全文获取类型
收费全文 | 240873篇 |
免费 | 14577篇 |
国内免费 | 662篇 |
专业分类
林业 | 22930篇 |
农学 | 13882篇 |
基础科学 | 2934篇 |
41278篇 | |
综合类 | 24013篇 |
农作物 | 17006篇 |
水产渔业 | 16067篇 |
畜牧兽医 | 88700篇 |
园艺 | 6230篇 |
植物保护 | 23072篇 |
出版年
2021年 | 2417篇 |
2020年 | 2813篇 |
2019年 | 3605篇 |
2018年 | 3708篇 |
2017年 | 4072篇 |
2016年 | 4595篇 |
2015年 | 4199篇 |
2014年 | 5553篇 |
2013年 | 16103篇 |
2012年 | 5428篇 |
2011年 | 7090篇 |
2010年 | 6916篇 |
2009年 | 7564篇 |
2008年 | 6552篇 |
2007年 | 5595篇 |
2006年 | 6431篇 |
2005年 | 5711篇 |
2004年 | 5609篇 |
2003年 | 5394篇 |
2002年 | 4816篇 |
2001年 | 4995篇 |
2000年 | 4702篇 |
1999年 | 4602篇 |
1998年 | 4037篇 |
1997年 | 4097篇 |
1996年 | 3847篇 |
1995年 | 4391篇 |
1994年 | 3807篇 |
1993年 | 3521篇 |
1992年 | 4007篇 |
1991年 | 4145篇 |
1990年 | 3907篇 |
1989年 | 3935篇 |
1988年 | 3494篇 |
1987年 | 3522篇 |
1986年 | 3409篇 |
1985年 | 3792篇 |
1984年 | 3549篇 |
1983年 | 3368篇 |
1982年 | 2844篇 |
1981年 | 2786篇 |
1980年 | 2738篇 |
1979年 | 3140篇 |
1978年 | 2826篇 |
1977年 | 2640篇 |
1976年 | 2494篇 |
1975年 | 2359篇 |
1974年 | 2598篇 |
1973年 | 2494篇 |
1972年 | 2363篇 |
排序方式: 共有10000条查询结果,搜索用时 19 毫秒
311.
Morphologic changes in the mammary gland of megestrol acetate-treated and untreated cats: a retrospective study 总被引:4,自引:0,他引:4
Abnormal mammary gland growth is a side effect of progestin therapy in some cats. In this retrospective study, the nature and significance of morphologic changes in the mammary gland of 17 megestrol acetate (MA)-treated cats were compared to mammary lesions in 97 untreated cats. Fourteen out of 17 MA-treated cats had non-neoplastic mammary lesions including fibroepithelial hyperplasia (nine cats), lobular hyperplasia (three cats), and duct ectasia (two cats); whereas three MA-treated cats had mammary neoplasms including one adenoma and two carcinomas. Although MA has been causally linked to mammary cancer in cats, only mammary fibroepithelial hyperplasia was clearly associated with MA therapy in this study. Fibroepithelial hyperplasia occurred in older (average age 8.1 years) neutered male and female cats in the MA-treated group and in younger (average age 2.1 years) female cats in the untreated group. Morphologically, both intraductal and solid fibroepithelial growth patterns were seen. Intraductal fibroepithelial hyperplasia was further subdivided into papillary and circumferential types. An apparent greater association between MA therapy and the intraductal types of fibroepithelial hyperplasia was noted. Furthermore, it appears likely that mammary lobular hyperplasia also is linked to MA therapy. Possible mammatrophic effects of MA and other growth-promoting agents in the cat are discussed. 相似文献
312.
