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   1557篇
  免费   2篇
化学工业   8篇
金属工艺   2篇
机械仪表   1篇
能源动力   4篇
轻工业   3篇
无线电   14篇
一般工业技术   4篇
冶金工业   1513篇
原子能技术   1篇
自动化技术   9篇
  2021年   1篇
  2019年   2篇
  2018年   4篇
  2017年   1篇
  2015年   3篇
  2014年   1篇
  2012年   1篇
  2011年   1篇
  2009年   1篇
  2008年   3篇
  2007年   2篇
  2006年   4篇
  2005年   2篇
  2003年   3篇
  2002年   1篇
  2001年   1篇
  1999年   49篇
  1998年   526篇
  1997年   265篇
  1996年   175篇
  1995年   92篇
  1994年   78篇
  1993年   110篇
  1992年   14篇
  1991年   20篇
  1990年   14篇
  1989年   15篇
  1988年   16篇
  1987年   10篇
  1986年   13篇
  1985年   11篇
  1984年   1篇
  1983年   3篇
  1982年   9篇
  1981年   8篇
  1980年   7篇
  1977年   36篇
  1976年   52篇
  1975年   3篇
  1956年   1篇
排序方式: 共有1559条查询结果,搜索用时 31 毫秒
991.
OBJECTIVE: We tested the hypothesis that serum free (nonesterified) fatty acid and triglyceride concentrations are increased in nulliparous women with preeclampsia relative to women with uncomplicated pregnancies and that these lipids decrease post partum, consistent with the known resolution of clinical symptoms. The relationships between serum concentrations of these lipids and the lipid peroxidation metabolite malondialdehyde were also examined. STUDY DESIGN: Predelivery and 24 to 48 hour postpartum venous blood samples were collected from eight women with preeclampsia and nine women with uncomplicated pregnancies after an 8- to 10-hour fast. Sera were analyzed for concentrations of triglycerides, free fatty acids, total cholesterol, high-density lipoprotein cholesterol, low-density lipoprotein cholesterol, and malondialdehyde. RESULTS: Antepartum serum triglyceride and free fatty acid concentrations were increased approximately twofold in women with preeclampsia relative to uncomplicated pregnancies (p <0.02 and 0.004, respectively). Total, high-density lipoprotein, and low-density lipoprotein cholesterol concentrations did not differ between groups. Concentrations of all lipids decreased significantly in both groups within 48 hours post partum. However, triglyceride and free fatty acid concentrations remained higher in women with preeclampsia (p<0.006, both variables). Triglyceride and free fatty acid concentrations correlated positively, both ante partum (R2 0.42, p<0.01) and post partum (R2 0.39, p<0.02). Antepartum concentrations of malondialdehyde were 50% higher in women with preeclampsia (p<0.01) and decreased post partum (p <0.02) but did not decrease in controls (p = 0.07). Antepartum serum triglycerides and free fatty acids correlated positively with malondialdehyde concentrations (R2 0.38, p <0.02, both cases). CONCLUSION: Triglycerides and free fatty acids, but not cholesterol, are increased in preeclampsia and correlate with the lipid peroxidation metabolite malondialdehyde. We speculate that these interactions may contribute to endothelial cell dysfunction in preeclampsia.  相似文献   
992.
High-dose megestrol acetate has been reported to be effective salvage therapy for women with ovarian carcinoma. The Eastern Cooperative Oncology Group performed this phase II study of oral megestrol acetate, 200 mg four times daily until disease progression, in 33 patients either with stage III or IV histologically confirmed ovarian carcinoma or with unresectable tumor in the pelvis with measurable or evaluable disease who progressed after treatment with one prior chemotherapy regimen. Thirty and 31 patients were evaluable for response and toxicity, respectively. No patient had an objective response and none had subjective improvement after a median treatment period of 1.4 months. Nausea or vomiting occurred in most patients, usually grade 1-2. Megestrol acetate is ineffective salvage therapy for patients with inoperable, previously treated ovarian carcinoma.  相似文献   
993.
