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   2043篇
  免费   189篇
  2021年   25篇
  2019年   23篇
  2018年   22篇
  2017年   20篇
  2016年   34篇
  2015年   46篇
  2014年   50篇
  2013年   80篇
  2012年   100篇
  2011年   92篇
  2010年   54篇
  2009年   66篇
  2008年   78篇
  2007年   81篇
  2006年   73篇
  2005年   85篇
  2004年   65篇
  2003年   81篇
  2002年   85篇
  2001年   69篇
  2000年   57篇
  1999年   44篇
  1998年   31篇
  1997年   31篇
  1996年   19篇
  1995年   22篇
  1994年   16篇
  1993年   19篇
  1992年   41篇
  1991年   32篇
  1990年   28篇
  1989年   16篇
  1988年   32篇
  1987年   35篇
  1986年   35篇
  1985年   35篇
  1984年   30篇
  1983年   37篇
  1982年   27篇
  1981年   31篇
  1980年   16篇
  1979年   16篇
  1978年   21篇
  1977年   21篇
  1976年   16篇
  1975年   21篇
  1974年   19篇
  1973年   16篇
  1972年   24篇
  1968年   18篇
排序方式: 共有2232条查询结果,搜索用时 31 毫秒
51.
Recently published data on mortality in the European Economic Community and Scandinavia convincingly showed that mortality among men and women aged 45-64 was considerably higher in the United Kingdom than elsewhere. This applied to deaths due to circulatory and respiratory disease, cancer, and all causes. For example, in 1980 in Scotland twice as many, or more, women aged 55-64 per 100 000 died of heart disease than in Belgium, Denmark, France, Greece, West Germany, the Netherlands, Norway, and Sweden. Reductions in mortality from all causes during 1950-80 in the United Kingdom did not match those in other countries, such as Finland and France. Whether the public in the United Kingdom knows about its relatively poor mortality state is doubtful. To secure improved funding of appropriate preventive and treatment services directed at reducing premature mortality, public awareness should be raised urgently so that politicians and political parties will respond quickly in a way that the problem demands.  相似文献   
52.
H. U. Cameron  M. Ford 《CMAJ》1978,119(10):1207-1209
  相似文献   
53.
54.
Two studies were performed to assess the accuracy of non-invasive methods in detecting intra-abdominal metastases from breast cancer. Firstly, the sites of spread detected at the time of first presentation with metastases were compared with the sites of spread shown at necropsy in the same patients. Although about two-thirds of the patients with bone and lung metastases at necropsy had had metastases detected at these sites when they first presented with metastases, only a third of the patients with liver metastases and none of those with other intra-abdominal metastases had had evidence of disease at first presentation with metastases. The second study confirmed a poor detection rate of liver and other intra-abdominal metastases in patients with breast cancer undergoing laparotomy and oophorectomy who were staged immediately before operation.Pre-mastectomy staging laparotomy should be considered in those patients with primary breast cancer who are most likely to have disseminated disease beyond the regional nodes. In the presence of occult gross metastases detected by staging laparotomy, mastectomy will not provide additional protection against loca recurrence of disease. Patients with occult gross metastases should also be excluded from studies on adjuvant chemotherapy (designed to treat micrometastases). Aggressive methods of staging are justified to protect the patient as far as possible against unnecessary mastectomy and to identify those patients who should be treated by therapeutic chemotherapy rather than adjuvant chemotherapy.  相似文献   
55.
