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   12678篇
  免费   877篇
  2024年   16篇
  2023年   96篇
  2022年   95篇
  2021年   192篇
  2020年   166篇
  2019年   210篇
  2018年   415篇
  2017年   410篇
  2016年   555篇
  2015年   748篇
  2014年   726篇
  2013年   1001篇
  2012年   1251篇
  2011年   1138篇
  2010年   654篇
  2009年   515篇
  2008年   748篇
  2007年   704篇
  2006年   682篇
  2005年   580篇
  2004年   571篇
  2003年   510篇
  2002年   462篇
  2001年   119篇
  2000年   109篇
  1999年   90篇
  1998年   68篇
  1997年   45篇
  1996年   52篇
  1995年   49篇
  1994年   27篇
  1993年   35篇
  1992年   45篇
  1991年   41篇
  1990年   40篇
  1989年   35篇
  1988年   31篇
  1987年   20篇
  1986年   32篇
  1985年   33篇
  1984年   40篇
  1983年   20篇
  1982年   21篇
  1981年   20篇
  1980年   17篇
  1979年   18篇
  1978年   13篇
  1977年   15篇
  1976年   14篇
  1974年   14篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
861.
Mitochondrial DNA (mtDNA) mutations increase with age. However, the number of cells with predominantly mutated mtDNA is small in old animals. Here a new hypothesis is proposed: mtDNA fragments may insert into nuclear DNA contributing to aging and related diseases by alterations in the nucleus. Real-time PCR quantification shows that sequences of cytochrome oxidase III and 16S rRNA from mtDNA are present in highly purified nuclei from liver and brain in young and old rats. The sequences of these insertions revealed that they contain single nucleotide polymorphisms identical to those present in mtDNA of the same animal. Interestingly, the amount of mitochondrial sequences in nuclear DNA increases with age in both tissues. In situ hybridization of mtDNA to nuclear DNA confirms the presence of mtDNA sequences inside nuclear DNA in rat hepatocytes. Bone marrow metaphase cells from both young and old rats show mtDNA at centromeric regions in 20 out of the 2n = 40 chromosomes. Consequently, mitochondria can be a major trigger of aging but the final target could also be the nucleus.  相似文献   
862.
863.
Background and objectiveThe aim of this study was to assess the utility of arterial calcium stimulation with hepatic venous sampling (ASVS) in the localization of tumors in patients with endogenous hyperinsulinism not detected with other methods.Patients and methodsWe performed a retrospective study of 26 patients admitted to our hospital for hypoglycemia who underwent ASVS because the source of hyperinsulinism was not clearly identified by other imaging techniques. The histopathological result in patients who underwent a surgical procedure was considered the reference for statistical study of the accuracy of this technique. Statistical analysis was performed by comparing proportions with the chi-squared test with Yates’ correction for contingency tables, and Cohen′s kappa coefficient as a measure of interrater agreement between two observations.ResultsSurgery was performed in 17 patients, 13 with positive ASVS and the remaining four with negative results. An insulinoma was removed in 12 patients, and 10 of these were detected in the ASVS. A total of 76.9 % of positive ASVS tests corresponded to a histological diagnosis of insulinoma, and 83% of these insulinomas were positive in ASVS. This association was statistically significant (chi cuadrado = 7.340; p = 0.012). Two of three patients with nesidioblastosis had a positive response in the ASVS. A good and statistically significant agreement was obtained between histopathologic diagnosis and ASVS results (κ=0.556, p = 0.007).ConclusionsASVS is a useful procedure in the localization diagnosis of endogenous hyperinsulinism not detected by other imaging tests. This technique allows tumors in the pancreatic gland to be identified and may be useful in the choice of the surgical technique to be used.  相似文献   
864.
865.
