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   89735篇
  免费   5529篇
  国内免费   46篇
林业   3735篇
农学   3466篇
基础科学   632篇
  11245篇
综合类   13526篇
农作物   3359篇
水产渔业   4937篇
畜牧兽医   47630篇
园艺   1165篇
植物保护   5615篇
  2020年   732篇
  2019年   954篇
  2018年   1582篇
  2017年   1845篇
  2016年   1660篇
  2015年   1404篇
  2014年   1718篇
  2013年   3716篇
  2012年   2931篇
  2011年   3587篇
  2010年   2339篇
  2009年   2331篇
  2008年   3470篇
  2007年   3208篇
  2006年   3135篇
  2005年   2668篇
  2004年   2575篇
  2003年   2591篇
  2002年   2372篇
  2001年   3389篇
  2000年   3304篇
  1999年   2553篇
  1998年   983篇
  1997年   1051篇
  1996年   883篇
  1995年   1022篇
  1994年   895篇
  1993年   896篇
  1992年   1845篇
  1991年   1919篇
  1990年   1856篇
  1989年   1859篇
  1988年   1607篇
  1987年   1594篇
  1986年   1627篇
  1985年   1526篇
  1984年   1225篇
  1983年   1032篇
  1982年   713篇
  1979年   1060篇
  1978年   816篇
  1975年   815篇
  1974年   897篇
  1973年   864篇
  1972年   805篇
  1971年   795篇
  1970年   800篇
  1969年   780篇
  1968年   708篇
  1967年   734篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
Forty-six cats with clinical haemobartonellosis were studied; 75 per cent of the cats of known age were two-and-a-half years old or younger, 50 per cent were intact males and 19.5 per cent were castrated males. The predominant signs of the disease were tachypnoea, lethargy, depression, anorexia, infestation with fleas, pale mucous membranes, icterus, emaciation, dehydration, splenomegaly, anaemia, leucocytosis, increased activities of alanine aminotransferase and aspartate aminotransferase, and azotaemia. Thirty-eight per cent of the cats that were tested for feline leukaemia virus (FeLV) antigen were positive, and 22 per cent of those tested for feline immunodeficiency virus (FIV) antibodies were positive. The prevalence of both FeLV and FIV was much higher than in the general Israeli cat population. The cats infected with both Haemobartonella felis and FeLV had a significantly lower body temperature, were more anaemic and the mean cell volume of their erythrocytes was greater than in the cats with haemobartonellosis alone.  相似文献   
992.
Morphological and biochemical aspects of apoptosis,oncosis and necrosis   总被引:39,自引:0,他引:39  
Recent investigations have demonstrated the need for a precise differentiation of various forms of cell death such as apoptosis, oncosis, necrosis and programmed cell death. Apoptosis is marked by cellular shrinking, condensation and margination of the chromatin and ruffling of the plasma membrane with eventually breaking up of the cell in apoptotic bodies. Cell death marked by cellular swelling should be called oncosis, whereas the term necrosis refers to the morphological alterations appearing after cell death. Apoptosis and oncosis are therefore pre-mortal processes, while necrosis is a post-mortal condition. The term programmed cell death refers to the 'fixed' pathway followed by dying cells, whether or not with the characteristic morphology of apoptosis. Three mechanisms are actually known to be involved in the apoptotic process: a receptor-ligand mediated mechanism, a mitochondrial pathway and a mechanism in which the endoplasmic reticulum plays a central role. All three mechanisms activate caspases which are responsible for the characteristic morphological changes observed during apoptosis. A review of the different methods used for detecting apoptotic cells demonstrates that most of these techniques are not entirely specific.  相似文献   
993.
994.
