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   3564篇
  免费   5篇
电工技术   12篇
综合类   2篇
化学工业   49篇
金属工艺   1篇
机械仪表   6篇
建筑科学   18篇
矿业工程   1篇
能源动力   2篇
轻工业   19篇
无线电   34篇
一般工业技术   28篇
冶金工业   3380篇
自动化技术   17篇
  2022年   2篇
  2021年   4篇
  2020年   5篇
  2018年   2篇
  2017年   6篇
  2016年   3篇
  2015年   2篇
  2014年   6篇
  2013年   8篇
  2012年   4篇
  2011年   2篇
  2010年   6篇
  2009年   5篇
  2008年   3篇
  2007年   6篇
  2006年   15篇
  2005年   9篇
  2004年   6篇
  2003年   12篇
  2002年   4篇
  2000年   2篇
  1999年   107篇
  1998年   1005篇
  1997年   601篇
  1996年   383篇
  1995年   247篇
  1994年   175篇
  1993年   212篇
  1992年   34篇
  1991年   65篇
  1990年   53篇
  1989年   67篇
  1988年   60篇
  1987年   50篇
  1986年   34篇
  1985年   37篇
  1984年   2篇
  1983年   10篇
  1982年   21篇
  1981年   16篇
  1980年   20篇
  1979年   3篇
  1978年   5篇
  1977年   95篇
  1976年   131篇
  1975年   8篇
  1973年   2篇
  1972年   3篇
  1970年   2篇
  1968年   3篇
排序方式: 共有3569条查询结果,搜索用时 15 毫秒
71.
BACKGROUND: This study examined the results of surgical treatment of leiomyosarcoma of the esophagus. METHODS: Between January 1920 and December 1996, 17 patients (9 men and 8 women) with leiomyosarcoma of the esophagus were treated surgically at the Mayo Clinic. Median age was 58 years and ranged from 26 to 76 years. Symptoms included dysphagia in 11 patients (64.7%) and odynophagia in 6 (35.3%). The tumor was located in the middle third of the esophagus in 10 patients (58.8%) and in the cervical esophagus in 7 (41.2%). Procedures performed included esophagogastrectomy in 9 patients (Ivor Lewis in 5, left thoracoabdominal in 3, and transhiatal in 1), enucleation in 3, transgastric excision in 1, and exploration without resection in 4. RESULTS: The procedure was considered curative in 11 patients (64.7%). There was one operative death (mortality, 5.9%). Complications occurred in 3 patients (17.6%) and included anastomotic leak in 2 and bleeding requiring reoperation in 1. Growth pattern was infiltrating in 7, polypoid in 5, and intramural in 5. Histologically, the tumor was grade 1 in 6 patients, grade 2 in 2, grade 3 in 7, and grade 4 in 2. The tumor was postsurgically classified as stage I in 2 patients, stage IIA in 7, stage IIB in 1, stage IIIA in 5, stage IV in 1, and unknown in 1. Six patients (35.3%) received adjuvant treatment. Follow-up was complete in 16 patients (94.1%) and ranged from 1 to 182 months (median, 48 months). Five- and 10-year actuarial survivals were 47.0% and 31.0%, respectively. Seven patients (41.2%) are currently alive (median survival, 72 months); all underwent curative resection. Factors affecting survival included completeness of resection, growth pattern, postsurgical stage, tumor grade, and tumor location (p < 0.05). CONCLUSIONS: We conclude that leiomyosarcoma of the esophagus is rare. Complete resection provides long-term survival.  相似文献   
72.
PURPOSE: During radical prostatectomy for prostate cancer tumor at the surgical margin is a relatively frequent finding. We summarize the literature on the incidence, etiology, location, prevention and treatment of positive surgical margins after radical prostatectomy. MATERIALS AND METHODS: The literature was reviewed for data on positive margins during radical prostatectomy for prostate cancer. RESULTS: Positive surgical margins may result from artifacts induced by tissue processing, incising inadvertently into the prostate or incising into extraprostatic tumor that has extended beyond the limits of resection. Patients with 10 ng./ml. or greater preoperative prostate specific antigen, biopsy Gleason score 7, multiple positive biopsies, or clinical stage T2b, T2c or T3 cancer have a higher risk of positive margins. Preoperative endorectal magnetic resonance imaging may be useful in staging a select group of patients. Neoadjuvant androgen deprivation reduces the incidence of positive margins but does not appear to delay progression or improve survival. The surgical approach, retropubic or perineal, may influence the location and etiology of positive margins. In general, nerve and bladder neck sparing procedures do not compromise tumor removal in appropriately selected patients. Positive margins increase the risk of progression and correlate with decreased cancer specific and overall survival. There is no consensus on the management of positive margins. External beam radiation and androgen deprivation may be administered as adjuvant therapy or at the time of recurrence. CONCLUSIONS: Tumor at the specimen edge is an adverse prognostic factor. With appropriate patient selection and meticulous surgical technique some positive margins can be prevented. Controlled prospective randomized studies of postoperative therapy are needed before definitive recommendations can be made for treating positive margins.  相似文献   
73.
