首页 | 官方网站   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   3060篇
  免费   55篇
  国内免费   1篇
工业技术   3116篇
  2023年   6篇
  2022年   6篇
  2021年   3篇
  2020年   23篇
  2019年   15篇
  2018年   14篇
  2017年   26篇
  2016年   13篇
  2015年   12篇
  2014年   7篇
  2013年   13篇
  2012年   30篇
  2011年   33篇
  2010年   17篇
  2009年   8篇
  2008年   19篇
  2007年   17篇
  2006年   15篇
  2005年   11篇
  2004年   8篇
  2003年   5篇
  2002年   2篇
  2001年   3篇
  2000年   5篇
  1999年   87篇
  1998年   915篇
  1997年   492篇
  1996年   333篇
  1995年   184篇
  1994年   139篇
  1993年   148篇
  1992年   18篇
  1991年   43篇
  1990年   32篇
  1989年   33篇
  1988年   33篇
  1987年   44篇
  1986年   36篇
  1985年   20篇
  1984年   1篇
  1983年   3篇
  1982年   10篇
  1981年   16篇
  1980年   20篇
  1979年   1篇
  1978年   2篇
  1977年   56篇
  1976年   133篇
  1975年   5篇
  1955年   1篇
排序方式: 共有3116条查询结果,搜索用时 890 毫秒
51.
52.
53.
The paper presents the results of complex radiation diagnosis of 118 cases of retroperitoneal tumors and tumor-like states, including 71 patients with tumors of mesenchymal origin, 7 with those of neurogenic origin, 6 with cysts, 9 with lymphogranulomatosis, 8 with retroperitoneal fibrosis, 12 with metastases, and 5 with hematomas. A wide complex of routine and up-to-date high-technology radiation diagnostic techniques was used to study the patients. The authors show the merits and dismerits of each technique. Greatest attention is given to the assessment of current techniques, primarily to ultrasonography (US) and computed tomography (CT). The studies have indicated that despite some specific features, many retroperitoneal tumors and tumor-like states bear similarities when various radiation methods are employed. The authors describe some syndromes which can be used to make a diagnosis. Despite the high capacities of US and CT in studying the normal and pathological structures of the human body, there are still great difficulties in differentiating different abnormalities when even the latest techniques are applied, these cases need paracentic aspiration or open biopsy.  相似文献   
54.
OBJECTIVES: The Physician Orders for Life-Sustaining Treatment (POLST), a comprehensive, one-page order form, was developed to convey preferences for life-sustaining treatments during transfer from one care site to another. This study examined the extent to which the POLST form ensured that nursing home residents' wishes were honored for Do Not Resuscitate (DNR) and requests for transfer only if comfort measures fail. DESIGN: The study used chart record data to follow prospectively a sample of nursing home residents with the POLST. SETTING: Eight geographically diverse, long-term, adult-care facilities in Oregon in which the POLST was in use. PARTICIPANTS: Nursing home residents (n = 180), who had a POLST recording DNR designation and who indicated a desire for transfer only if comfort measures failed, were followed for 1 year. MEASUREMENTS: For all subjects: treatment and disposition after significant health status changes; orders for narcotics and for provision or limitation of aggressive interventions. For hospitalized subjects: diagnosis, medical interventions, and DNR orders. For those who died: cause and location of death, life-sustaining treatments attempted, and comfort measures provided. RESULTS: No study subject received CPR, ICU care, or ventilator support, and only 2% were hospitalized to extend life. Of the 38 subjects who died during the study year, 63% had an order for narcotics, and only two (5%) died in an acute care hospital. A total of 24 subjects (13%) were hospitalized during the year. Hospitalized subjects' mean length of stay was 4.9 days, and the mean rate of hospitalizations for all subjects was 174 per 1000 resident years. In 85% of all hospitalizations, patients were transferred because the nursing home could not control suffering. In 15% of hospitalizations (n = 4), the transfer was to extend life, overriding POLST orders. CONCLUSIONS: POLST orders regarding CPR in nursing home residents in this study were universally respected. Study subjects received remarkably high levels of comfort care and low rates of transfer for aggressive life-extending treatments.  相似文献   
55.
Two geographic races of the house musk shrew (Suncus murinus) were crossed and intercrossed in the laboratory. Many cases of male sterility were detected among the hybrids. Segregation analysis of the pedigree data showed that the inheritance of male sterility in interracial hybrids of S. murinus can be described within the framework of monogene polyallele model with sterility of a single allele combination. This model is similar if not identical to that proposed by Dobzhansky and Muller.  相似文献   
56.