ObjectiveTo evaluate the extent to which metabolic targets in type 2 diabetes (DM-2) are achieved in the Endocrinology and Clinical Nutrition Unit of the Hospital Puerta del Mar in Cadiz (Spain) from 2005 to 2008.MethodThe database included in the computer application HP-Doctor used for all patients attended in our unit (admissions, consultations and peripheral centers) was analyzed. All patients with a principal or secondary diagnosis of DM-2 were included. Clinical characteristics, chronic complications, drug treatment and the percentage of patients who achieved annual mean targets of glycosylated hemoglobin (HbA1c) and low-density lipoprotein cholesterol (LDLc) were analyzed.ResultsFrom 2005 to 2008, the number of DM-2 patients with computerized records increased by 108.7%. In 2008, 1,177 patients were evaluated. A total of 10.8% were active smokers, 53% had hypertension, and 51.2% and 12.6% presented with retinopathy and cardiovascular disease, respectively. During the study period, the percentage of patients with a mean HbA1c <7% was similar (2005: 31.7% 2008: 30.4%), those with LDLc <100 mg/dl increased from 19.2% to 25.6% and only 9.2% of patients achieved both targets, HbA1c <7% and LDLc <100 mg/dl.ConclusionsIn 2008, only 30% of DM-2 patients achieved a mean HbA1c < 7% and only 25% had LDLc < 100 mg/dl. Metabolic control in DM-2 patients should be improved.  相似文献   
866.
867.
This article reports the lipase-catalyzed ring-opening copolymerization of l-lactide (LLA) and glycolide using the commercially available ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate as solvent media. Candida antarctica lipase B, immobilized in an acrylic support was used as biocatalyst. The reaction temperature had a direct influence on yields and molecular weights of the copolymers as well as LLA incorporation. The materials presented semi-crystalline structures assessed by DSC and powder XR diffraction analyses.  相似文献   
868.
A PCR screening approach was used to search for sequences homologous to a previously described hAT transposon found in Drosophila simulans and Drosophila sechellia, named here as hosimary. In this study, 52 Drosophilidae species were analyzed and these sequences seem to be restricted to some species of the melanogaster group and Zaprionus indianus. These species present variable number of copies and most of those appear to be putatively encoding. The high hosimary sequences similarity among different species and the patchy distribution presented by this transposon strongly support the hypothesis that hosimary was horizontally transferred between the melanogaster group species and Z. indianus.  相似文献   
869.
The Lower Pleistocene TD6 level at the Gran Dolina site in the Sierra de Atapuerca (Burgos, Spain) has yielded nine ribs that represent a minimum of three individuals of the species, Homo antecessor. We present a detailed morphological and metric study of these costal elements, including the siding and anatomical position of all of the rib remains. The adult or nearly adult ribs are also metrically compared with other fossil hominins and with modern comparative samples. The costal elements recovered to date from the TD6 level at Gran Dolina can neither confirm nor reject the hypothesis that H. antecessor had a large thorax, similar to that of Neandertals. However, the fragmentary evidence of the H. antecessor thoracic skeleton is not inconsistent with this suggestion based on other skeletal elements, such as clavicles.

Resumen

En el nivel TD6 del Pleistoceno inferior del yacimiento de Gran Dolina, en la Sierra de Atapuerca (Burgos, España) se han recuperado nueve costillas que pertenecen a un mínimo de tres individuos de la especie Homo antecessor. Presentamos un detallado estudio métrico y morfológico incluyendo el lado y la determinación anatómica. Las costillas pertenecientes a individuos adultos o casi adultos también son comparadas métricamente a muestras modernas de comparación y otros homininos fósiles. Basándonos en el registro de costillas de Homo antecessor recuperado hasta el momento no podemos probar ni refutar la hipótesis de que esta especie presentaba un tórax grande similar al de los Neandertales. Sin embargo, el registro de costillas no es inconsistente con la hipótesis de un tórax grande como sugiere la gran longitud de sus clavículas.  相似文献   
870.
Background The purpose of this study is to better characterize the hematological, biochemical, respiratory, cardiovascular and electroneurophysiological parameters in young adult Cercopithecus aethiops sabaeus of both sexes. The rhesus and cynomolgus monkeys are widely used as experimental primate models. However, only few articles have been published testing toxicological effects of pharmaceuticals on African green monkey. Methods The present study was carried out with the recompilation of all parameters recorded before the first drug administration in five sub‐chronic or chronic toxicological studies performed on 66 Cercopithecus aethiops sabaeus, born in Cuba. Results This study provides hematological, biochemical, respiratory, cardiovascular and electroneurophysiological data for both choosing animals to be included into experiments and monitoring these parameters during the study. Conclusions We conclude that this study provides valuable integrated data for determining the health status, including electroneurophysiological parameters, data not previously reported for this species, of the African green monkey.  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