When the Japanese quail is held in constant darkness, retinal responses (ERG b-waves) increase during the animal's subjective night and decrease during its subjective day. Rod photoreceptors dominate the b-wave responses (lambdamax = 506 nm) to all stimulus intensities at night but only to those intensities below the cone threshold during the day. Above the cone threshold, cones dominate b-wave responses (lambdamax, approximately 550-600 nm) during the day regardless of the state of retinal adaptation. Apparently a circadian oscillator enables cone signals to block rod signals during the day but not at night. The ERG b-wave reflects the activity of bipolar cells that are postsynaptic to rods and cones. The ERG a-wave reflects the activity of both rods and cones. The amplitude of the isolated a-wave (PIII) changes with time of day, as does that of the b-wave, but its spectral sensitivity does not. The PIII responses are maximal at approximately 520 nm both day and night and may reflect multiple receptor mechanisms. The shift in rod-cone dominance detected with the ERG b-wave resembles the Purkinje shift of human vision but, unlike the Purkinje shift, does not require a change in ambient light intensity. The shift occurs in constant darkness, with a period of approximately 24 hr indicative of a circadian rhythm in the functional organization of the retina.  相似文献   
994.
995.
The evolution of the health system in Chile, since its beginning and during the National Health Service period is described. The transformations that this system underwent from 1980 on, during the military regime, and the changes introduced by the democratic governments from 1990 on, with the purpose of correcting distortions introduced in the previous period, are commented on. The production of the health services is compared as between the public and private sub-sectors. The indicators used are outpatient attendance per thousand insured inhabitants, use of, lab texts on the same populational basis, hospitalization rates, discharges per thousand insured and surgery rates per thousand. Differences between the two sub-systems are commented on as also are those related to user satisfaction in the two sub-systems. The chief challenges to the public system are considered as well as their possible responses to them on the basis of current policies and the solutions implemented. The present situation in the Public/Private mix in health care in Chile gives rise to the positive expectation that the public subsystem may succeed in maintaining its historical leadership in the orientation and health services offered in this country. This positive view is supported by the rigorous geographic and demographic logic of the public subsystem, the stable availability of financial resources (result of the 7% contribution for health applied on all salaries, complemented by tax funds), a not completely modern but reliable network of public health facilities and the considerable motivation shown by its staff. Some features of the Chilean health system's development are mentioned as potentially useful to the development of health systems in other countries, especially in Latin America, such as organization and management of regional health services, innovative methodologies in health service payment (diagnosed related payment, management agreements and per capita financing for primary care); the health surveilance system (especially for food and drugs) and the state regulation of private health care financers.  相似文献   
996.
The secretory immune response in saliva to colonization by Actinomyces naeslundii genospecies 1 and 2 was studied in 10 human infants from birth to 2 years of age. Actinomyces species were not recovered from the mouths of the infants until approximately 4 months after the eruption of teeth. However, low levels of secretory immunoglobulin A1 (SIgA1) and SIgA2 antibodies reactive with whole cells of A. naeslundii genospecies 1 and 2 were detected within the first month after birth. Although there was a fivefold increase in the concentration of SIgA between birth and age 2 years, there were no differences between the concentrations of SIgA1 and SIgA2 antibodies reactive with A. naeslundii genospecies 1 and 2 over this period. When the concentrations of SIgA1 and SIgA2 antibodies reactive with whole cells of A. naeslundii genospecies 1 and 2 were normalized to the concentrations of SIgA1 and SIgA2 in saliva, the A. naeslundii genospecies 1- and 2-reactive SIgA1 and SIgA2 antibodies showed a significant decrease from birth to 2 years of age. The fine specificities of A. naeslundii genospecies 1- and 2-reactive SIgA1 and SIgA2 antibodies were examined by Western blotting of envelope proteins. Similarities in the molecular masses of proteins recognized by SIgA1 and SIgA2 antibodies, both within and between subjects over time, were examined by cluster analysis and showed considerable variability. Taken overall, our data suggest that among the mechanisms Actinomyces species employ to persist in the oral cavity are the induction of a limited immune response and clonal replacement with strains differing in their antigen profiles.  相似文献   
997.
A 24-year-old man with congenital bladder diverticula leading to bilateral hydronephrosis diagnosed on radionuclide scintigraphy is described. Diagnostic findings of sonogram and CT scan are also mentioned.  相似文献   
998.