In adult life, the type 2 isozyme of 11beta-hydroxysteroid dehydrogenase (11betaHSD2) protects the mineralocorticoid receptor (MR) from glucocorticoid by inactivating cortisol to cortisone. 11betaHSD2 activity has been reported in human fetal tissues, where glucocorticoids may impair fetal growth yet are also required for normal fetal development. Using digoxigenin-labeled complementary ribonucleic acid (RNA) probes and an in-house 11betaHSD2 antiserum, we have analyzed the expression of 11betaHSD2, MR, and glucocorticoid receptor (GR) in human fetal tissues of gestational age 6-17 weeks (n=15). 11BetaHSD2 expression was absent at gestational age 6+ weeks, but was expressed in abundance in many fetal tissues between 8-12 weeks. At this time, 11betaHSD2 colocalized with GR messenger RNA (mRNA) expression in metanephros, gut, muscle, spinal cord and dorsal root ganglia, periderm, sex chords of testis, and adrenal. In particular within fetal kidney, intense expression of 11betaHSD2 and GR mRNA was observed over Bowman's capsule and the vascular tufts of developing glomeruli as they migrated from the surface of the kidney to the inner cortex. Only lung and adrenal medullary rests demonstrated high levels of GR mRNA but low levels of 11betaHSD2. 11BetaHSD2 mRNA and immunoreactivity staining patterns were similar, with the exception of the fetal adrenal, where mRNA was localized to the outer definitive zone but immunoreactivity was localized to the inner fetal zone. Colocalization of 11betaHSD2 (and GR mRNA) with MR mRNA was observed principally within epithelial cells of collecting ducts, particularly after 16 weeks gestation when the pattern of distribution of 11betaHSD2 became more adult in nature. High levels of MR mRNA were observed within developing bone. The data indicate that 11betaHSD2 in fetal life principally modulates ligand access to the GR in most fetal tissues, notably glomeruli and tubules in the developing kidney, testis, and periderm, and this may be have ramifications for fetal sodium homeostasis and differentiation. The development of tissues previously shown to have a critical requirement for glucocorticoids, such as lung and adrenal medulla, is facilitated by the expression of GR mRNA, but not 11betaHSD2. The expression of MR mRNA in high abundance in bone suggests a role for corticosteroids in human bone development, and the low/absent expression of 11betaHSD2 at this site suggests that it is functionally acting as a GR.  相似文献   
57.
Nucleotide sequences from two nuclear loci, alcohol dehydrogenase and internal transcribed spacer-1 of the nuclear ribosomal DNA repeats, and two mitochondrial genes, cytochrome oxidase I and cytochrome oxidase II, were determined from nine species in the Drosophila saltans species group. The partition homogeneity test and partitioned Bremer support were used to measure incongruence between phylogenetic hypotheses generated from individual partitions. Individual loci were generally congruent with each other and consistent with the previously proposed morphological hypothesis, although they differed in level of resolution. Since extreme conflict between partitions did not exist, the data were combined and analyzed simultaneously. The total evidence method gave a more resolved and highly supported phylogeny, as indicated by bootstrap proportions and decay indices, than did any of the individual analyses. The cordata and elliptica subgroups, considered to have diverged early in the history of the D. saltans group, were sister taxa to the remainder of the saltans group. The sturtevanti subgroup, represented by D. milleri and D. sturtevanti, occupies an intermediate position in this phylogeny. The saltans and parasaltans subgroups are sister clades and occupy the most recently derived portion of the phylogeny. As with previous morphological studies, phylogenetic relationships within the saltans subgroup were not satisfactorily resolved by the molecular data.  相似文献   
58.
The electroencephalographic abnormalities seen in Landau-Kleffner syndrome (LKS) (language deterioration) are non-specific, and consist of a variety of epileptiform discharge patterns including continuous slow spike-wave discharges during sleep, focal sharp waves with spikes, and centrotemporal (rolandic) spikes. Similarly, the EEG abnormalities seen in autistic epileptiform regression (language and social/behavioral deterioration) are non-specific and overlap with those seen in LKS. By contrast, distinct epilepsy syndromes in otherwise normal children occur in the EEG-defined benign focal epilepsies of childhood. Occipital spikes or spike-wave present either in the older child with visual symptoms and headache or in the younger child with autonomic symptoms followed by brief or prolonged partial motor seizures. Seven young children (five from a consecutive series of 42) presenting clinically with autism or autistic regression and possible or definite seizures, whose EEGs revealed occipital spikes or spike-wave characteristic of the benign epilepsies, are reported. Although occipital spikes are commonly seen in young children as an age-dependent EEG-defined benign focal epilepsy, their high frequency in this population with cognitive difficulties suggests a possible causal relation. The effects of the epileptiform discharge on cognitive functioning presumably reflect extension into temporal and parietal lobes, rather than occipital disturbances per se.  相似文献   
59.
60.
The congenital bilateral perisylvian syndrome is characterized by pseudobulbar palsy, moderate delay in mental and motor development and epilepsy. Three characteristic case stories are presented. Epileptic seizures are most frequently generalized: tonic, astatic, atypical absences and tonic-clonic seizures. Partial seizures are less frequent. Seizure control is often unsatisfactory. Neuroimaging demonstrates thickening of the cerebral cortex in the perisylvian area bilaterally; these changes together with the clinical picture establish the diagnosis. The etiology is unknown.  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-23

京公网安备 11010802026262号