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   40667篇
  免费   2359篇
  国内免费   9篇
  2023年   237篇
  2022年   284篇
  2021年   613篇
  2020年   391篇
  2019年   435篇
  2018年   515篇
  2017年   485篇
  2016年   791篇
  2015年   1308篇
  2014年   1485篇
  2013年   2222篇
  2012年   2235篇
  2011年   2282篇
  2010年   1527篇
  2009年   1424篇
  2008年   1862篇
  2007年   1750篇
  2006年   1546篇
  2005年   1545篇
  2004年   1492篇
  2003年   1281篇
  2002年   1170篇
  2001年   300篇
  2000年   242篇
  1999年   307篇
  1998年   335篇
  1997年   246篇
  1996年   305篇
  1995年   271篇
  1994年   260篇
  1993年   358篇
  1992年   322篇
  1991年   289篇
  1990年   288篇
  1989年   459篇
  1988年   417篇
  1987年   523篇
  1986年   444篇
  1985年   450篇
  1984年   396篇
  1983年   341篇
  1982年   379篇
  1981年   387篇
  1980年   377篇
  1979年   351篇
  1978年   412篇
  1977年   440篇
  1976年   468篇
  1975年   475篇
  1974年   335篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Turkey poults were given either of two different dosages of two different gliotoxin-producing strains ofAspergillus fumigatus. Infected lung tissue was examined postmortem for the presence of gliotoxin. Gliotoxin was found in lung tissue of ten poults infected with one strain and in seven of ten poults infected with the other strain. Concentrations of gliotoxin in the tissue exceeded 6 ppm in some of the infected tissues. The concentration of gliotoxin found in infected tissue did not appear to be correlated with the dosage of organism given. Considering the pathologic changes observed in turkey poults with aspergillosis and the production of gliotoxin during the pathogenic state in turkey poults, gliotoxin is considered likely to be involved in avian aspergillosis. Disclaimer: Names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by USDA implies no approval of the product to the exclusion of others that may also be suitable.  相似文献   
992.
We have tested the therapeutic potency of peritumorally injected low doses of interleukin-2(IL-2). Seventy tumours of the bovine ocular squamous-cell carcinoma (BOSCC), 1–3 cm in diameter, were treated with 5000, 20 000 or 200 000 U IL-2 from Eurocetus (Chiron) to find the optimal dose for treatment. Injections were given peritumorally on Monday to Friday on 2 consecutive weeks. The size of the tumours was measured before treatment and 1, 3, 4, 9 and 20 months after treatment. After 9 months complete regression was observed in 89% of the tumours treated with 5000 U IL-2, 80% treated with 20 000 U and 67% treated with 200 000 U. After 20 months, there was complete regression of 35%, 31% and 67% of the tumours respectively. The 9-and 20-month results of the 200 000-U treatment are significantly better than those of the 5000-U and 20 000-U treatments taken together. This protocol may be useful to treat advanced inoperable tumours (e.g. of the nasopharynx or skin) of human patients.  相似文献   
993.
The effect ofl-pyroglutamic acid, a metabolite that accumulates in pyroglutamic aciduria, on different neurochemical parameters was investigated in adult male Wistar rats. Glutamate binding, adenylate cyclase activity and G protein coupling to adenylate cyclase were assayed in the presence of the acid.l-pyroglutamic acid decreased Na+-dependent and Na+-independent glutamate binding Basal and GMP-PNP stimulated adenylate cyclase activity were not affected by the acid. Furthermore, rats received unilateral intrastriatal injections of 10–300 nmol of bufferedl-pyroglutamic acid. Vehicle (0.25 M Tris-Cl, pH 7.35–7.4) was injected into the contralateral striatum. Neurotoxic damage was assessed seven days after the injection by histological examination and by weighing both cerebral hemispheres. No difference in histology or weight could be identified between hemispheres. These results suggest that, although capable of interfering with glutamate binding, pyroglutamate did not cause a major lesion in the present model of neurotoxicity.  相似文献   
994.
