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   7100篇
  免费   388篇
  国内免费   9篇
耳鼻咽喉   92篇
儿科学   246篇
妇产科学   210篇
基础医学   1053篇
口腔科学   365篇
临床医学   644篇
内科学   1338篇
皮肤病学   235篇
神经病学   703篇
特种医学   309篇
外科学   1064篇
综合类   53篇
一般理论   10篇
预防医学   388篇
眼科学   36篇
药学   386篇
中国医学   8篇
肿瘤学   357篇
  2021年   96篇
  2020年   68篇
  2019年   73篇
  2018年   101篇
  2017年   86篇
  2016年   102篇
  2015年   111篇
  2014年   146篇
  2013年   208篇
  2012年   328篇
  2011年   325篇
  2010年   190篇
  2009年   191篇
  2008年   306篇
  2007年   325篇
  2006年   297篇
  2005年   312篇
  2004年   268篇
  2003年   261篇
  2002年   293篇
  2001年   261篇
  2000年   224篇
  1999年   203篇
  1998年   82篇
  1997年   71篇
  1996年   67篇
  1995年   56篇
  1994年   57篇
  1993年   53篇
  1992年   116篇
  1991年   100篇
  1990年   103篇
  1989年   111篇
  1988年   97篇
  1987年   120篇
  1986年   102篇
  1985年   88篇
  1984年   77篇
  1983年   72篇
  1982年   52篇
  1978年   57篇
  1977年   51篇
  1976年   61篇
  1973年   55篇
  1971年   48篇
  1969年   49篇
  1968年   47篇
  1966年   51篇
  1933年   45篇
  1932年   47篇
排序方式: 共有7497条查询结果,搜索用时 15 毫秒
1.
2.
3.
4.
Feeding behavior in Aplysia fasciata and A. oculifera is modified by pairing the behavior with reinforcing consequences. Successful and unsuccessful attempts to transfer food from the buccal cavity to the crop act as positive and negative reinforcers, respectively. A number of changes in feeding behavior occur as a result of pairing of feeding with the negative reinforcer: feeding responses become less effective in leading to the entry of food into the buccal cavity; when food does enter the buccal cavity, it exits sooner; swallowing responses after food entry are less likely to occur; Aplysia eventually cease responding to food. Pairing successful transfer of food into the crop with feeding behavior produces opposite effects. Behavioral change is specific to pairing, as shown by lack of change when reinforcement is explicitly unpaired with feeding behavior. Behavioral change is specific to foods with a particular taste and texture; generalization to alternate foods was not observed. In spite of cessation of feeding, animals remain aroused, as shown by low response latency to alternate foods. Memory of response change persists for at least 48 hr.  相似文献   
5.
The expression of four cytochrome (cyt.) P-450 isoenzymes has been studied in preneoplastic and neoplastic lesions during the course of nitrosamine-induced hepatocarcinogenesis in the female Wistar rat. Following exposure to diethylnitrosamine (50 or 100 ppm in the drinking water) for 10 days, animals were taken sequentially, and the livers were analyzed for the evolution of adenosine triphosphatase deficient focal lesions. These lesions were subdivided into different phenotypes with regard to their cyt. P-450 isoenzyme expression using serial frozen sections. Our results demonstrate that about 40% of the adenosine triphosphatase-deficient lesions show concomitant alterations in their cyt. P-450 isoenzyme contents. Of these lesions, islets which are characterized by decreased levels of at least three cyt. P-450 isoenzymes show a dramatic increase in their volumetric fraction of liver tissue with progression of time. Although only very few lesions express this phenotype, the contribution to the volumetric fraction of islet tissue raises from about 2% at 10 weeks to about 60% at 35 weeks after cessation of diethylnitrosamine treatment. By contrast, lesions which express less than two alterations in cyt. P-450 isoenzyme levels develop relatively slowly. Similar results were obtained when animals were exposed continuously to diethylnitrosamine for a period of up to 8 weeks. Following treatment of islet-bearing animals with phenobarbital, an induction of cyt. P-450 isoenzymes and NADPH-cyt. P-450-reductase was observed within preneoplastic and neoplastic lesions. This induction was most pronounced in large, expansively growing nodules, a type of lesion which displayed decreased levels of these enzymes in livers of animals not treated with phenobarbital. The elevation of the cyt. P-450 isoenzymes disappeared within 2 to 3 weeks after cessation of inducer treatment. Our results indicate that a high proportion of rapidly growing lesions has assumed a constitutive deficiency in cyt. P-450 isoenzyme expression during nitrosamine-induced hepatocarcinogenesis. This deficiency, however, is not an irreversible quality, since individual cyt. P-450 isoenzymes can be markedly induced by treatment with an enzyme inducer like phenobarbital. Thus, the observed decrease in cyt. P-450 expression during development of malignancy does not result from alterations in the cyt. P-450 encoding structural genes but may rather be related to abnormalities in the function of regulatory systems of a higher order which may play a central role in the maintenance of cell homeostasis.  相似文献   
6.
