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   1990篇
  免费   167篇
  国内免费   78篇
耳鼻咽喉   5篇
儿科学   99篇
妇产科学   72篇
基础医学   278篇
口腔科学   27篇
临床医学   261篇
内科学   406篇
皮肤病学   28篇
神经病学   85篇
特种医学   302篇
外科学   197篇
综合类   55篇
一般理论   1篇
预防医学   180篇
眼科学   6篇
药学   153篇
中国医学   4篇
肿瘤学   76篇
  2021年   18篇
  2019年   11篇
  2018年   21篇
  2016年   26篇
  2015年   25篇
  2014年   31篇
  2013年   53篇
  2012年   63篇
  2011年   43篇
  2010年   49篇
  2009年   59篇
  2008年   63篇
  2007年   105篇
  2006年   52篇
  2005年   55篇
  2004年   49篇
  2003年   65篇
  2002年   53篇
  2001年   52篇
  2000年   53篇
  1999年   44篇
  1998年   89篇
  1997年   93篇
  1996年   59篇
  1995年   76篇
  1994年   43篇
  1993年   45篇
  1992年   49篇
  1991年   41篇
  1990年   42篇
  1989年   55篇
  1988年   47篇
  1987年   47篇
  1986年   45篇
  1985年   79篇
  1984年   34篇
  1983年   32篇
  1982年   34篇
  1981年   27篇
  1980年   36篇
  1979年   26篇
  1978年   23篇
  1977年   24篇
  1976年   23篇
  1975年   24篇
  1974年   10篇
  1973年   13篇
  1972年   15篇
  1971年   10篇
  1970年   10篇
排序方式: 共有2235条查询结果,搜索用时 31 毫秒
21.
22.
The effects of co-culture of human spermatozoa with human immortalized endometrial cells - epithelial or stromal - on sperm movement characteristics, including hyperactivation, were studied using computer- assisted sperm analysis (CASA). Epithelial and stromal cell types could be separated following 8-10 days of culture of endometrial cells originating from human biopsies. Both cell types were immortalized by the SV 40 large T antigen. Co-incubation of sperm with epithelial and stromal monolayers enhanced the rate of hyperactivation: 24.9% (P <0.05) and 17.8% (P = 0.05) versus 9.5% as control, respectively, whereas the majority of motility parameters remained unchanged. Conditioned media had no effect upon sperm parameters, including hyperactivation. Co-incubation with either monolayer was able to maintain sperm motility over a longer period than incubation in control medium alone. In four patients whose spermatozoa did not exhibit hyperactivation, co-incubation with epithelial cells, but not conditioned medium, allowed normal rates of hyperactivation (range: 6.9- 15.6%).   相似文献   
23.
24.
25.
Superoxide, an agent which attenuates the half-life of nitric oxide, is metabolized and synthesized by superoxide dismutase (SOD) and xanthine oxidase, respectively. Over the last few years much work has focused on the role of nitric oxide in human parturition. The aim of this study was to determine whether the onset of human parturition is associated with a change in the expression of copper/zinc superoxide dismutase (Cu/Zn SOD), manganese superoxide dismutase (Mn SOD) or xanthine oxidase within the uterus. Samples of myometrium, placenta, decidua and fetal membranes were obtained from women before and after the onset of labour at term. Immunocytochemistry was used to localize Cu/Zn SOD, Mn SOD and xanthine oxidase and measure SOD enzyme activity. Cu/Zn and Mn SOD-like immunoreactivity was detected in syncytiotrophoblast cells, villous stromal cells and endothelial cells of blood vessels in the placenta. In the myometrium Cu/Zn and Mn SOD were localized to myocytes and endothelial cells and to some vascular smooth muscle cells. In the fetal membranes we observed staining for Cu/Zn SOD and Mn SOD in the amnion, chorion, extravillous trophoblast and decidua. There was no difference in SOD enzyme activity or staining intensity for SOD between different cell types before and during labour. Xanthine oxidase immunoreactivity was identified in each of the tissues examined and again there was no difference in immunostaining in tissues obtained from women delivered before or after the onset of labour. These results show that the pregnant uterus is capable of both synthesizing and degrading superoxide and suggest that superoxide dismutase and xanthine oxidase may play a role in the maintenance of uterine quiescence during pregnancy, but not in the initiation of parturition.   相似文献   
26.
Oxygen radical generation is known to be detrimental to sperm function, especially motility, through the lipid peroxidation of the membranes. Generation of reactive oxygen species can be induced by leukocyte contamination, sperm centrifugation and the presence of abnormal spermatozoa with excess residual cytoplasm. This study aims to evaluate the effect on sperm motility of incubation in an antioxidant-containing solution, during liquefaction and centrifugation. Thirty semen samples were each divided into two equal parts: one mixed with Tyrode's solution, the other with a salt solution containing antioxidants (Sperm- Fit; Ellios Bio-Media, Paris, France). All the procedures were identical in the two groups. The ratio of leukocytes to spermatozoa was significantly correlated with the motility after liquefaction and after a 24 h incubation in routine in-vitro fertilization (IVF) medium and with the number of motile spermatozoa recovered after Percoll preparation. Moreover, when this ratio was > or = 0.2, all motility parameters were lowered. Incubation with Sperm-Fit allowed a higher percentage of motility after Percoll preparation when the ratio was > or = 0.2 (48 +/- 5% versus 41 +/- 6% for Sperm-Fit and Tyrode's solution respectively; P < 0.05) and a greater number of motile spermatozoa recovered after Percoll preparation, whatever the ratio (3.2 +/- 1.0 x 10(6) versus 2.4 +/- 0.7 x 10(6) for Sperm-Fit and Tyrode's solution respectively when ratio > or = 0.2; 18.1 +/- 3.4 x 10(6) versus 14.4 +/- 2.9 x 10(6) for Sperm-Fit and Tyrode's solution respectively when ratio < 0.2; P < 0.05). These results show that incubation with antioxidants during liquefaction and centrifugation increases recovery of motile spermatozoa.   相似文献   
27.
