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   64540篇
  免费   6954篇
  国内免费   5236篇
  2023年   526篇
  2022年   1331篇
  2021年   2171篇
  2020年   1560篇
  2019年   1996篇
  2018年   1881篇
  2017年   1494篇
  2016年   2139篇
  2015年   3165篇
  2014年   3749篇
  2013年   4076篇
  2012年   4925篇
  2011年   4476篇
  2010年   2886篇
  2009年   2618篇
  2008年   3199篇
  2007年   2967篇
  2006年   2650篇
  2005年   2310篇
  2004年   2049篇
  2003年   1986篇
  2002年   1784篇
  2001年   1467篇
  2000年   1385篇
  1999年   1209篇
  1998年   699篇
  1997年   638篇
  1996年   598篇
  1995年   501篇
  1994年   498篇
  1993年   456篇
  1992年   814篇
  1991年   733篇
  1990年   689篇
  1989年   736篇
  1988年   624篇
  1987年   642篇
  1986年   526篇
  1985年   623篇
  1984年   501篇
  1983年   414篇
  1982年   427篇
  1981年   381篇
  1980年   338篇
  1979年   455篇
  1978年   391篇
  1977年   343篇
  1975年   339篇
  1974年   390篇
  1973年   372篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
OBJECTIVE--To investigate non-invasive detection of cancer by testing for unusual CD44 gene activity in a clinical sample as an indicator of exfoliated tumour cells. DESIGN--Case-control study. SUBJECTS--44 unselected, consecutive patients with bladder cancer and 46 people with no evidence of neoplasia. MAIN OUTCOME MEASURE--Presence or absence of large CD44 gene products containing exon 6 derivatives in urine samples. RESULTS--Novel abnormalities in the pattern of expression of this gene, seen specifically in tumour tissue, led to cloning of a newly recognised coding region in it (exon 6). This was tested as a probe for detection of exfoliated malignant cells in naturally voided urine. CD44 gene products extracted from the urine and amplified with polymerase chain reaction contained predicted electrophoretic band of 735 base pairs in 40 of the 44 patients with bladder cancer (sensitivity 91%). Products from 38 of the 46 people with no evidence of neoplasia showed no such band (specificity 83%). CONCLUSIONS--Unusual activity of the CD44 locus in neoplasia and malignancy is confirmed, and techniques for the analysis of such activity can enable non-invasive investigation of patients for primary or recurrent bladder cancer or for other tumours that shed neoplastic cells into body fluids.  相似文献   
992.
OBJECTIVES--To determine whether the period spent on the true inpatient waiting list is a valid indication of the total time that patients have to wait for an operation; and to assess the feasibility of monitoring the total "postreferral waiting time" by using existing computerised information systems. SETTING--Three randomly selected Scottish hospitals. SUBJECTS--Waiting list patients admitted to hospital for operations during June to August 1993 in six major specialties, separate attention being focused on cataract operations and hip and knee replacements. MAIN OUTCOME MEASURE--The total time that patients have to wait for an operation after the initial general practitioner referral--the postreferral waiting time--compared with that spent at the final stage of the process on the true inpatient waiting list. RESULTS--In the specialties investigated roughly half (58 days; 53%) of the average postreferral wait of 110 days was spent on the true inpatient waiting list, one third (35 days; 32%) being spent on the outpatient waiting list and one sixth (17 days; 15%) waiting between waiting lists. Only a quarter of cataract patients (73/292) were treated within three months of general practitioner referral compared with over three quarters (228/292) within three months of being placed on the inpatient waiting list. Nevertheless, within a year over 99% of patients (290) had been treated whichever date was taken as the starting point. CONCLUSIONS--Monitoring postreferral waiting times would provide a much more accurate picture for purchasers and patients of waiting times for treatment than is obtained by focusing exclusively on the true inpatient waiting list and facilitate fairer comparisons between NHS trusts in national league tables. Stringent national and local monitoring is essential to ensure (a) that future reductions in the time waiting on true inpatient waiting lists are not gained at the expense of longer periods waiting to be placed on the lists, and (b) that no increases occur in the number of patients placed instead on deferred waiting lists or exempted from the normal maximum waiting time guarantees.  相似文献   
993.
Review of a 1993 survey of the 29 United Kingdom departments of general practice (or equivalent) identified seven master''s degree courses available for general practitioners. Up to another 11 are planned within the next five years. Around 50 general practitioners undertake all such courses at any one time. Possible reasons for this low uptake include cost, lack of flexibility of courses, and the prospect of writing a thesis. Appropriate master''s courses are essential to the future development of general practice, and this paper postulates the characteristics of an "ideal" course.  相似文献   
994.
