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   30362篇
  免费   1284篇
  国内免费   8篇
林业   1073篇
农学   894篇
基础科学   174篇
  4572篇
综合类   4001篇
农作物   1177篇
水产渔业   1150篇
畜牧兽医   16497篇
园艺   335篇
植物保护   1781篇
  2021年   228篇
  2020年   266篇
  2019年   332篇
  2018年   601篇
  2017年   667篇
  2016年   567篇
  2015年   368篇
  2014年   555篇
  2013年   1667篇
  2012年   864篇
  2011年   1114篇
  2010年   811篇
  2009年   763篇
  2008年   1078篇
  2007年   1055篇
  2006年   893篇
  2005年   716篇
  2004年   722篇
  2003年   663篇
  2002年   589篇
  2001年   745篇
  2000年   746篇
  1999年   545篇
  1998年   245篇
  1997年   227篇
  1995年   269篇
  1992年   400篇
  1991年   416篇
  1990年   432篇
  1989年   440篇
  1988年   344篇
  1987年   354篇
  1986年   384篇
  1985年   342篇
  1984年   333篇
  1983年   304篇
  1979年   483篇
  1978年   328篇
  1977年   370篇
  1976年   468篇
  1975年   485篇
  1974年   525篇
  1973年   480篇
  1972年   439篇
  1971年   354篇
  1970年   372篇
  1969年   434篇
  1968年   441篇
  1967年   446篇
  1966年   438篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
The aim of this research was to observe the effects of cyclophosphamide and its uroprotective agents, mesna and hyperbaric oxygen (HBO), on the motility of urinary bladder muscle in guinea pigs. In the experimental groups, mesna and cyclophosphamide were intraperitoneally injected at a dose of 21.5 mg/kg and 68.1 mg/kg, respectively. For the combination of mesna and cyclophosphamide, one dose of mesna was injected 20 min before cyclophosphamide administration and three additional injections of mesna were repeated every three hours. A total of 8 HBO exposures were performed at 2.8 ATA for 90 min twice daily for another experimental group. In the HBO and cyclophosphamide combined group 5 HBO exposures were given prophylactically before cyclophosphamide. The combination of mesna, HBO and cyclophosphamide was administered by the same procedure. The contractions obtained in response to acetylcholine (ACh, 10(-4) M) in the control group were reduced using cyclophosphamide and HBO individually, but not by mesna. However, the contractions belonging to the various combinations of these three agents were not different from those seen in the control group. On the other hand, the combinations of cyclophosphamide, mesna and HBO showed higher responses to ACh than the groups in which cyclophosphamide and HBO were used individually, while the responses elicited by the cyclophosphamide and HBO combination were greater than those seen in the group treated with HBO only.  相似文献   
102.
The effects of ractopamine (RAC) and ractopamine stereoisomers (RR, RS, SR, and SS) on cyclic AMP (cAMP) production, total protein, and DNA concentrations in mouse skeletal muscle cells (C2C12) were evaluated. The RAC (10 microM) caused an approximately 30% increase in cell number, protein, and DNA concentrations in myoblasts after 48 h; no differences were found in myotubes. The RAC-stimulated increase of these variables in myoblasts was blocked by the presence of equimolar concentrations of propranolol. At a later passage, myoblasts failed to exhibit an increase in cell number, protein, or DNA upon exposure to RAC. Both myoblasts and myotubes increased cAMP production in response to 10 microM RAC. The RAC isomers ranked RR > SR > RS approximately SS in ability to stimulate cAMP production, with essentially no response to SS. The SR produced about 50% of the RR response. Coincubation of propranolol (10 microM) and RAC (10 microM) prevented RAC-stimulated cAMP production in myotubes but not in myoblasts (approximately 35% of cAMP produced by RAC alone). Turkey satellite cells (derived from biceps femoris of 12-wk-old toms) produced essentially no increased cAMP when exposed to 10 microM RAC stereoisomers. Stability of RAC was evaluated under laboratory storage and culture conditions. The RAC was stable for more than 4 mo when stored in deuterated DMSO (>98% purity) at room temperature or in aqueous solutions at -80 degrees C, as determined from sequential nuclear magnetic resonance studies. Radiolabeled RAC was incubated for 72 h in the presence of serum-containing medium, with or without C2C12 cells. Ninety-eight percent of the parent compound found in the medium at time zero was present in the medium as parent at the end of 72 h. The cellular cAMP response to RAC through beta-adrenergic receptors seems to be stereospecific. If the state of myoblasts and myotubes in vitro reflects the in vivo state, then the ractopamine effect in vivo on cellular processes (including cell division and protein and DNA accumulation) may be independent of beta-adrenergic receptors in muscle.  相似文献   
103.
The reference strains of the 12 serotypes of Actinobacillus pleuropneumoniae express one or two of three different RTX exotoxins designated Apx I, Apx II and Apx III. The toxins are important virulence factors. In the present study, ELISAs with purified Apx I, Apx II and Apx III, respectively, as antigen were evaluated as candidates for serological diagnosis of Actinobacillus pleuropneumoniae infection in pigs. The pigs were inoculated with biotype 1, serotypes 1-12, and biotype 2, serotype 14, respectively. A strong humoral antibody response was seen to all the three antigens in most pigs irrespective of the serotype used for inoculation. However, titers to the exotoxins secreted by the serotype used for inoculation were generally highest. The results show that toxin proteins of Actinobacillus pleuropneumoniae are antigenically related and that a correlation between serotype and secretion of exotoxin is not revealed serologically in the ELISA test.  相似文献   
104.
105.

