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   253545篇
  免费   15251篇
  国内免费   648篇
林业   23392篇
农学   13980篇
基础科学   2946篇
  42622篇
综合类   25302篇
农作物   17130篇
水产渔业   16682篇
畜牧兽医   97245篇
园艺   6304篇
植物保护   23841篇
  2021年   2469篇
  2020年   2857篇
  2019年   3657篇
  2018年   3941篇
  2017年   4335篇
  2016年   4772篇
  2015年   4292篇
  2014年   5656篇
  2013年   16223篇
  2012年   6270篇
  2011年   8182篇
  2010年   7262篇
  2009年   7869篇
  2008年   7558篇
  2007年   6612篇
  2006年   7268篇
  2005年   6489篇
  2004年   6328篇
  2003年   6119篇
  2002年   5451篇
  2001年   6137篇
  2000年   5734篇
  1999年   5402篇
  1998年   4143篇
  1997年   4190篇
  1996年   3916篇
  1995年   4507篇
  1994年   3888篇
  1993年   3630篇
  1992年   4347篇
  1991年   4408篇
  1990年   4142篇
  1989年   4190篇
  1988年   3658篇
  1987年   3757篇
  1986年   3716篇
  1985年   3994篇
  1984年   3668篇
  1983年   3480篇
  1982年   2884篇
  1981年   2812篇
  1980年   2768篇
  1979年   3096篇
  1978年   2818篇
  1977年   2632篇
  1976年   2460篇
  1975年   2307篇
  1974年   2492篇
  1973年   2470篇
  1972年   2205篇
排序方式: 共有10000条查询结果,搜索用时 640 毫秒
811.
The discriminating ability of 15 parameters alone or in combinations, including results from analysis of plasma endotoxin, the Nycomed plasma D-Dimer test and phospholipase A2, were analyzed to predict morbidity and mortality in equine gastrointestinal colic. Endotoxaemia was a characteristic feature of the colic horses. The problem of adequately predicting non-survivors among colic horses required several parameters to be included in the logistic model: if the “classical parameters”, (heart rate, respiratory rate, PCV, anion gap) were included in the model, addition of plasma D-dimer, phospholipase A2, and Cl- significantly improved the predictive value of the logistic model. Increasing heart rate and D-dimer together with decreasing chloride was a risk factor for nonsurvival. The sensitivity of this three-parameter logistic model to predict nonsurvival was 78% and specificity 77%. The Nycomed D-Dimer test is recommended as a horse-site test to predict disseminated intravascular coagulation and nonsurvival in equine colic.  相似文献   
812.
The administration of 20 mg/kg of acetylsalicylic acid in 18 clinically healthy dogs resulted in a thrombocytopathy with lengthened capillary bleeding time and irreversible aggregation inhibition. Through the set up of individual dilution series, one could conclude the proportional percentage of aggregation functional transfused thrombocytes. The capillary bleeding time was shortened after the intravenous injection of prednisolone (5 mg/kg) without measurable influence on the thrombocytes. Compared to the singular use of cortisone or blood transfusion alone, the effect on capillary bleeding time became magnified when one combined transfusion and corticosteroids. The fresh blood conserves (12 hours) were, with respect to the haemostyptical properties, superior to blood stored for 5 days. The component of aggregation-functional thrombocytes was, due to the prophylactic cortisone application prior to transfusion, not increased.  相似文献   
813.
1. The effect of bacterial endotoxin injection was studied in growing pullets of different ages. Commercial chicks were divided into 5 groups according to age. Bacterial endotoxins (E. coli and S. typhimurium) were injected intravenously and rectal temperature was measured over a period of 300 min. 2. The results showed no significant effect of age on the febrile response induced by bacterial endotoxins, but a slight tendency towards a reduced fever peak was observed with increasing age. The response latency also increased with age.  相似文献   
814.
Dairy goats were given subcutaneous implants with 3 mg of norgestomet (NOR) and IM injections of 0.625 mg of estradiol valerate and 0.375 mg of norgestomet on day 0 of the estrous cycle (estrus; NOR 0, n = 18), on postestrus day 4 (NOR 4, n = 18), or on postestrus day 11 (NOR 11, n = 15). Ear implants were removed after 9 days. Mean (+/- SE) hours from removal of ear implants to onset of estrus and proportion of goats responding were 36 +/- 3.8 and 83%, 33 +/- 4.0 and 61%, and 36 +/- 2.7 and 93% for groups NOR 0, NOR 4, and NOR 11, respectively. There were no significant differences between treatment groups in time to onset of estrus. The percentage of goats in group NOR 11 that had signs of estrus was significantly greater than the percentage of goats in group NOR 4. Of the goats in groups NOR 0, NOR 4, and NOR 11 that had signs of estrus, 53, 55, and 86%, respectively, had onset of behavioral estrus between 24 and 48 hours after implant removal. All goats that had signs of estrus had onset of behavioral estrus between 12 and 72 hours after implant removal. Mean (+/- SE) hours from removal of ear implants to time of peak concentrations of luteinizing hormone (LH) were 49 +/- 4.1, 49 +/- 3.8, and 49 +/- 4.0 for groups NOR 0, NOR 4, NOR 11, respectively (not different). The percentage of goats in group NOR 11 that had LH peaks was significantly greater than the percentage of goats in group NOR 4.(ABSTRACT TRUNCATED AT 250 WORDS)  相似文献   
815.
