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   60939篇
  免费   5176篇
  国内免费   49篇
  2023年   173篇
  2022年   508篇
  2021年   1070篇
  2020年   612篇
  2019年   814篇
  2018年   1260篇
  2017年   1011篇
  2016年   1736篇
  2015年   2835篇
  2014年   3147篇
  2013年   3716篇
  2012年   4778篇
  2011年   4521篇
  2010年   2903篇
  2009年   2563篇
  2008年   3642篇
  2007年   3412篇
  2006年   3132篇
  2005年   2809篇
  2004年   2711篇
  2003年   2448篇
  2002年   2069篇
  2001年   1746篇
  2000年   1608篇
  1999年   1270篇
  1998年   569篇
  1997年   500篇
  1996年   431篇
  1995年   415篇
  1994年   326篇
  1993年   308篇
  1992年   671篇
  1991年   540篇
  1990年   504篇
  1989年   502篇
  1988年   430篇
  1987年   404篇
  1986年   335篇
  1985年   344篇
  1984年   284篇
  1983年   237篇
  1982年   201篇
  1981年   179篇
  1980年   171篇
  1979年   228篇
  1978年   205篇
  1977年   184篇
  1976年   177篇
  1974年   202篇
  1972年   163篇
排序方式: 共有10000条查询结果,搜索用时 625 毫秒
111.
112.
Abstract Morphological features of mature larvae of Petelia rivulosa (Butler), Exangerona prattiaria (Leech) and Culcula panterinaria (Bremer & Grey) of Ennominae are described and illustrated. All specimens examined are deposited in the Insect Collection of Department of Forest Resources Protection, Kangwon National University, Korea.  相似文献   
113.
BOOK REVIEWS: 4     
J. C. Lee 《Biometrics》2003,59(3):737-738
  相似文献   
114.
115.
N L Brown  D A Rouch  B T Lee 《Plasmid》1992,27(1):41-51
Copper is an essential trace element that is utilized in a number of oxygenases and electron transport proteins, but it is also a highly toxic heavy metal, against which all organisms must protect themselves. Known bacterial determinants of copper resistance are plasmid-encoded. The mechanisms which confer resistance must be integrated with the normal metabolism of copper. Different bacteria have adopted diverse strategies for copper resistance, and this review outlines what is known about bacterial copper resistance mechanisms and their genetic regulation.  相似文献   
116.
PURPOSE: Evaluation of the fertility of a cohort of formerly bilaterally cryptorchid men in comparison with a group of formerly unilaterally cryptorchid men, and a group of control men. MATERIALS AND METHODS: Using a detailed questionnaire concerning paternity and factors related to paternity, a cohort of formerly bilateral cryptorchid men were studied and compared with men who had undergone orchiopexy for unilateral cryptorchidism, and a group of control men. All study subjects had had surgery at the Children's Hospital of Pittsburgh, Pittsburgh, Pa., between 1955 and 1975. A subset of the full cohort underwent clinical evaluation that included a physical examination, serum hormonal determination and semen analyses. RESULTS: Paternity rates are significantly lower among the formerly bilaterally cryptorchid men who have attempted to father a child (65.3%) as compared to the formerly unilaterally cryptorchid (89.7%; p < 0.001) and control men (93.2%; p < 0.001). Differences in the ability to father children are also apparent when semen and hormone levels are compared between the three groups. The bilateral group has significantly lower sperm density and inhibin B levels, and higher FSH and LH levels, than the unilateral and control groups. CONCLUSIONS: Men born with bilateral cryptorchidism have severely compromised fertility in adulthood. This reduction in fertility is clearly shown in comparisons of both paternity rates, and in semen and hormone analyses, between the formerly bilateral, formerly unilateral, and control groups.  相似文献   
117.
118.
119.
Two hundred eighteen usable serum samples were collected from hunter-killed collared peccaries (Tayassu tajacu) during March 1986, in three areas of Arizona. Evaluations for antibodies against vesicular stomatitis virus (VSV) New Jersey (NJ) type, VSV Indiana type, pseudorabies virus, brucellosis, and leptospirosis revealed positive test results in 8%, 0%, less than 1%, 0%, and 23% of the sera, respectively. Exposure of peccaries to VSV (NJ) was widespread, but variation in the prevalence of seropositive peccaries was not found between the three areas sampled. The exposure of peccaries to VSV (NJ) probably was related to the recent epizootics in livestock in the vicinity. Exposure to Leptospira interrogans serovars also was widespread, and geographic variation in the prevalence of peccaries with antibodies against L. interrogans was found.  相似文献   
120.
In this study we present evidence to suggest that gastroduodenal mucosal defects may occur in gastric fistula dogs actively immunized with PGE2-thyroglobulin conjugate. One of four PGE2-immunized dogs developed a chronic pyloroduodenal ulcer with penetration into the pancreas and the other three had endoscopic evidence of gastric and/or duodenal erosions. In contrast, no gastroduodenal mucosal defects were seen in control dogs immunized with thyroglobulin alone. Occurrence of gastroduodenal ulcers or erosions was temporally related to formation of specific antibody to PGE2 suggesting that PGE2 antibody may be responsible for lesion formation. An increase in gastric acid secretion was not observed in PGE2-immunized dogs. Thus, it is likely that mucosal defects occur as a result of an impairment of PGE2-mediated mucosal defense mechanisms. Since gastroduodenal lesions can be visualized by endoscopy, the dog may prove to be useful in studying the role of endogenous PG in ulcer diseases.  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