Largepterosaurwithamaximumwingspanofmorethan5metres.Theskullislowerandlonger,withoutmiddlecrestorsupraoccipita1crest.ThenasaIandpre-orbita1fenestraareconfluentcompletely,andoccupyaboutha1foftheskulI1ength.Thetoothlessbeakisslenderandpointed.The1ongneckiscomposedbyse-venslendercervicalvertebrae.Thenotariumconsistsofsixco-ossifiedanteriordorsalvertebrae-ThetaiIisextremlyshort.lthassixpairsof"A"shapegastralia.Theanteriorlimbsarestrong;thehumerusarethickandshort;thewing-metacarpalbonearelongert…  相似文献   
995.
广西崇左县冲塘新石器时代人骨微量元素的初步研究   总被引:4,自引:1,他引:3  
本文对广西崇左县冲塘新石器时代的3个个体(M02,S08,S09)的肢骨进行了化学元素(Sr,Zn,Ca,P,Cd,Ph)的含量测定.并根据研究结果对当时人类的生活环境、食物性质作了推测和分析.  相似文献   
996.
A 29-year-old male with Angelman syndrome and an unbalanced reciprocal translocation, 45,XY,-8,-15,+der(8),t(8;15)(p23.3;q11)pat, was evaluated with DNA studies. These showed the underlying mechanism to be paternal uniparental disomy. This is the second case reported of Angelman syndrome that has resulted from a familial unbalanced reciprocal translocation.  相似文献   
997.
Previous studies have shown that recognition of melanoma by cytotoxic T lymphocytes may be restricted by HLA-A1, A2 and other HLA antigens. The present study examined the cytotoxic specificity and major histocompatibility complex restriction of cloned cytotoxic T lymphocytes (CTL) isolated from a patient with the HLA phenotype A3,31 who had been immunized with a vaccine prepared from HLA-A1,3 melanoma cells. Cytotoxic assays against HLA-typed allogeneic melanoma cells indicated that cloned CTL from the patient were able to kill allogeneic melanoma cells expressing HLA-A1 but not other HLA-A1-positive cells. Studies on a representative clone indicated that proliferation and cytokine (tumour necrosis factor ) production in response to melanoma cells was also associated with HLA-A1 on melanoma cells. Response to the melanoma cells was associated with interleukin-4 (IL-4) rather than IL-2 production. The antigen recognized in the context of HLA-A1 on allogeneic melanoma cells was detected in cytotoxic assays on cells from 9 of 12 HLA-A1+ melanoma cell lines and did not appear to be the product of the MAGE-1 or-3 genes. These findings suggest that T cells can recognize melanoma antigens in the context of alloantigens and that allogeneic vaccines containing immunodominant alloantigens may generate CTL that are ineffective against autologous melanoma. The study does not, however, exclude the possibility that CTL with specificity to the latter may be activated by allogeneic vaccines, and further studies are needed to answer this question.  相似文献   
998.
The mouse autosomal recessive mutant gene weaver (wv) results in abnormalities in cerebellum, substantia nigra and testis. Although a subtracted cDNA library prepared by removing P31 (wv/wv) sequences from a P1 (wv/+) library should contain mainly nonrepetitive neonatal sequences, unfortunately, repetitive sequences still appear during screening. Two clones, one repetitive, the other not, are used to illustrate the problems encountered in attempting to isolate the weaver gene from a subtracted cDNA library.Special issue dedicated to Dr. Sidney Ochs.  相似文献   
999.
三褶脉紫菀中的新二萜甙   总被引:1,自引:0,他引:1  
三褶脉紫菀(AsterageratoidesTurcz.)系菊科多年生草本植物,遍布全国,是民间常用的中药,有清热解毒、祛痰镇咳的功效[1,2]。化学工作者们曾从其同属植物紫菀(AstertararicusL.f.)中分离到紫菀酮(shionone)、槲皮素(quercetin)、无羁萜(friedelin)、表无羁萜(epifriedelinel)、毛叶醇(lachnophyllol)、乙酸毛叶酯(lachnophyllolacetate)、茴香醚(anethole)以及紫菀三萜皂甙[2—4]…  相似文献   
1000.
边界层阻力在叶片气体交换过程中的作用   总被引:1,自引:0,他引:1  
作者对现有的气体交换法的叶室作了改进。从实验上检验了作者根据电学类比分析和模拟检验得到的结论:在已有的气体交换测定装置中,空气流速不变,叶片的边界层导度也固定不变,所得到的结论用于边界层导度变化的情形时,会造成比较大的误差。现加一障碍物使得流过叶片的风速减小,从而改变叶片的边界层导度。通过测量在不同边界层导度下同一叶片的光强曲线可以考察不同边界层导度下的气孔导度、光合速率、蒸腾速率以及它们之间的相互关系。实验结果与作者以前的电学类比分析和模拟得到的结论是基本一致的  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