Background

Population characteristics and outcome of cats with arterial thromboembolism (ATE) managed in general practice (GP) have been poorly described.

Hypothesis

Cats with ATE presenting to GP are usually euthanized at presentation, but survival times >1 year are possible.

Animals

Cats with ATE managed by 3 GP clinics in the United Kingdom.

Methods

Records of cases presenting to GP over a 98‐month period (2004–2012) were reviewed. Cats with an antemortem diagnosis of limb ATE were included. Outcome information was obtained.

Results

Over 98 months, 250 cats were identified with ATE. Prevalence was approximately 0.3%. At presentation, 153 cats (61.2%) were euthanized, with 68/97 (70.1%) of the remaining cats (27.2% of the total population) surviving >24 hours after presentation. Of these, 30/68 (44.1%) survived for at least 7 days. Hypothermia (HR, 1.44; 95% CI, 1.002–2.07; P = .049) and management by Clinic 2 (HR, 5.53; 95% CI, 1.23–24.8; P = .026) were independent predictors of 24‐hour euthanasia or death. For cats surviving >24 hours, hypothermia (HR, 2.25; 95% CI, 1.12–4.48; P = .021) and failure to receive aspirin, clopidogrel, or both (HR, 8.26; 95% CI, 1.39–50; P = .001) were independent predictors of euthanasia or death within 7 days. For cats that survived ≥7 days, median survival time was 94 (95% CI, 42–164) days, with 6 cats alive 1 year after presentation.

Conclusions

Although 153/250 cats were euthanized at presentation, 6 cats survived >12 months. No factors were identified that predicted euthanasia on presentation.  相似文献   
106.
The investigations were carried out on 37 skulls from the Van-Yoncatepe early Iron Age necropolis (in eastern Anatolia) (first group) and 11 skulls from the scythians castle from Bileskoje Iron Age (second group). The major distance between both occipital condyles and both bases of the jugular processes, the major width of the foramen magnum, height of the foramen magnum, height of the skull, height of the occipital triangle and the length of the dorsal notch were measured. A dorsal notch was present in seven dogs (six male and one female) from Van-Yoncatepe. The average length of the dorsal notch was 2.5 mm. The investigation states that the dorsal notch occurred in dogs from the Iron Age. This anatomical formation had no association with their death.  相似文献   
107.
1.?An experiment determined the effect of 6 different lysine concentrations (40, 52, 58, 65, 90 and 130 g/kg CP) in two protein concentrations (210 and 260 g/kg) on the efficiency of energy utilisation of broiler chickens and turkeys. Ninety-six male Ross 308 broiler chicks and 96 male BUT Big 6 turkey poults were fed at 90% of ad libitum during a 12 d bioassay period. At the end of the bioassay, birds were slaughtered and carcase crude protein, fat and gross energy were measured and nutrient metabolisability determined.