Glutathione and total carnitine (i.e., free carnitine plus acid-soluble carnitine esters) were measured in an affected (superior frontal gyrus; SFG) and unaffected (cerebellum: CBL) region of Alzheimer disease (AD) and control brains. Average glutathione content in AD SFG (n=13) and AD CBL (n=7) (7.9±2.1 and 11.9±4.0 nmol/mg protein, respectively (mean ±S.D.)) was similar to that in control SFG (n=13) and CBL (n=6) (7.7±2.0 and 11.6±2.6 nmol/mg protein, respectively). However, glutathione increased significantly with age in AD brain (p=0.003) but not in control brain. Average total carnitine in AD SFG (84±47 pmol/mg protein; n=10) and AD CBL (108±86 pmol/mg protein; n=7) was not significantly different from that in the corresponding regions of control brain (148±97 (n=10) and 144±107 (n=6) pmol/mg protein, respectively). However, a significant decline of total carnitine with age in both regions was noted for AD brain, but not for control brain. Carnitine acetyltransferase activity in the AD SFG (n=13) was not significantly different from that of control SFG (n=13) (1.83±1.05 and 2.04±0.82 nmol/min/mg protein, respectively). However, carnitine acetyltransferase activity of AD CBL (n=7) was significantly lower than that of control CBL (n=6) (1.33±0.88 versus 2.26±0.66 nmol/min/mg protein; p=0.05).  相似文献   
995.
Abstract Mosaic penicillin-binding proteins (PBP) 1A, 2X and 2B genes were cloned from four clinical isolates of Streptococcus pneumoniae with levels of susceptibility to penicillin ranging from 1.5 to 16 μg benzylpenicillin ml−1. In each instance it was possible to transform either the penicillin-sensitive laboratory strain R6 or a sensitive clinical isolate 110K/70 to the full level of penicillin resistance with these three penicillin-binding proteins alone. Until now it has not been possible to clearly determine whether alterations to PBP1A, 2X and 2B alone were sufficient to attain high level penicillin resistance.  相似文献   
996.
In the mid-western Himalaya (altitude 1350 m, rainfall 1100 mm), multipurpose trees found as escapees in agricultural fields or naturally growing in the forests, play an important role in providing fuel, fooder and small timber to the farmers. Shoot elogation, and tree architecture of 4 year old trees of Grewia optiva, Robinia pseudoacacia and Celtis australis (early successionals), and Quercus leucotrichophora, Q. glauca and Ilex odorata (late successionals), were analyzed. All the late successional species showed a proleptic type of bud and branch production, while the early successional trees made growth through syllepsis. The shoot elongation differed significantly (P <0.05) with the crown position, and ranged from 11 to 30 cm in different species. Early successional species tended to maintain a comparatively narrow crown and showed a significantly (P <0.05) higher ramification ratio, and multilayered canopy. The late successionals, in contrast, showed a wide crown with monolayered canopy, adapted to the weak light intensity. There was only one flush of leaves in Q. leucotrichophora and Q. glauca while in the rest of the species there were two distinct flush periods. The results are important for the management of agroforestry trees.  相似文献   
997.
998.
Of 131 young (17 to 44 years) and middle-aged (45 to 55 years) adults who had brain infarction or hemorrhage, the most common etiologic factors were rheumatic heart disease, migraine and oral contraceptive use among the younger group. In contrast, atherosclerotic, hypertensive and diabetes-associated cerebrovascular were the most common causes in the middle-aged group. Patients who have a stroke before age 45 should have prompt, complete laboratory and radiologic testing to define a possible treatable cause.  相似文献   
999.
1000.
The distribution of immunoglobulin allotypes of the Gm and Km systems was examined in 51 patients with antinuclear antibodies (ANA), which reacted with two saline-extractable non-DNA nuclear antigens, anti-La (SS-B) and anti-RNP, which characterize certain multisystem autoimmune diseases. Forty-six percent of the 26 patients with anti-La were positive for the Km(1) allotype compared with 14% of the 35 with anti-RNP and 16% of 1204 of healthy subjects (corrected P value < 0.005). The high frequencies of the Km(1) allotype (46%), female sex (100%), and the HLA-B8, DR3 phenotype (> 90%) in patients with anti-La are indicative of a substantial inherited predisposition to the development or expression of this autoantibody. The strong association between the Km(1) allotype and the anti-La response may be due to linkage disequilibrium between genes coding for the constant region of immunoglobulin kappa light chains and genes coding for the variable regions of kappa light chains which confer antibody specificity for the special configuration of the ribonucleoprotein known as La.  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