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   173582篇
  免费   9490篇
  国内免费   120篇
林业   8192篇
农学   5841篇
基础科学   1083篇
  22679篇
综合类   26199篇
农作物   6835篇
水产渔业   8921篇
畜牧兽医   89785篇
园艺   2378篇
植物保护   11279篇
  2019年   1692篇
  2018年   2648篇
  2017年   3003篇
  2016年   2764篇
  2015年   2327篇
  2014年   2904篇
  2013年   6950篇
  2012年   5290篇
  2011年   6517篇
  2010年   4228篇
  2009年   4290篇
  2008年   6297篇
  2007年   6000篇
  2006年   5668篇
  2005年   5306篇
  2004年   5002篇
  2003年   5130篇
  2002年   4603篇
  2001年   5630篇
  2000年   5669篇
  1999年   4501篇
  1998年   1929篇
  1997年   1830篇
  1996年   1723篇
  1995年   2019篇
  1994年   1775篇
  1993年   1642篇
  1992年   3446篇
  1991年   3655篇
  1990年   3625篇
  1989年   3534篇
  1988年   3232篇
  1987年   3221篇
  1986年   3233篇
  1985年   3128篇
  1984年   2570篇
  1983年   2224篇
  1982年   1529篇
  1979年   2336篇
  1978年   1880篇
  1977年   1510篇
  1976年   1597篇
  1975年   1657篇
  1974年   1985篇
  1973年   1981篇
  1972年   1875篇
  1971年   1776篇
  1970年   1693篇
  1969年   1697篇
  1967年   1524篇
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
41.
A systematic immunohistochemical study of the ultimobranchial tubule (UBT) has been carried out in 45 Wistar rats of different ages (0, 5, 10, 15, 20, 25, 30, 60 and 120 days). The existence of calcitonin immunoreactive cells in the UBT wall has been demonstrated in a 5-days old rat. In addition, immunohistochemical studies for thyroglobulin revealed positive staining in follicular cells connected to the UBT and, occasionally, in isolated cells lying within solid clusters from the UBT. These last results together with the continued and repeated existence of numerous mitosis and PAS (+) microfollicles, apparently rising from the UBT, support the hypothesis that the ultimobranchial body (UBB) may contribute partially to the formation of a part of the follicular component.  相似文献   
42.
The oscillatory potentials (OP) on the b-wave of the canine ERG are characterized. Normal values for an OP-index, implicit times and periodicity are given. The OP-index increases during dark adaptation as well as with increasing stimulus intensities for white light flashes. Scotopic blue and red stimuli, although balanced for the maximum b-wave amplitude, surprisingly result in higher OP-indices for red flashes. Implicit times decrease for all OP with increasing stimulus intensities. Scotopic balanced red light results in OP with markedly reduced implicit times compared to blue light stimulation. The intervals between two OPs remain constant during dark adaptation, however, intervals between OPs with longer implicit times tend to be shorter. In contrast, the b-waves for scotopic balanced stimuli show rhythmic oscillations with constant intervals between them, the intervals being shorter for blue light than for red light. The intervals between OPs tend to become shorter with increasing stimulus intensities. This investigation indicates that OPs are influenced but not generated by photoreceptors. The examination of a dog with optic nerve hypoplasia indicates that the ganglion cells do not contribute to the biogenesis of OPs. The prognostic value of OPs in the early diagnosis for hereditary retinal degenerations in the dog is discussed.  相似文献   
43.
Porcine urogenital disease is the result of an imbalance of the normal microflora of the urinary and reproductive tracts brought about by hormonal, environmental, and management-related stress factors. Production and economic losses can be substantial, and diagnosing and treating the problem can be frustrating. Through proper hygiene, facility design, and culling procedures, the severity of the problem can be minimized.  相似文献   
44.
Losses due to mortality and rejection of carcases and viscera in a population of 2,959,607 pigs admitted for slaughter in Singapore abattoirs between 1984 and 1986 were studied. Mortality losses were 2822 pigs (9.5 per 10,000 admissions) while 3039 whole carcases (10.3 per 10,000 admissions) were condemned at post-mortem examination. The main reason for rejection of carcases was pyaemia (30.3%). Kidneys and livers were the two main organs of economic value rejected. Rejection of kidneys was primarily due to nephritis (54.8%) while liver condemnation was mainly due to cirrhosis (38.6%). The financial loss from abattoir rejection was S$5.27 millions or S$1.78 per pig admitted. The value of abattoir condemnation data as a tool in preventive medicine is discussed.  相似文献   
45.
A sensitive and precise immunoassay for equine serum amyloid A protein (SAA) was established and used to determine, for the first time, the circulating concentration of this protein in health and disease. As in other species, equine SAA was present only at trace levels in healthy animals but behaved as an extremely sensitive and rapidly responding acute phase reactant following most forms of tissue injury, infection and inflammation, objectively reflecting the extent and activity of disease. Measurements of SAA should make a significant contribution to diagnosis and management of viral and bacterial infection in horses, and routine serial assays could provide an objective criterion for monitoring prospectively the general health of horses in training and racing.  相似文献   
46.