Meta-analyses aim to provide a full and comprehensive summary of related studies which have addressed a similar question. When the studies involve time to event (survival-type) data the most appropriate statistics to use are the log hazard ratio and its variance. However, these are not always explicitly presented for each study. In this paper a number of methods of extracting estimates of these statistics in a variety of situations are presented. Use of these methods should improve the efficiency and reliability of meta-analyses of the published literature with survival-type endpoints.  相似文献   
999.
A number of O-alkynyloximes of tropinone and N-methyl-4-piperidinone have been synthesized and evaluated for muscarinic activity. The affinities of these oximes were tested in homogenates of cerebral cortex, heart, and submandibulary glands from rats using [3H]pirenzepine and [3H]-N-methylscopolamine as radioligands. The oximes bind to the cortical muscarinic receptors with pKi values varying from 3 to 7. Higher binding affinities were observed for the O-alkynyl tropinone oximes than the corresponding piperidinone analogues. Binding to the muscarinic sites in the heart and submandibulary glands was also observed but with lower affinities. Good M1 subtype selectivity (10-fold or greater) was observed with some oximes (26a, 28a, 32a) at the cortical sites. These oximes also attenuated scopolamine-induced impairment of the water mask task in mice. Functional assays for M3 activity on the rat aorta showed that all oximes possessed M3 agonist action but M2 agonist activity was not observed at the endothelium-denuded rabbit aorta. Analysis of the quantitative structure-activity relationship (QSAR) indicated that the Connolly surface area is an important determinant of activity, accounting for 70% of the variation in cortical binding affinity among the oximes.  相似文献   
1000.
STUDY DESIGN: A 15-year clinical follow-up of tuberculous lesions of the lumbosacral region. OBJECTIVES: To verify the hypothesis that the lumbar lordosis and the specific biomechanics of the lumbosacral region influence and alter the healing pattern and progress of the disease when compared with their effects in other regions of the spine. SUMMARY OF BACKGROUND DATA: An estimated 2 million or more patients have active spinal tuberculosis, and the global incidence of the disease is increasing. The involvement of the lower lumbar region and the lumbosacral junction is relatively rare, with few reports in English literature. METHODS: Of a total of 304 patients forming a part of a controlled clinical trial comparing two forms of therapy in spinal tuberculosis, 53 patients had involvement of L3 and below. The following data were studied in these patients: age at start of treatment, number of vertebra involved, vertebral body loss, progress of the angle of kyphosis, and anterior and posterior growth of the involved segment during a period of 15 years. Student's t test for independent samples was used for statistical analysis. RESULTS: The fourth lumbar vertebra was the most common vertebral segment involved, and the lumbosacral junction was affected in 12 patients. The average pretreatment kyphosis was 6.4 degrees and increased to 10.2 degrees at the end of 15 years. The average kyphotio angle per vertebral body loss was 4.9 degrees, far less than in the dorsolumbar region in which kyphotic angles of 27-30 degrees have been reported. Children younger than 10 years old differed in clinical appearance and progress compared with those older than 17 years. They not only showed more extensive involvement but also had more deformity with the same vertebral loss. Twelve patients less than 10 years old had an average involvement of 3.1 vertebral bodies and an average vertebral loss of 2.2 bodies. In comparison, the average number of vertebrae involved was 1.9 (P < 0.01) and the vertebral body loss was only 0.87 (P < 0.01) in patients older than 17 years. Also, the average kyphosis was 6.4 degrees compared with only 4.2 degrees (P < 0.01) in adults. In patients older than 17 years, there was no change after 2 years, by which time the collapse was complete. Four of 12 patients less than 10 years old, showed progressive kyphosis caused by continued growth of posterior parts of the body (i.e., sequestrated hemivertebrae). CONCLUSIONS: In tuberculosis of the lumbosacral region, the development of kyphosis is minimal in patients older than 17 years, when growth has already stopped, and deformity is expressed more as foreshortening of the trunk. Children younger than 10 years old have more severe involvement with increased tendency toward greater kyphosis. They are also prone to progressive deformity through the years when the anterior growth plates are destroyed. Surgery is indicated in this group to prevent greater deformity.  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