BACKGROUND. About one third of all pregnancies are unplanned and 20% of all pregnancies end in abortion. More than 170,000 legal abortions are performed in the United Kingdom annually. Nearly all general practitioners provide contraceptive advice; the most commonly used form of reversible contraception is the oral contraceptive pill. AIM. The aim of this study was to determine factors associated with women's knowledge of taking the contraceptive pill correctly and of emergency contraception, and to investigate if their knowledge could be improved in general practice by providing women with Family Planning Association information leaflets. METHOD. An uncontrolled intervention study was performed in one rural and one urban English general practice, using a self-completion questionnaire that was initially administered to women attending their general practitioner for oral contraception over six months from 1 October 1992. The questionnaire asked for: sociodemographic information; knowledge of how late women can be taking an oral contraceptive pill and still be protected against unplanned pregnancy; for how many days after being late with a pill they need to use other precautions; sources and methods of emergency contraception; and for how long the methods are effective after the primary contraceptive failure. After completing the questionnaire women were given two leaflets: one about how to take their prescribed contraceptive pill correctly and one about emergency contraception. Three to 12 months later the same questionnaire was administered in the same manner. RESULTS. Of 449 women completing the first questionnaire, 233 (52%) completed the second questionnaire. Initially 71% of 406 women taking an oestrogen/progestogen combined pill knew about the '12-hour rule' and 17% knew about the 'seven-day rule'; giving women information about the pill they were taking increased the extent of knowledge about these rules among 212 respondents to 82% (P < 0.01) and to 25% (P < 0.05), respectively. The proportion of respondents who knew that they could obtain emergency contraception from their own general practitioner, from any general practitioner and from family planning clinics all increased after they had received the leaflets (from 84% to 92%, from 34% to 47% and from 82% to 90%, respectively, all P < 0.01). There were significant improvements in the proportion of women knowing the duration of effectiveness of emergency contraception. However, after receiving the leaflet on emergency contraception the majority of women still did not know for how long after unprotected intercourse the high-dose combined pill and the intrauterine contraceptive device were effective (80% and 93% of 233 women, respectively). Improvements in knowledge depended upon women's social class, previous use of emergency contraception and with which practice they were registered. CONCLUSION. Providing women with leaflets about taking the contraceptive pill correctly and about emergency contraception appears to improve significantly their extent of such knowledge. If such practice was adopted elsewhere this increased knowledge might reduce the number of unplanned pregnancies in the UK. The effect of general practitioners personally providing such leaflets, with or without verbal instruction, warrants further study.  相似文献   
28.
The lipopolysaccharide (LPS) O-antigen side chains of Klebsiella serotype O1 have been studied by using mutants selected by resistance to a Klebsiella bacteriophage designated O1-A. Two classes of LPS mutants were identified. The major group (90%) synthesized rough LPS. The remaining 10% of the mutants produced a novel LPS profile that lacked the highest-molecular-weight O-substituted molecules (HMW-LPS) but still produced lower-molecular-weight O-substituted species (LMW-LPS). By using antisera raised against mutant Klebsiella strains and antiserum specific for Pasteurella haemolytica serotype 4, it was demonstrated that HMW-LPS and LMW-LPS contain shared epitopes. HMW-LPS also contained an epitope absent in LMW-LPS. This unique epitope was recognized by a monoclonal antibody (O1-52.6) and appears to be responsible for the serological cross-reaction between the O antigens of Klebsiella O1 and Escherichia coli O19. This HMW-LPS epitope was present in eight other Klebsiella O1 isolates which were examined. Electron microscopy demonstrated that HMW-LPS excluded overlying capsular polysaccharide for a distance of 25 to 40 nm. The distance was reduced to 10 to 18 nm in strains which synthesized only LMW-LPS and to zero in rough LPS strains. The HMW-LPS of Klebsiella O1 was shown to be an important virulence determinant, since this molecule was responsible for the resistance of the bacterium to nonspecific, complement-mediated serum killing.  相似文献   
29.
Based on the dissection of 30 hemi-mandibles, the authors report a study of the inferior alveolar artery in its intraosseous course. On morphologic considerations they propose a classification of the collaterals into two groups: the principal collaterals destined for the teeth and the bony alveolar tissue and the secondary collaterals destined for the sheath and the nerve as well as the bony tissue around the canal. Loss of the teeth and absorption of the alveolar bone modify the caliber of the inferior alveolar arterial axis, the distribution of its collaterals and possibly its mode of termination. These facts suggest a consideration of the vascularization of the mandible in terms of four sectors. They arrive at practical conclusions that may be drawn from this study in stomatology.  相似文献   
30.
During a recent outbreak of Lassa fever in Sierre Leone, a 20-year-old woman developed an acute febrile disease with tonsillar exudates and hemorrhagic manifestations. Lassa virus was isolated in cell cultures from pharyngeal secretions and pleural fluid and was identified by complement fixation. Typical arenavirus particles were observed in these infected cell cultures. In a liver biopsy specimen, diffuse hepatocellular damage and focal necroses were evident, with a spectrum of liver cell change, ranging from slight vacuolizaiton to frank lysis. Virus was frequently observed in nearby extracellular spaces and was clearly associated with hepatocytes rather than sinusoidal cells. The demonstration for the first time of Lassa virus particules in human tissue provides direct evidence that the virus is responsible for the observed pathologic changes.  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