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   129568篇
  免费   7234篇
  国内免费   87篇
林业   4902篇
农学   3777篇
基础科学   803篇
  14477篇
综合类   23005篇
农作物   4667篇
水产渔业   6117篇
畜牧兽医   69642篇
园艺   1490篇
植物保护   8009篇
  2018年   1697篇
  2017年   1871篇
  2016年   1685篇
  2015年   1518篇
  2014年   1808篇
  2013年   4966篇
  2012年   3406篇
  2011年   4210篇
  2010年   2617篇
  2009年   2709篇
  2008年   4004篇
  2007年   3822篇
  2006年   3700篇
  2005年   3319篇
  2004年   3203篇
  2003年   3274篇
  2002年   3099篇
  2001年   3988篇
  2000年   4025篇
  1999年   3185篇
  1998年   1306篇
  1997年   1304篇
  1996年   1232篇
  1995年   1500篇
  1994年   1356篇
  1993年   1296篇
  1992年   2800篇
  1991年   3071篇
  1990年   2879篇
  1989年   2893篇
  1988年   2711篇
  1987年   2772篇
  1986年   2825篇
  1985年   2772篇
  1984年   2196篇
  1983年   1974篇
  1982年   1316篇
  1979年   2081篇
  1978年   1689篇
  1977年   1515篇
  1976年   1461篇
  1975年   1511篇
  1974年   1896篇
  1973年   1912篇
  1972年   1850篇
  1971年   1670篇
  1970年   1682篇
  1969年   1521篇
  1968年   1344篇
  1967年   1389篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
831.
The endocannabinoid anandamide may regulate intestinal motility through activation of CB1 receptors. Anandamide is then inactivated by fatty acid amide hydrolase (FAAH), a membrane bound enzyme. Under pathological conditions, inactivation of such enzymatic activity may lead to inhibition of the intestinal motility. Here, preliminary reports on the distribution of Fatty Acid Amide Hydrolase (FAAH) immunoreactivity in the mouse gastrointestinal neurons, and the pharmacological effects of N‐arachidonoylserotonin (AA‐5HT), a selective inhibitor of FAAH, are reported. FAAH was revealed by an indirect immunofluorescence. Laminar preparations containing the myenteric or the submucous plexus adhered, were peeled off after the whole gut wall had been stretched out and fixed in 4% paraformaldehyde. They were subsequently incubated with a polyclonal anti‐serum directed against a region near the N‐terminus of the human FAAH and revealed by a FITC‐conjugated goat anti‐rabbit secondary anti‐serum. FAAH‐immunoreactive neurons were observed within the myenteric ganglia throughout the GIT. The positive nerve cells varied in size and density of immunoreactivity. Stomach and large intestine showed the highest neuronal density. AA‐5HT significantly reduced both gastric emptying and gastrointestinal tract transit. Such inhibitory effect was reduced by the C1 receptor antagonist SR141716A. Both morphological and pharmacological results suggest that FAAH may play a critical role in controlling gut anandamide levels.  相似文献   
832.
The magnitude of perinatal mortality in 50 species of captive wild ungulates born at the Zoological Society of London's collections at Regent's Park and Whipsnade between 1975 and 1985 is reviewed. Thirty-five per cent of 2471 ungulates born during this 11 year period died before six months old and most deaths occurred in the first week after birth. Similar findings have been reported at other zoos and in the wild. The seasonal distribution of births is described in 43 species. Significantly higher perinatal mortality was found in species which breed throughout the year (notably axis deer and sitatunga) than in seasonal breeders, and differences associated with system of management were apparent in some species (eg, mouflon and scimitar-horned oryx) kept at Regent's Park and Whipsnade. Considerable advances have been made in the management of captive wild ungulates in recent years but it is likely that perinatal mortality rates could be further reduced by improved management and veterinary care of the dams and neonates.  相似文献   
833.
Monoclonal antibodies were produced against bovine lymphoid cells. The reactivities of the antibodies for membrane determinants were examined on both cell suspensions and cryostat tissue sections prepared from bovine blood, thymus, spleen and lymph nodes. The antibodies were putatively grouped into sets which reacted with monomorphic and polymorphic determinants associated with bovine class I and class II major histocompatibility complex (MHC) antigens (MAbs P12 and P3, and R1 and P2 respectively), or associated with differentiation antigens expressed on T cells and monocytes (MAb P5) or exclusively on monocytes (MAb P8). The antibodies were used to identify the surface phenotypes of cells which stimulate (R1+ P5+ P8+) and proliferate (R1- P5+ P8-) in the bovine mixed leukocyte cultures, and cells which proliferate in response to the mitogen, concanavalin A (R1- P5+).  相似文献   
834.
835.