7.
From 1930 to 1989 postoperative rupture of abdominal wound occurred in 0.42 per cent (n = 82) in the Department of Obstetrics and Gynaecology of the University of Rostock. 60 per cent of the patients were younger than 50 years. Patients of reduced general condition and anaemia were predisposed to this complication. We observed postoperative alterations of the respiratory tract in 13 per cent and alterations of the gastro-intestinal tract in 20 per cent which lead to increased intra-abdominal pressure. We observed postoperative wound rupture only in 8 cases after Pfannenstiel's incision, but all other times after median section, predominantly during the 7th and 12th postoperative day. All patients were treated by secondary suture, immediately. 8 patients came to death, three of them had no direct link to wound rupture. For prophylaxis the use of non-absorbable suture material, safety sutures, intraoperative blood and protein substitution and prevention of affections of the respiratory system and gastrointestinal tract are recommended.  相似文献   
8.
Up to 30% of patients with temporal lobe epilepsy (TLE) remain without remarkable changes in MRI. In this study we investigated the role of (1)H-MR spectroscopy ((1)H-MRS) in lateralizing the affected hemisphere in the mentioned patient group. Twenty-two consecutive patients diagnosed with TLE were investigated by high resolution MRI and (1)H-MRS. We examined the incidence and diagnostic accuracy of temporal metabolite alterations determined by Linear Combination of Model Spectra (L C Model) via water reference. Metabolite values of each hemisphere of TLE patients were compared with healthy controls. Results of metabolite alterations were related to intensive video EEG focus localization. Reduction of N-acetylaspartate + N-acetylaspartyl-glutamate (tNAA) in the affected hemisphere revealed identification in six of nine patients (66%) with unilateral TLE. Group comparison revealed a significant reduction of tNAA (6.1+/-0.8*) in the involved temporal lobe compared with controls (6.67+/-0.4*, P=0.026). Choline levels were significantly increased in the affected hemisphere (1.42+/-0.17*) compared with healthy controls (1.22+/-0.17*, P=0.035). The results of our study show that (1)H-MRS is able to identify the affected hemisphere of MRI negative TLE patients and can be used as an additive tool in multimodal focus localization.  相似文献   
9.
10.
Fetal tissues from 16 spontaneous abortions, two terminations, and one perinatal death, 18 of which were associated with maternal human parvovirus B19 infection, were examined for B19 infection by histology and in situ hybridization using a digoxigenin-labeled B19-DNA probe. In 15 spontaneous abortions and one termination, erythroblasts with intranuclear inclusions (lantern cells) reacted with B19-DNA by in situ hybridization. No internal or external fetal malformations were observed. Because 13 (86.7%) spontaneous abortions with lantern cells occurred between the 20th and 28th weeks of gestation, it is postulated that B19 infection may be a particular threat to the fetus during this stage of gestation.  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