H. Frackowiak 《Anatomia, histologia, embryologia》1989,18(4):342-348
Seventeen lion heads were examined. Their arteries were injected with colored supervinyl-chloride (dissolved in acetone) after which the heads were macerated biologically. The rete mirabile of the maxillary artery was located extracranially and was formed by numerous arteries that arose from the dorsal and medial surfaces of the maxillary artery. The rete released branches (rami retis) which entered the cranial cavity through the orbital fissure where they anastomosed with the intracranial segment of the internal carotid artery. The extracranial portion of the internal carotid was found to be obliterated. Some other arteries leaving the rete anastomosed with the internal ophthalmic artery before it entered the optic canal. The following further arteries left the rete: A. ethmoidalis externa, A. ophthalmica externa, rami musculares and ramus pterygoideus. The rete mirabile of the maxillary artery in the lion provides the principal source of the blood for the brain; it also supplies the orbit. 相似文献
313.
314.
315.
Thyme ( Thymus vulgaris L.) plants were spaced at 15, 30 or 45 cm distances in a clay-loamy soil. They received different levels of nitrogen and phosphorus, besides a constant level of potassium. The results showed, that the wider spacing promoted the growth and production of herb and oil per plant, however dense cultivation significantly increased the yields of herb and oil per unit area. In all cases, the applied fertilization treatments significantly increased the productivity per unit area. Dense cultivation accompanied with higher levels of fertilization proved to be very usefull. The essential oil content was not influenced by either the plant spacing or fertilization treatments applied in this study. 相似文献
316.
317.
Abstract The abundances of Aeromonas salmonicida subsp. salmonicida in the water and in the surface microlayer was studied during the initial phase of a cohabitant infection experiment with Atlantic salmon, Salmo salar L., smolt. Aeromonas salmonicida was detected in the water samples only until the intraperitoneally infected smolt were dead and had been removed. In the lipid rich surface microlayer, A. salmonicida was detected in high concentrations from the day of the first fish mortality and throughout the rest of the experiment. The significance of the high cell surface hydrophobicity is discussed as a possible reason for enrichment of A. salmonicida at the air-water interface. 相似文献
318.
319.
Seedling ryegrass and oat plants in various proportions were exposed to oviposition by frit fly in the laboratory. The growth stage of the ryegrass ranged from single-leaved (unattractive as oviposition sites) to three-leaved (almost immune to serious damage). Oat plants attracted most oviposition and their inclusion usually reduced oviposition on ryegrass, by up to 90%. Where the frit fly population density was so high that potential oviposition exceeded the availability of oviposition sites, inclusion of oats did not reduce the number of eggs laid on ryegrass.
This approach does not, therefore, show promise of a reliable control method against severe primary attack in the field, but is potentially useful over an important range of population densities at which further control measures might otherwise be required. The inclusion of oats would also provide an early warning of damage. 相似文献
This approach does not, therefore, show promise of a reliable control method against severe primary attack in the field, but is potentially useful over an important range of population densities at which further control measures might otherwise be required. The inclusion of oats would also provide an early warning of damage. 相似文献
320.
George J. Brewer MD William Schall DVM Robert Dick BS Vilma Yuzbasiyan-Gurkan PhD Michael Thomas DVM George Padgett DVM 《Journal of veterinary internal medicine / American College of Veterinary Internal Medicine》1992,6(1):41-43
Inherited canine copper toxicosis is a serious problem in Bedlington terriers and West Highland White terriers, and may also be a problem in other less-studied breeds. Affected dogs become ill at midlife with progressive and ultimately fatal liver disease. Treatments for removal of copper and prevention of copper accumulation are available, but are most effective if begun before the dog becomes ill. Until recently diagnosis has not been available until the dog is 1 year of age, and then only by an invasive liver biopsy with determination of liver copper concentration. The authors studied the use of 64copper for early diagnosis of canine copper toxicosis. Two procedures were evaluated. The first involved measuring the concentration of 64copper in blood 24 hours after oral administration of the radioisotope. At this time, 64copper was associated primarily with ceruloplasmin secreted into the blood by the liver. This procedure is useful in the diagnosis of the human counterpart, Wilson's disease. However, the authors found it to be nondiscriminatory between affected and unaffected dogs. In contrast, the second procedure, which involved measuring 64copper excreted in stool during 48 hours after an intravenous dose of radioisotope, yielded results that differentiated most affected and unaffected dogs. 相似文献