Blood flow to each uterine horn of cows during the oestrous cycle and early pregnancy was determined daily by use of electromagnetic blood flow probes placed around both middle uterine arteries. The pattern of blood flow to uteri of pregnant and non-pregnant cows was similar until Day 14 after mating or oestrus. Between Days 14 and 18 of pregnancy blood flow to the uterine horn containing the conceptus increased (P less than 0.01) 2- to 3-fold, whereas blood flow to the non-gravid uterine horn in these cows remained constant. No corresponding increase in blood flow to the uterine horn ipsilateral to the ovary bearing the CL was observed in non-pregnant cows during this 4-day period. By Day 19 of pregnancy, blood flow to the gravid uterine horn had returned to a level similar to that observed on Day 13. Blood flow to both uterine horns of pregnant cows remained constant from Days 19 to 25 and then increased to the gravid horn (P less than 0.01) markedly until Day 30 whereas blood flow to the non-gravid horn remained low. Uterine blood flow during the oestrous cycle of non-pregnant cows was positively correlated (P less than 0.01) with systemic concentrations of oestradiol and the ratio of oestradiol (pg/ml) to progesterone (ng/ml). There was no association between oestradiol concentrations and blood flow to the gravid uterine horn. These data indicate local control of uterine blood flow by the bovine conceptus which may function to create optimal conditions for the continuation of pregnancy.  相似文献   
56.
Summary Human chromosome 12 has been used as a model for studying the distributions of sites of induced and spontaneous breaks. The breakpoints were determined from (1) translocations involving chromosome 12, (2) spontaneous breaks in untreated cultures, (3) radiation-induced breaks, and (4) spontaneous breaks in Fanconi's anaemia.Statistical analysis showed discordance in the results both between the eleven individual bands and between the four assessments. Also, the distribution of breaks for all bands was significantly diferent from random in each assessment. Certain bands added considerable bias to the results, and when analysed individually, only four bands (p11.1, q13, q24, and p13) showed distributions over the four assessments that were significantly different from random. These four bands are Giemsa-negative bands, and two (p13 and q24) are adjacent to telomeres, while p11.4 is adjacent to the centromere. The fourth band, q13, is a known fragile site.It is concluded that bands adjacent to centromeres, which are not C-banded, are peculiarly sensitive to breakage. Telomeric bands are variable in their response to different conditions of breakage, and both the physical structure of the telomere and the specific gene sequences of individual telomeres are probably of importance in determining this response. The fragile site q13 responds as if breakage at this site is due to the base composition of the DNA.  相似文献   
57.
58.
Summary Ribosomal RNA (rRNA) sequence homology (as determined by comparisons of T1 oligonucleotide catalogs of32P-labeled 16S rRNAs) has been used to assess phylogenetic relationships within the filamentous and unicellular blue-green bacteria, and to identify regions of evolutionary conservatism within blue-green bacterial 16S rRNAs.Nostoc andFischerella, representatives of two morphologically distinct and highly differentiated orders, are shown to be as closely related (on the basis of RNA sequence homology) as typical members of the non-blue-green bacterial genusBacillus. They are further shown to be (on the same basis) indistinguishable from typical unicellular members of a subgroup of the unicellular blue-green bacterial order Chroococcales. These results have general implications for studies of the origin of differentiated prokaryotes and of evolutionary change in prokaryotic macromolecules. In particular, they provide indirect evidence that the divergences of contemporary major prokaryotic groups are truly ancient ones.  相似文献   
59.
60.
Internal viscoelastic loading in cat papillary muscle.   总被引:4,自引:0,他引:4       下载免费PDF全文
The passive mechanical properties of myocardium were defined by measuring force responses to rapid length ramps applied to unstimulated cat papillary muscles. The immediate force changes following these ramps recovered partially to their initial value, suggesting a series combination of viscous element and spring. Because the stretched muscle can bear force at rest, the viscous element must be in parallel with an additional spring. The instantaneous extension-force curves measured at different lengths were nonlinear, and could be made to superimpose by a simple horizontal shift. This finding suggests that the same spring was being measured at each length, and that this spring was in series with both the viscous element and its parallel spring (Voigt configuration), so that the parallel spring is held nearly rigid by the viscous element during rapid steps. The series spring in the passive muscle could account for most of the series elastic recoil in the active muscle, suggesting that the same spring is in series with both the contractile elements and the viscous element. It is postulated that the viscous element might be coupled to the contractile elements by a compliance, so that the load imposed on the contractile elements by the passive structures is viscoelastic rather than purely viscous. Such a viscoelastic load would give the muscle a length-independent, early diastolic restoring force. The possibility is discussed that the length-independent restoring force would allow some of the energy liberated during active shortening to be stored and released during relaxation.  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