The gene rafY from the plasmid pRSD2, which enables Escherichia coli to grow on raffinose, was transferred into expression plasmid pUSL77. The protein was expressed in the porin-deficient Escherichia coli strain KS26 and was isolated and purified to homogeneity. The pure protein was reconstituted into lipid bilayer membranes. It formed an ion-permeable channel with a single-channel conductance of 2.9 nS of the open state in 1 M KCl, which is approximately twice of that of the general diffusion pores OmpF and OmpC of E. coli outer membrane. At lower pH the channel exhibited rapid flickering between three substates of the open channel. The RafY channel appears to be wide and water filled and has a small selectivity for cations over anions. Although RafY is part of an uptake and fermentation system for raffinose it does not contain a binding site for carbohydrates. Our results suggest that RafY is a general diffusion pore with a diameter, larger than that of the general diffusion porins OmpF and OmpC, that allows the diffusion of high-molecular-mass carbohydrates through the outer membrane.  相似文献   
74.
Segments 20-22 of alpha-spectrin and 1-3 of beta-spectrin are required for high avidity interchain binding at the tail-end of the molecule. Here, sequence analysis guided by the crystal structure of spectrin's repeating segments was used to redefine the boundaries of a repetitive beta segment that is critical for interchain binding and demonstrate the contribution of non-repetitive spectrin segments in high avidity interchain binding. Our results show that several motifs together are required for high avidity binding, indicating that interchain binding at the tail-end of the spectrin molecule depends on the long distance coordination of several different elements. We also explored the role of unusual motifs contained in beta segments involved in interchain binding. A row of basic residues and a row of small hydrophobic residues were found not to be required for interchain binding, suggesting that their conservation among species reflects functions unrelated to interchain binding. The octamer between segments beta 2 and beta 3 that maintains a specific register between true binding sites was found to have an indirect role in interchain binding by stabilizing neighboring segments. A 5-residue domain in segment beta 2 (EKPPK) was required for interchain binding because it sustains normal helix-helix interactions within segments beta 2.  相似文献   
75.
We present the results of a meta-analysis using clinical data obtained from seven of our patients and 24 previously reported patients with idiopathic thrombocytopenic purpura complicated by intracranial hemorrhage. Twenty-four had an intracerebral hemorrhage (ICH) and seven had a subdural hematoma (SDH). Mean age of the patients with ICH was significantly younger than those with SDH. The mortality rate of ICH associated with ITP was similar to that of spontaneous ICH. All seven patients with SDH improved without sequelae.  相似文献   
76.
77.
Twenty-nine selected styles of subprimals or sections of veal were obtained from a commercial facility to assist in the development of a support program for retailers. They were fabricated into bone-in or boneless retail cuts and associated components by trained meat cutters. Each style selected (n = 6) was used to generate mean retail yields and labor requirements, which were calculated from wholesale and retail weights and processing times. Means and standard errors for veal ribs consisting of five different styles (n = 30) concluded that style #2, 7-rib 4 (10 cm) x 4 (10 cm), had the lowest percentage of total retail yield (P < .05) owing to the greatest percentage of bone. Furthermore, rib style #2 required the longest total processing time (P < .05). Rib styles #3, 7-rib chop-ready, and #5, 6-rib chop ready, yielded the greatest percentage of total retail yield and also had the shortest total processing time (P < .05). Within veal loins, style #2, 4 (10 cm) x 4 (10 cm) loin kidney fat in, had the greatest percentage fat (P < .05). Loin styles #2 and #3, 4 (10 cm) x 4 (10 cm) loin special trimmed, generated more lean and fat trimmings and bone, resulting in lower percentage of total retail yields than loin style #1, 0 (0 cm) x 1 (2.5 cm) loin special trimmed (P < .05). Results indicated that bone-in subprimals and sections required more processing time if fabricated into a boneless end point. In addition, as the number of different retail cuts increased, processing times also increased.  相似文献   
78.
The present study has examined the distribution of axons of differing sizes in the optic pathway of the ground squirrel. Axon diameters were measured from electron micrographs at various locations across sections of the optic nerve and tract, and total distributions and numbers were estimated. In both the nerve and tract, roughly 1.2 million optic axons were present. The population of optic axons had a unimodal size distribution, peaking at 0.9 microm in diameter and having an extended tail toward larger diameters. Local axon diameter distributions in the optic tract indicated distinct (though partially overlapping) axon diameter classes, including one of fine sizes peaking at 0.8-0.9 microm, a second of medium sizes peaking around 1.7-1.8 microm, and a third composed of the larger fibers with diameters up to 4.8 microm. The fine-caliber axons were found at all locations in the tract, and were the only axons present immediately adjacent to the pia, while the medium- and coarse-caliber axons were found at deeper locations. Curiously, the larger axons were found primarily in the medial parts of the tract, where axons from the dorsal retina normally course. A similarly restricted distribution of the larger axons was observed in the dorsotemporal parts of the optic nerve, suggesting that this difference in the tract may relate to an asymmetric distribution of ganglion cells on the retina giving rise to these axons. Measurements of axonal size taken within the optic fiber layer in dorsal and ventral parts of the retina confirmed this asymmetry, consistent with previous demonstrations of soma size differences in the dorsal versus ventral retina. The partial segregation of axons by size in the optic tract of the ground squirrel then reflects both the asymmetric distribution of retinal ganglion cell classes and the chronotopic reordering of optic axons that occurs within the chiasmatic region.  相似文献   
79.
Magnesium (Mg) infusions over 24 hours were given to patients with suspected acute myocardial infarction (AMI) at least 2 hours after thrombolysis. Patients showed no benefit and even some increased risk in contrast to reduction in mortality obtained by Mg therapy in smaller trials. Results of all of the studies were pooled and statistically analyzed, according to a fixed-effects model that is inappropriate for studies of different protocols. The panel concluded that further study of Mg in AMI is not needed. This conclusion has been questioned.  相似文献   
80.
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