Immunomodulation with killed Propionibacterium acnes was attempted in guinea pigs simultaneously vaccinated with Brucella abortus strain 19. Two groups, each comprised of 9 guinea pigs, were injected by different routes (s.c. and or i.v.) with 1.4 mg of P. acnes and 5 X 10(8) CFU of B. abortus, S-19, while 3 other groups each received either P. acnes, B. abortus S-19, or saline (s.c.). The antibody titers to B. abortus measured at 6, 10 and 14 weeks after vaccination indicated no significant (P less than 0.01) response in the 2 groups immunopotentiated with P. acnes concurrent with B. abortus S-19 vaccination. The delayed hypersensitivity response to 3 Brucella antigens conducted 8 weeks after immunization did not show a significant difference between the B. abortus S-19 vaccinated group compared with the 2 groups immunopotentiated and vaccinated. However, the proliferative response of lymphocytes to the B. abortus soluble antigen diluted 1:100 indicated significantly enhanced blastogenesis in the (s.c.) immunopotentiated and immunized guinea pigs compared with the B. abortus S-19 vaccinated group. A slightly enhanced response was also observed in the group immunopotentiated (i.v.) and vaccinated (s.c.). The guinea pigs were challenged with B. abortus strain 2308 and necropsied 4 weeks later. The mean splenic CFU of the Brucella in the group immunopotentiated (i.v.) and vaccinated (s.c.) was significantly decreased when compared with the guinea pigs vaccinated with B. abortus S-19 alone. These findings indicated that P. acnes administered simultaneously with B. abortus S-19 vaccine was able to augment the immune response in guinea pigs. Immunomodulation as evidenced by enhanced clearance of B. abortus from the spleens of immunopotentiated animals was presumably brought about by activated macrophages or a T-cell mediated cytolytic mechanism or both.  相似文献   
836.
The role of the IgA antibody to Streptococcus agalactiae found in the whey of milks 12 hours after the first intramammary infection of six Friesian first lactation heifers was assessed using an in vitro bactericidal assay. The mean percentage kill of the streptococci by neutrophils in the presence of these wheys was 36.2% while the equivalent figure for the non-infected quarter whey was 0%. When the IgA antibody was absorbed from the infected quarter wheys using class specific IgA antiserum cross linked with glutaraldehyde the percentage kill of the test system fell to 0%. Elution of the absorbed antibody partially restored the activity to a mean percentage kill of 18.2%. The results indicated that the IgA antibody found in infected quarter whey during the acute stages of intramammary infection with Streptococcus agalactiae was responsible for the opsonic activity which pertained at that time.  相似文献   
837.
The concentration of testosterone, 17-beta oestradiol and aflatoxin B1 were studied in the semen plasma of 21 boars of four breeds for the period of twelve months. The following spermiological parameters were investigated: semen volume, sperm concentration, percentage of abnormal spermatozoa, and survival of spermatozoa. The fertilizing capacity of ejaculates was evaluated according to the conception rate of sows and gilts after the first insemination, according to the average number of piglets per litter and average number of live-born piglets per litter. The highest aflatoxin B1 residues in sperm were recorded in March to May and were related with aflatoxin concentration in feed ration. The group of boars with fertility disorders had more aflatoxin in their sperm (up to 100 pmol . l-1), lower sperm concentration, lower survival of spermatozoa, and a larger proportion of abnormal spermatozoa. The year season had a significant influence on the concentration of the hormones. The highest average value of testosterone (10.2 +/- 1.28 nmol) was obtained in autumn and lower values were recorded in winter. The changes in 17-beta estradiol concentration were similar to the changes in testosterone content, with the maximum value in November (0.249 nmol X 1(-1]. The boars with reproduction disorders had a significantly lower concentration of 17-beta oestradiol. Significant correlations were found between the concentration of the hormones, semen volume, and sperm concentration. 17-beta oestradiol also had a significant positive correlation to abnormal spermatozoa and to the activity of aspartate aminotransferase.  相似文献   
838.
The results obtained indicate, that catfish peripheral blood lymphocytes recognize and respond to human IL-1. The second part of this report is dealing with a substance produced by carp epidermal cells with functional similarities to mammalian IL-1.  相似文献   
839.
A dog with signs of weakness, labored breathing, and generalized edema was examined. It was heavily infested with fleas and had wounds resulting from a recent fight. Hematologic findings were not remarkable, except for parasites in the blood. The dog was treated for fleas and given antibiotics, but was euthanatized when it failed to respond. Blood and tissue specimens were found to contain Trypanosoma cruzi, and the serum contained antibodies to the organism. We believe this is the first confirmed case of T cruzi infection in dogs from Oklahoma. The public health implications of this finding are underscored by a report on the detection of T cruzi in raccoons in the same season and geographic area.  相似文献   
840.
Effect of liquid test meal volume on gastric emptying was investigated in healthy nonoperated dogs fed meal sizes of 6.6 to 110 ml/kg of body weight. For meals less than or equal to 66 ml/kg, the amount of test meal emptied from the stomach in 30 minutes vs meal size could be described as following a linear, exponential, or square root pattern. Deviation from these patterns was seen with test meals of 77, 88, and 110 ml/kg. A mean +/- SEM maximum of 0.99 +/- 0.06 ml/kg/min of liquid test meal was emptied from the stomach of dogs fed test meals of 77, 88, and 110 ml/kg. Massive gastric distension induced in dogs during this study did not alter gastric emptying of the liquid test meal.  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