2.?Broiler chickens had lower AME and AMEN than turkeys. AME was higher in the 260 g/kg than in the 210 g/kg protein diet; however, no difference was evident in AMEN between any of the diets in both species. A significant species × lysine interaction indicated that the decrease in AME and AMEN with increasing lysine was greater in turkeys than in broiler chickens.

3.?Turkeys had lower weight gains and efficiencies of energy retention than broiler chickens. The effects of different lysine concentrations on growth and the efficiency of energy retention were significant and growth responses were quadratic but there were negative linear responses in the efficiency of energy retention in both species at both crude protein concentrations. The efficiency of energy retention was increased with decreasing lysine concentrations throughout the wide range of concentrations used in the experiment.  相似文献   

108.
109.
Objective  To investigate the relationship of the iridocorneal angle as it appears on ultrasound biomicroscopy (UBM) to increases in IOP post-phacoemulsification in the canine eye.
Animals studied  47 eyes of 28 dogs of various age, sex, and breed.
Methods  The ciliary process and limbus were used as a reference points to measure the angle opening distance (AOD), which was set by multiplying 0.45 by the distance between the ciliary process and limbus (when measured from the ciliary process). Pressure measurements were taken at five set points: Before, immediately post-, one day post-, 1 week post-, and 1 month post-phacoemulsification.
Results  A weak relationship between the AOD and the IOP one day post-phacoemulsification (Intercept: 2.83, Slope: –1.24, P -value: 0.0155) was observed. No relationship immediately post-operative (Intercept: 3.45, Slope: –1.34, P -value: 0.0651), 1 week post-phacoemulsification (Intercept: 2.31, Slope: –0.01, P -value: 0.9829), 1 month post-phacoemulsification (Intercept: 1.85, Slope: –0.49, P -value: 0.1533) was observed. The pre-operative measurements made with UBM were: Distance from limbus to ciliary process (DLCP): (Minimum: 1.152, Maximum: 2.992, Mean: 1.91, SD: 0.468); AOD (Minimum: 0.104, Maximum: 0.764, Mean: 0.40, SD: 0.172).
Conclusions  The relationship between AOD as measured by UBM pre-operatively is weakly associated with IOP pressure elevations at day one post-phacoemulsification. Further study is required prospectively to establish the importance of this relationship. Initial measurements of the canine iridocorneal angle were created, suggesting a method to be used in the future to establish true canine normal measurements.  相似文献   
110.
An approximately eight-year-old female grey parrot (Psittacus erithacus erithacus) was presented with a two months history of blindness. The radiographic examination showed a dilatation of the proventriculus, ventriculus and gut. Ophthalmoscopy and electroretinography revealed degeneration of the retina. A proventricular dilatation disease was suspected. The bird was euthanased because of deteriorating condition and poor prognosis. The pathological examination showed an atrophy of the ventricular muscles and lymphoplasmacytic infiltrates of the myenteric plexus of the proventriculus, ventriculus and gut as well as moderate lymphoplasmacytic infiltrates of the cerebrum with moderate neuronophagia. Lymphoplasmacytic infiltrates in the retina, indicating proventricular dilatation disease, and subsequent retinal degeneration were found. A potential common aetiology for proventricular dilatation disease and blindness is discussed.  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