SUMMARY Bronchoalveolar lavage (BAL) was performed at weekly intervals in 10 Thoroughbred horses in race training (group 1) and in 4 rested horses (group 2) for 10 weeks. Lavages were continued on a weekly basis in 4 group 1 horses for an additional 9 weeks (group 3). Cytological analysis of samples included leukocyte counts, erythrocyte counts, differential leukocyte counts, and haemosiderophage score. The mean leukocyte concentration was significantly lower in group 1(92.1 ± 4.6 cells/μL) when compared with group 2 (133.5 ± 8.2 cells/μL), P = 0.037. The differential leukocyte data were not significantly different between groups. There was a large amount of variability in the percent-age of macrophages and lymphocytes in the differential counts over time with no obvious trends. The proportion of neutrophils demonstrated a tendency to decrease over time for both groups 1 and 2. Erythrocyte counts and haemosiderin scores were significantly higher in the exercising group than the rested horses. Neither exercise nor the technique itself evoked an inflammatory response in the BAL fluid.  相似文献   
816.
817.
Moderately PSE (pale, soft, exudative) and moderately DFD (dark, firm, dry) pork was examined by x-ray diffraction for interfilament separation, by differential interference contrast microscopy for interfiber area, and was centrifuged to measure water holding capacity (WHC). Internal reflectance spectra were measured by fiber optics. For PSE to DFD pork, filament separation ranged from 39 to 48 nm, interfiber area from 42 to 3%, and WHC from 49 to 64%, respectively. The correlation of reflectance with interfilament separation varied considerably with wavelength (reaching r = -.83 at 680 nm, P less than .005). The correlation of reflectance with interfiber area was more uniform across the spectrum (reaching r = .90 at 450 nm, P less than .005), as was the correlation of reflectance with WHC (reaching r = -.80 at 400 nm, P less than .005). At 24 h postmortem, fiber-optic spectrophotometry may be used as a rapid, nondestructive method to predict WHC and potential fluid losses from commercial pork with a moderate range from PSE to DFD. Interfiber area was correlated negatively with filament lattice area and WHC, but no significant correlation was found between filament lattice area and WHC. Filament separation was decreased only slightly by centrifugation. These results indicate that at 24 h postmortem the extra fluid released from PSE pork already has been lost from the myofilament lattice and is awaiting release from compartments downstream such as interfiber and interfascicular spaces.  相似文献   
818.
We have adapted an enzyme-linked immunoblot assay (ELIBA) for the detection of a c-ras proto-oncogene and oncogene protein products in human cell lines and tumors of 21,000 daltons molecular weight (p21ras) to studies of tissues derived from sheep. In the ELIBA, a double antibody system is used in which p21ras proteins are initially immunoprecipitated from protein extracts with monoclonal antibodies, and subsequently identified using additional anti-ras antibodies. Binding is identified with a non-radioactive enzyme-linked colorimetric detection system. In the present study, the ELIBA system was used to study twenty-seven ovine lung specimens, representing normal lung, inflammatory, and neoplastic lesions. We detected p21ras protein expression in every tissue examined, but the nature and amount of the protein product varied significantly among the tissues examined. Some tissues expressed multiple ras species. Broncho-alveolar carcinoma specimens were most likely to express c-Ki-ras proteins. Mutant proteins of c-N-ras and c-Ki-ras were detected in several bronchoalveolar carcinoma specimens, based on migrational differences between mutant and normal proteins in 15% polyacrylamide gels. The results of this study demonstrate the utility of the ELIBA system for detection of c-ras expression in ovine lung tissues, and demonstrate the ability of the system to discriminate specific ras protein species. The prognostic significance of ras expression in sheep pulmonary carcinoma has yet to be determined.  相似文献   
819.
Germinating seeds and young plants of winter rape var. Górczañski were vernalized for 56–63 days under conditions of 9-hour day, at the temperature 2 and 5 °C and in continuous darkness at the temperature 2 °C. After vernalization the plants grew under conditions enabling to complete vernalization: in a glass-house at the temperature day/night 15/10 °C and in semi natural conditions of open vegetation hall in the period from June till August. After sub-optimal vernalization further growth of the plants at lowered temperature increased its effectiveness (completion of vernalization). Depending on the degree of the vernalization of the plants the completion of their vernalization was both obligatory, i.e. conditioning the acquisition of the ability of generative development, and facultative i.e. accelerating this development. It has been demonstrated that the population of plants of the examined variety is strongly differentiated not only with respect of vernalization requirements in the particular plants, but also what regards the effectiveness of vernalization completion. New observations have been made indicating that the mechanisms controlling the successive phases of generative development, i.e. phase of forming flower buds and the flowering phase are not identical which may be interpreted as indicating that the "flowering factor" is polymorphous.  相似文献   
820.
Ten, anesthetized dogs were instrumented with three pulse oximeter probes; two lingual transmittance probes and one rectal reflective probe. Arterial oxygen desaturation was produced by decreasing the inspired oxygen concentration. Hypotension was produced with an infusion of nitroprusside. Simultaneous pulse oximeter readings (SpO2) were compared to co-oximeter measured arterial saturation (SaO2) collected over a range of SaO2 (50–100%) and mean arterial pressures (40–100mmHg). Each of the monitors and means of evaluating SpO2 studied provided accurate SpO2 measurements over a range of mean arterial pressure from 40–100mmHg. All of the monitors tested tended to overestimate the SaO2 when the arterial saturation was less than 70%.  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