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   1867篇
  免费   177篇
  国内免费   1篇
林业   49篇
农学   38篇
基础科学   16篇
  191篇
综合类   282篇
农作物   95篇
水产渔业   63篇
畜牧兽医   1138篇
园艺   39篇
植物保护   134篇
  2023年   14篇
  2022年   21篇
  2021年   28篇
  2020年   38篇
  2019年   54篇
  2018年   57篇
  2017年   65篇
  2016年   55篇
  2015年   40篇
  2014年   69篇
  2013年   84篇
  2012年   103篇
  2011年   112篇
  2010年   65篇
  2009年   45篇
  2008年   90篇
  2007年   82篇
  2006年   73篇
  2005年   45篇
  2004年   65篇
  2003年   66篇
  2002年   60篇
  2001年   30篇
  2000年   25篇
  1999年   21篇
  1998年   15篇
  1997年   14篇
  1995年   26篇
  1994年   17篇
  1993年   16篇
  1992年   24篇
  1991年   20篇
  1990年   30篇
  1989年   19篇
  1988年   20篇
  1987年   20篇
  1986年   32篇
  1985年   20篇
  1984年   13篇
  1983年   12篇
  1982年   18篇
  1979年   13篇
  1976年   17篇
  1974年   21篇
  1973年   21篇
  1972年   18篇
  1971年   14篇
  1970年   17篇
  1967年   12篇
  1966年   13篇
排序方式: 共有2045条查询结果,搜索用时 15 毫秒
1.
Precise localization of the needle tip during CT-guided percutaneous biopsy is considered to be a key element of a successful procedure. To ensure accuracy, the true needle tip must be differentiated from a false or simulated tip which appears when the CT slice encompasses only the shaft of an angled needle. By obtaining images of an aspiration biopsy needle inserted vertically into a phantom and then incrementally tilting the gantry, the authors were able to compare the characteristic features of the true tip to the simulated tip. The true tip was abrupt and distinct and had an adjacent flame-like low density artifact. The simulated tip was indistinct and tapered, yet still produced the adjacent artifact. We concluded that the shape and distinctness of the end portion of the needle itself, rather than the attendant artifact, were the most reliable criteria for accurate needle tip identification.  相似文献   
2.
Data from 321 spring-calving cows (mean calving date March 27) were used to assess the effects of body condition, live weight, cow age (from 4 to 13 yr), and breed (237 Hereford x Friesians and 84 Blue-Greys) and time of calving on the proportion of cows that became pregnant, the number of days from the start of mating to pregnancy, and calving interval. Mating started at turn-out to pasture in mid-May and lasted 9 to 10 wk. Body condition at calving and breed were the most significant animal factors affecting reproductive performance. Cows calving in higher body condition had shorter (P less than .001) calving intervals (11.2 d per unit of body condition at calving). Blue-Grey cows became pregnant in a higher proportion (90%) and calving interval was shorter (364 d) than in Hereford x Friesians (83%; 374 d). Body condition at the start of mating was less important and body condition at the end of mating had no effect. Live weight at calving and changes in live weight from calving to the start of mating and during the mating period had no significant effect. The proportion of cows becoming pregnant decreased significantly with age in Hereford x Friesian cows older than 7 yr. The variance in calving interval accounted for by calving date, body condition at calving, breed, and age was 42%.  相似文献   
3.
4.
Clinical, radiological and ultrasonographic findings from 22 cats with gastrointestinal lymphosarcoma were reviewed. The most common clinical findings were anorexia, weight loss, vomiting, lethargy, depression, fever, anemia and a palpable abdominal mass. Abdominal radiographs of 12 cats revealed a mass associated with the gastrointestinal tract and/or mesentery, and decreased serosal detail, suggestive of peritoneal effusion. The most common ultrasonographic finding was transmural gastric or intestinal thickening associated with loss of normal wall layering, reduced wall echogenicity and localized hypomotility. As is noted in people, other patterns such as transmural-segmental, transmural-nodular, transmural-bulky and mucosal infiltration were represented. In 3 cats, mesenteric lymph-adenopathy was responsible for most of the mass-effect. Fine-needle aspiration biopsy, automated microcore biopsy, endoscopic biopsy or necropsy confirmed the diagnosis of lymphosarcoma in all cats. Chemotherapy was instituted in 19 cats. Only 6 cats had a complete response to treatment. The results of this study indicate that ultrasonography is a valuable tool for the diagnosis of feline alimentary lymphosarcoma and that biopsies can be obtained under ultrasonographic guidance.  相似文献   
5.
