of the colliculus. It was suggested that directional selectivity of many cells in the superficial layers of the tectum of the hamster is organized cortically.8. A clear spatial correspondence was observed for the receptive-fields of visual, somatosensory, and auditory neurones.9. As has been suggested for other species, the hamster's superior colliculus appears to play an important role in orienting the animal toward visual, somatosensory, and auditory stimuli.  相似文献   
138.
1. The distribution of radioactivity among lipid classes of myelin and other subcellular brain fractions of young rats (18-21 days) was determined after in vivo injection of (3-(14)C-labelled ketone bodies, [U-(14)C] glucose or [2-(14)C] glucose. 2. The incorporation ratios (sterol/fatty acids) were 0.67, 1.48, 0.25, 0.62 and 0.54 for whole brain, myelin, mitochondria, microsomes and synaptosomes, respectively, with (3-(14)C)-labelled ketone bodies as substrate and 0.37, 0.89, 0.19, 0.34 and 0.29 with [U-(14)C] glucose as substrate. These data show that, both in whole brain and in subcellular brain fractions, acetyl groups derived from ketone bodies are used for sterol synthesis to a large extent than acetyl groups originating from glucose. 3. The specific radioactivity of cholesterol is much higher in myelin than in whole brain or in the other brain fractions, particularly after administration of labelled ketone bodies as substrate. 4. The incorporation patterns of acetoacetate and D-3-hydroxybutyrate were very similar, indicating that both ketone bodies contribute acetyl groups for lipid synthesis via the same metabolic route. 5. Our data suggest that a direct metabolic path from ketone bodies towards cholesterol exists - possibly via acetoacetyl-CoA formation in the cytosol of brain cells - and that this process is most active in oligodendrocytes.  相似文献   
139.
In 1957 California began placing responsibility for public mental health services on county programs still vary widely in emphasis and size. The authors explored determinants of counties' unequal program developments using per capita expenditures as the measure of county services. Sociodemographic variables that help explain states' social program expenditures correlate weakly or not at all with county mental health expenditures. The most powerful predictor of these expenditures is number of psychiatrists per 10,000 population. Equal per capita expenditures, however, are not a reasonable goal; these can mask wide variations in program content, efficiency, and quality of care.  相似文献   
140.
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