1. The incidence of microscopically detectable degenerative characteristics in 5 skeletal muscles (m. pectoralis thoracicus, m. supracoracoideus, m. biceps femoris, m. semitendinosus, m. femorotibialis medius) of turkeys was investigated. 2. Samples were obtained from 30 Large White turkey males 14, 16 and 18 weeks old. Hyaline degeneration, infiltration of mononuclear cells and necrotic fibres were observed. 3. Individual fibres varied greatly in size and muscle fibre nuclei were often shrunken and pyknotic. 4. Weak and/or uniform reaction for Ca++-ATPase and SDH in all types of muscle fibres and loss of alkaline phosphatase activity in cell membranes were noted. A positive reaction for acid phosphatase occurred in regions of perivascular infiltration and in necrotic muscle fibres. The majority of muscle fibres possessed high activity for phosphorylase a and b. 5. Based on the use of fluorescein alpha-bungarotoxin conjugate, motor end-plates appeared to be morphologically intact. Direct immunofluorescence with anti-chicken IgG showed positive reaction in muscle fibres undergoing necrosis and in the involved connective tissue. 6. Degenerative changes varied with age and were most marked in the oldest birds. 7. Because gross degenerative symptoms were absent from both the birds and the meat from them, the condition appears to be either different from or a precursor to the degenerative myopathy characterised by other authors.  相似文献   
47.
White leghorn hens were experimentally infested with northern fowl mites (Ornithonyssus sylviarum) and antibody responses to mite immunogens were monitored over 12 weeks. Mite burdens increased during the early phase of infestation and declined over the latter weeks of the study. Antigen was prepared from homogenized whole mites, which were then sonicated and extracted with non-ionic detergent. Antigen extract was fractionated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and antibody-reactive polypeptides were identified by immunoblotting. At the start of infestation, hens had natural, pre-existing antibodies that reacted with several mite-extract components. Individual hens had different natural antibody reactivities; however, all birds had immunoglobulins reactive with extract polypeptides of 117,000, 77,000 and 36,000 molecular weight. A variety of mite extract components reacted with hen antibodies generated in response to experimental infestation. The number of antibody-reactive mite polypeptides increased through week 8 of infestation and then decreased by week 12. Fifteen polypeptides of northern fowl mite extract were reactive with antibodies developed by the majority of infested birds. These commonly reactive polypeptides had molecular weights ranging from 40,000 to 160,000. Glycoconjugates of fractionated mite extract were identified by blotting with lectins that have different carbohydrate binding specificities. Also identified were lectins that bound extract components with the same molecular weights as those moieties complexed by immunoglobulins of infested birds.  相似文献   
48.
The effect of food deprivation on ova transport, hormonal profiles and metabolic changes was studied in 20 crossbred multiparous sows during their second oestrus after weaning. To determine the time of ovulation, transrectal ultrasonographic examination was performed. The sows were divided into 2 groups, one control group (C-group), which was fed according to Swedish standards, and one experimental group (E-group). The E-group sows were deprived of food from the first morning meal after ovulation until slaughter. Blood samples were collected every second hour from about 12 h before expected ovulation in the second oestrus after weaning until slaughter and were analysed for progesterone, prostaglandin F2 alpha-metabolite, insulin, glucose, free fatty acids and triglycerides. All sows were slaughtered approximately 48 h after ovulation and the genital tract was recovered. The isthmic part of the oviduct was divided into 3 equally long segments and flushed separately with phosphate buffered saline (PBS). Uterine horns were also flushed with PBS. A significantly greater number of ova were found in the first and second part of the isthmus in the E-group (p = 0.05) while in the C-group most of the ova were found in the third part of the isthmus or the uterus (p = 0.01). The level of prostaglandin F2 alpha-metabolite was significantly higher in the E-group compared with the C-group. The concentration of progesterone increased in both groups after ovulation but there were no significant differences between the groups. The other blood parameters showed that the food-deprived sows were in a catabolic state. The 48 h period of fasting results, directly or indirectly in an delayed ova transport, which may be due to a delayed relaxation in the smooth circular muscle layer of the isthmus.  相似文献   
49.
Fifteen and eight mature beagles, without (normal group) and with experimental mitral regurgitation (MR group), respectively, were given 0.02 mg/kg/day digoxin powder for 10 days orally. The optimum time for sample collection after administration of digoxin was observed to be 8-18 hr and 10-22 hr in the normal and MR groups, respectively. In both groups, a stable concentration was reached after 3-5 days of treatment. No differences in plasma level were observed between sexes. The optimum concentration of digoxin was attained at an earlier stage than has been previously reported for both dogs and humans.  相似文献   
50.
The purpose of this study was to evaluate seasonal influences on thyroid hormone levels of healthy outdoor dogs in Hokkaido. We surveyed serum basal total thyroxine (tT4), free thyroxine (fT4), and canine thyroid-stimulating hormone (cTSH) levels, and tT4 levels after administration of TSH for a year. Basal tT4 levels decreased in January, and increased in August and September. fT4 levels increased in January and November. No significant seasonal variation was found in cTSH. tT4 levels after administration of TSH in August and November increased. These results suggested that the thyroid gland may have been activated in November. We should take seasonal variation into consideration when thyroid function is tested.  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