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   7310篇
  免费   999篇
  国内免费   1篇
  8310篇
  2021年   87篇
  2016年   111篇
  2015年   195篇
  2014年   199篇
  2013年   285篇
  2012年   333篇
  2011年   328篇
  2010年   218篇
  2009年   215篇
  2008年   309篇
  2007年   297篇
  2006年   269篇
  2005年   229篇
  2004年   244篇
  2003年   235篇
  2002年   241篇
  2001年   234篇
  2000年   230篇
  1999年   191篇
  1998年   78篇
  1997年   85篇
  1996年   79篇
  1995年   63篇
  1994年   72篇
  1993年   65篇
  1992年   126篇
  1991年   134篇
  1990年   152篇
  1989年   132篇
  1988年   137篇
  1987年   120篇
  1986年   126篇
  1985年   140篇
  1984年   108篇
  1983年   124篇
  1982年   98篇
  1981年   81篇
  1980年   82篇
  1979年   113篇
  1978年   87篇
  1977年   75篇
  1976年   71篇
  1975年   88篇
  1974年   85篇
  1973年   82篇
  1972年   84篇
  1971年   66篇
  1970年   86篇
  1969年   76篇
  1967年   70篇
排序方式: 共有8310条查询结果,搜索用时 15 毫秒
91.
Summary A monoclonal antibody against substance P was used for immunocytochemical staining of the central ganglia of the snail Helix aspersa and several peripheral tissues including the gut, reproductive system, cardiovascular system, tentacle and other muscles.Within the central ganglia many neurones, and many fibres in the neuropile and the nerves entering the ganglia, were stained for the SP-like material. The largest numbers of reactive cell bodies were in the pleural ganglia and on the dorsal surfaces of the pedal ganglia. A group of cells was also found, surrounding the right pedal-cerebral connective, that did not fluoresce, but were enveloped by reactive processes terminating directly onto the neurone somata.Specific staining was observed in all peripheral tissues examined and always appeared to be concentrated in nerve terminals. Most particularly these occurred in the heart and aorta, the pharyngeal retractor muscle and the tentacle. Although mostly present in muscular tissues, some fluorescence was also observed in the nervous layer surrounding the retina. The tentacular ganglion also contained immunoreactive cell bodies.  相似文献   
92.
93.
94.
Observations on the separation of Theileria sporozoites from ticks   总被引:3,自引:0,他引:3  
Hyalomma anatolicum anatolicum ticks infected with Theileria annulata were partially fed on rabbits and then ground up with tissue culture medium. The ground up ticks were treated by centrifugation at 100 g, filtration through membranes of 8 μm pore diameter and centrifugation on a discontinuous density gradient of Percoll. Counts of sporozoites and tick debris were made from Giemsa stained slides of samples at each stage of the separation. Debris was removed during light centrifugation and filtration at a greater rate than sporozoites. After filtration approximately 41% of the original sporozoites remained in the suspension. After density gradient centrifugation most sporozoites were found in a distinct zone, at approx. 1·08 g/cm3 density, separate from most dense debris and light debris and soluble contaminants. After this final centrifugation approximately 24% of the original sporozoites remained in the recovered suspension.  相似文献   
95.
Legionella micdadei is a recently described opportunistic pulmonary pathogen that produces an acute, suppurative pneumonia in patients receiving steroid therapy. Most prospective diagnoses have been made by open lung biopsy. We present a case in which the diagnosis was made from cytologic material. The clinicopathologic features of L. micdadei pneumonia are discussed, and criteria for diagnosis from cytologic specimens are presented.  相似文献   
96.
The identification of the treponemas among prehistoric Amerindian populations is problematic. This paper presents the evidence for the presence of cardiovascular disease of syphilitic origin on the Plains of North America during prehistoric times. Comparative data from modern populations is used to arrive at a diagnosis.  相似文献   
97.
When intact mesophyll chloroplasts of Zea mays var Kelvedon Glory were illuminated, activation of NADP-malate dehydrogenase occurred. Activity declined rapidly on darkening. Light activation of the enzyme was very much greater in the presence of pyruvate (~10- to 20-fold) than with the electron acceptors 3-phosphoglycerate or oxaloacetate present (~2-fold). Following preillumination in the presence of pyruvate, addition of 3-phosphoglycerate, oxaloacetate, or nitrite substantially diminished the activity of NADP-malate dehydrogenase. In these circumstances, with pyruvate and 3-phosphoglycerate present, activity could be restored by the addition of nigericin or dihydroxyacetone phosphate. Nigericin also restored activity with both oxaloacetate and pyruvate present. The effect of nitrite was more marked in the presence of low concentrations of DCMU.