Transforming growth factor type beta (TGF-beta) and adipogenesis in pigs   总被引:1,自引:0,他引:1  
The present study was performed on s.c. adipose tissue of fetal pigs at 35 to 110 d of gestation to examine the distribution of TGF-beta-positive cells, to localize TGF-beta immunoreactivity at the cellular level using electron microscopy (EM), and to determine the effect of TGF-beta on primary cultures of pig adipose tissue cells. Tissues for EM were fixed and embedded in LR white resin. Sections then were incubated with a polyclonal antibody specific for TGF-beta and TGF-beta was located using 20 nm colloidal gold conjugated second antibody. Tissues were fixed and embedded in paraffin for localization of TGF-beta at the light microscope (LM) level. Tissues were incubated with anti-TGF-beta followed by localization using biotinylated second antibody. Using LM, only a few cells stained positively for TGF-beta within developing blood vessels at 35 d. By 50 d, more TGF-beta-positive cells were associated with forming capillary networks. Between 70 d and 110 d, positively stained adipocytes usually were clustered around blood vessels. Cells surrounding hair follicles stained positive for TGF-beta between 90 to 110 d. Electron microscopy revealed TGF-beta labeling within fat cells. Fibroblasts and endothelial cells did not exhibit TGF-beta immunoreactivity. The addition of TGF-beta to primary cultures of s.c. adipose tissue cells from newborn pigs prevented lipid filling in fat cells. This effect was dose-dependent, with half-maximal inhibition occurring at 3 pM maximum inhibition occurred at 40 pM. These results indicate that TGF-beta may regulate angiogenic activity and lipid filling in s.c. adipose tissue of fetal pigs. Although TGF-beta was present in adipocytes and in cells associated with developing capillary networks, the physiological role of TGF-beta during early adipose tissue development is not known.  相似文献   
6.
A 13-year-old neutered female Yorkshire terrier presented with a history of progressive episodic weakness and disorientation of 4 months duration. Physical and neurologic examinations were normal at presentation. Abdominal ultrasound revealed a mass involving the right adrenal gland. Standard planar scintigraphy was performed at 4, 18, and 24 hours after intravenous injection 185 MBq (5mCi) of 123I-labeled metaiodobenzylguanidine (123I-MIBG). An area of focal intense uptake was identified in the area of the right adrenal gland. A pheochromocytoma was confirmed histologically after surgical excision.  相似文献   
7.
Fungal pathogens are becoming increasingly important for human and small animal medicine. This article highlights many standards-of-care and new agents for treatment of these pathogens for small animals and people.  相似文献   
8.
9.
Inapparent of nonprogressive Aleutian disease virus (ADV) infection is a subclinical but persistent virus infection of mink. Mink with the inapparent type of ADV infection when subjected to stress did not develop the progessive form of the disease. However, when challenged with a large dose of the virus, these mink did develop progressive Aleutian disease indicating that they were not highly resistant to the virus. Sera of mink with either the progressive of the inapparent type of ADV infection did not neutralise the virus. The anti-ADV antibody activity in mink with inapparent type of ADV infection was in the IgG fraction of the serum the same as in mink with progressive Aleutian disease. These data indicate that the resistance of the mink with inapparent infection as compared to mink with progressive Aleutian disease was not due to a difference in the class of immunoglobulin response to the virus. However, mink with progressive Aleutian disease showed a greatly increased immunoglobulin response.  相似文献   
10.
Richness of Ancient Woodland Indicator plant species was analysed in 308 woodland patches that were surveyed during the Countryside Survey of Great Britain carried out in 1998. The Countryside Survey recorded vegetation plots and landscape structure in 569 stratified 1 km sample squares and developed a remotely-sensed land cover map of the UK. Using these datasets, we tested the hypothesis that Ancient Woodland Indicator species richness in woodland fragments was limited by patch area, shape and spatial isolation and that woodland patches located in the lowland region of Great Britain would respond differently than those in the upland region. The variation in Ancient Woodland Indicator species richness in the British lowlands (n = 218) was mainly explained by patch area and two measures of connectivity, the length of hedgerows and lines of trees in the 1 km square and the area of woodland within 500 m of the vegetation plot. By contrast, variation in Ancient Woodland Indicator species richness in the British uplands (n = 90) was related to Ellenberg scores of the vegetation communities sampled – a surrogate for habitat quality – and no significant effect of spatial structure was detected. It therefore appears that the degree of fragmentation of woodland in the British lowlands limits the distribution of Ancient Woodland Indicator species, while in the uplands, failed colonisation is a matter of habitat quality rather than a result of landscape structure.This revised version was published online in May 2005 with corrections to the Cover Date.  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