Pericardial tissue was obtained from 14 dogs with idiopathic pericarditis, and from three dogs with pericardial effusion associated with neoplastic disease, for histopathological assessment and characterisation of infiltrating leucocytes by immunohistochemistry. The major pathological change was extensive pericardial fibrosis which was generally accompanied by a mixed inflammatory response that was of greatest intensity at the cardiac surface of the tissue. Perivascular lymphoplasmacytic aggregates were present at the pleural surface and within the fibrosed pericardium. There were no features that clearly distinguished the samples from dogs with neoplastic disease from dogs with idiopathic pericarditis. The pericardial infiltrates were dominated by MAC 387+ monocyte-macrophages and plasma cells expressing immunoglobulin (Ig)A or IgG. CD3+ T lymphocytes and major histocompatibility complex (MHC) class II+ macrophages were less common, although the perivascular aggregates were mixtures of T and B lymphocytes and a proportion of fibroblasts expressed MHC class II. There was no vascular pathology or deposition of immunoglobulin or complement within vessel walls. These findings are consistent with an immune response dominated by humoral effector mechanisms (Th2 immunity) but do not clearly support a primary immune-mediated pathogenesis for idiopathic pericarditis.  相似文献   
995.
The aim of this study was to develop an immunological method for the identification of sheep infected with Echinococcus granulosus which would allow the monitoring of animals imported into countries free from hydatidosis and as an aid to countries where control schemes for the disease are in operation. Three enzyme-linked immunosorbent assays (ELISAs) were developed and validated, using as antigen either a purified 8 kDa hydatid cyst fluid protein (8kDaELISA), a recombinant EG95 oncosphere protein (OncELISA) or a crude protoscolex preparation (ProtELISA). Sera used for the assay validations were obtained from 249 sheep infected either naturally or experimentally with E. granulosus and from 1012 non-infected sheep. The highest diagnostic sensitivity was obtained using the ProtELISA at 62.7 and 51.4%, depending on the cut-off. Assay sensitivities were lower for the 8kDaELISA and the OncELISA. Diagnostic specificities were high, ranging from 95.8 to 99.5%, depending on the ELISA type and cut-off level chosen. A few sera from 39 sheep infected with T. hydatigena and from 19 sheep infected with T. ovis were recorded as positive. Western immunoblot analysis revealed that the dominant antigenic components in the crude protoscolex antigen preparation were macromolecules of about 70-150 kDa, most likely representing polysaccharides. This study demonstrated that the ProtELISA was the most effective immunological method of those assessed for detection of infection with E. granulosus in sheep. Because of its limited diagnostic sensitivity of about 50-60%, it should be useful for the detection of the presence of infected sheep on a flock basis and cannot be used for reliable identification of individual animals infected with E. granulosus.  相似文献   
996.
Studies have indicated variations in the degree of efficacy of certain commercial disinfectants used in poultry production facilities. We used an adequate method of in vitro testing to compare the efficacy of disinfectants while testing them in conditions similar to those of the poultry facilities. BioSentry 904, ethylenediaminetetracetic acid (EDTA)-Tris, and a combination of the two were tested by this method against five field isolates of Pseudomonas aeruginosa at 10(3), 10(6), and 10(9) colony-forming units (CFU)/ml. At the 10(9) CFU/ml concentration, most compounds failed to achieve a total kill with a contact time of 15 min. When tested at bacterial concentrations of 10(3) CFU/ml, the combination of EDTA-Tris mixed at a 1:1 ratio with BioSentry 904 killed the bacteria upon initial contact (< or = 0.05 min). This disinfectant mixture exhibited antagonistic, indifferent, or synergetic effects when exposed to different bacterial isolates at a concentration of 10(6) CFU/ml.  相似文献   
997.
A rostrocaudal (RCd) nasal view was developed in large breed mesaticephalic dogs using a complete, subsequently sectioned, skull and cadaver specimens to optimise the radiographic technique and evaluate normal anatomic features. Gelatin was placed in one nasal passage of the cadaver specimens to mimic the effects of nasal pathology. The latter specimens and 18 clinical cases with suspected nasal disease were evaluated to determine the usefulness of the RCd view compared to standard nasal views. An optimal RCd view was obtained with the dog in dorsal recumbency and the head symmetrically positioned with the hard palate perpendicular to the table using a table top technique with 8:1 grid, collimating to the nasal region and centring the primary beam on the philtrum. The dorsolateral aspects of the maxillary bone, the nasal bones, septal sulcus of the vomer, mucosa lined nasal septum and conchae could be seen. A centrodorsal more radiolucent area representing the ethmoid bone region was also visible. Gelatin soft tissue opacification of the nasal passage could be seen more clearly in RCd nasal view than in occlusal dorsoventral view. In clinical cases the RCd view was useful to build up a 3-dimensional image of nasal passage pathology as well as to detect nasal septum and osseous nasal border pathology not visible in other views. This view is particularly useful in cases where cross-sectional imaging modalities are not available or where the nasal investigation is limited by cost considerations.  相似文献   
998.