These observations are discussed in terms of the dependence of enzyme activity upon the redox state of ferredoxin and electron carriers; the redox state of the latter was estimated by analysis of the DCMU-induced relaxation kinetics of chlorophyll fluorescence quenching in the presence of different substrates.

  相似文献   
98.
Intact chloroplasts prepared from summer-grown spinach plants supported (aspartate plus 2-oxoglutarate)-dependent O2 evolution but not (glutamine plus 2-oxoglutarate)-dependent O2 evolution. The former activity, which was sensitive to amino oxyacetate, was attributed to transaminase activity and reduction of the resulting oxalo-acetate to malate using H2O as eventual electron donor. A reconstituted chloroplast system which included chloroplast stroma, thylakoid membranes, ferredoxin and NADP(H) supported O2 evolution in the presence ofl-glutamine and 2-oxoglutarate at rates of 15–22 μmol mg-1 chlorophyll h-1 although lower rates were obtained with material from winter-grown plants. Activity was not observed in the absence of ferredoxin and omission of NADP(H) decreased activity by 40%. The reaction was associated with the production of 0.49 mol O2 mol-1 2-oxoglutarate consumed and up to 0.46 mol O2 mol-1 glutamine supplied. The reaction, which was inhibited by azaserine but not by methionine sulphoximine or amino oxyacetate, was attributed to light-coupled glutamate synthase (EC 1.4.1.13) with H2O serving as eventual electron donor. Activity was not affected significantly byl-malate. The reconstituted system also supported O2 evolution in the presence of nitrite, oxaloacetate, (aspartate plus 2-oxoglutarate) and oxidised glutathione.  相似文献   
99.
The ultrastructure of the pituitary, the adrenal, and the lung was examined in the newborn wallaby, Macropus rufogriseus. Tissue from six wallaby neonates (less than 8 hr of age), two near-term fetuses (26 days after removal of suckling pouch young [RPY]), and a two-day-old pouch young was examined; and tissue levels of cortisol in the adrenal glands of five neonates and a near-term fetus (26 days) were measured by radioimmunoassay. At birth the adenohypophysis comprised the bulk of the pituitary gland. The pars distalis was well vascularized and many cells contained electron-dense, membrane-bound granules. The adrenal glands lacked specific zones but comprised two distinct populations of cells. The cytoplasm of one cell type contained electron-dense, membrane-bound granules, similar to those observed inside catecholamine-secreting cells of the adrenal medulla; the other cell type possessed large amounts of smooth endoplasmic reticulum and mitochondria with tubulo-vesicular cristae. These features are characteristic of cells which are actively synthesizing steroid hormones. The concentration of cortisol was 0.58 ng/adrenal in the wallaby at birth. The fetal lungs near term were at the glandular stage of development, and epithelial differentiation of type I and type II pneumocytes was imminent although attenuation was not evident. The canalicular neonatal lung did not contain true alveoli, but type II pneumocytes contained osmiophilic lamellar inclusions of surfactant. The fetal pituitary and adrenal are functional at birth and are thus capable of initiating parturition and of influencing lung maturation in the fetus.  相似文献   
100.
Mucus glycoproteins from newborn and adult rat small intestine were radiolabelled in vivo with Na2 35SO4 and isolated from mucosal homogenates by using Sepharose 4B column chromatography followed by CsCl-density-gradient centrifugation. Non-covalently bound proteins, lipids and nucleic acids were not detected in the purified glycoproteins. Amino acid, carbohydrate and sulphate compositions were similar to chemical compositions reported for other intestinal mucus glycoproteins, as were sedimentation properties. There were, however, important differences in the chemical and physical characteristics of the mucus glycoproteins from newborn and adult animals. The buoyant density in CsCl was higher for the glycoproteins from newborn rats (1.55 g/ml versus 1.47 g/ml). On sodium dodecyl sulphate/polyacrylamide/agarose-gel electrophoresis, the glycoprotein from newborn rats had a greater mobility than the adult-rat sample. Although both preparations had similar general amino acid compositions, variations were observed for individual amino acids. The total protein content was greater in the glycoprotein from newborn animals (27%, w/w, versus 18%, w/w). The molar ratio of carbohydrate to protein was less in the newborn, primarily owing to a decreased fucose and N-acetylgalactosamine content. Comparison of the molar ratio of fucose and sialic acid to galactose for both glycoproteins demonstrated a reciprocal relationship similar to that described by Dische [(1963) Ann. N.Y. Acad. Sci. 106, 259-270]. The sulphate content was greater in the glycoprotein from newborn rats (5.5%, w/w, versus 0.9%, w/w). Both had similar sedimentation coefficients in a dissociative solvent. These results suggest an age-related difference in the types of mucus glycoproteins synthesized by small intestine.  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