Delayed acute capture myopathy is the term used to describe the clinical syndrome observed in three roe deer captured by drive-nets and transported to an enclosure for scientific purposes. The animals died 48 h, 60 h and 8 days after being captured. The simultaneous deaths coincided with a previous episode of deliberate human disturbance. The histopathological findings were indicative of acute myopathy and myoglobinaemic nephrosis. These could be related to an ataxic myoglobinuric syndrome brought on by capture and transport operations. The lack of clinical signs and negative prognosis indicators in the period between capture and just before death. the absence of gross muscular lesions in the animal that died after 8 days post-capture, the simultaneous deaths of animals captured at different times and the evidence of deliberate human disturbance in the enclosure are suggestive of death triggered by a second stress episode.  相似文献   
999.
Three adult brown pelicans (Pelicanus occidentalis) were observed to be weak, anorexic and unresponsive to antibiotics, anti-inflammatory drugs, vitamins including vitamin E, and steroids. Blood chemistry revealed high activities of aspartate aminotransferase, creatinine kinase and lactate dehydrogenase. Radiographs of the birds' leg muscles revealed multiple opacities suggestive of calcification; the gross lesions included white streaks in the leg, wing, and heart muscles, and the microscopical lesions consisted of various degrees of degeneration and necrosis characterised by eosinophilia, variations in fibre size, loss of striations, myolysis, mineralisation, and proliferation of mononuclear cells in the skeletal muscles and the myocardium. The levels of heavy metals, selenium and vitamin E in the birds' livers were not abnormal. The level of peroxide in their diet of capelin fish was high, 69 meq/kg, (normal <20 meq/kg) consistent with rancid feed, and the level of vitamin E was very low, 0.5 iu/kg (normal 20 to 30 iu/kg). It was concluded that the myopathy was probably caused by vitamin E deficiency due to feeding the pelicans a rancid diet.  相似文献   
1000.
OBJECTIVE: To determine the effects of early postoperative rehabilitation on limb function in dogs after surgery for ruptured cranial cruciate ligament (RCCL). DESIGN: Prospective clinical study. ANIMALS: 51 client-owned dogs. PROCEDURE: Dogs weighing between 20 and 40 kg (44 to 88 lb) that had RCCL and medial meniscal injury were studied. After removal of the RCCL and complete medial meniscectomy, the stifle joint was stabilized by use of a lateral retinacular stabilization technique. Twenty-five dogs were included in a postoperative rehabilitation group, and 26 dogs were included in an exercise-restricted group. Limb function (peak vertical force [PVF] and vertical impulse [VI]) was determined before surgery and 6 months after surgery, using force platform gait analysis. RESULTS: Prior to surgery, mean PVF and VI in affected limbs were similar between groups. Six months after surgery, PVF and VI were significantly increased in dogs of both groups. However, PVF and VI in dogs in the rehabilitation group were significantly greater than those of dogs in the exercise-restricted group. At this time, differences in limb function (as measured by PVF and VI) between the repaired and normal limbs were not evident in dogs in the rehabilitation group. Conversely, limb function in the repaired limb of dogs in the exercise-restricted group was still significantly less than that of the normal limb. CONCLUSION AND CLINICAL RELEVANCE: Dogs that have surgery for RCCL and a torn medial meniscus benefit from postoperative rehabilitation; rehabilitation should be considered part of the postoperative management of these patients.  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