全文获取类型
收费全文 | 14425篇 |
免费 | 1459篇 |
国内免费 | 15篇 |
专业分类
15899篇 |
出版年
2022年 | 162篇 |
2021年 | 266篇 |
2020年 | 147篇 |
2019年 | 185篇 |
2018年 | 257篇 |
2017年 | 234篇 |
2016年 | 371篇 |
2015年 | 606篇 |
2014年 | 667篇 |
2013年 | 815篇 |
2012年 | 1057篇 |
2011年 | 1004篇 |
2010年 | 642篇 |
2009年 | 569篇 |
2008年 | 813篇 |
2007年 | 792篇 |
2006年 | 737篇 |
2005年 | 718篇 |
2004年 | 627篇 |
2003年 | 608篇 |
2002年 | 599篇 |
2001年 | 270篇 |
2000年 | 223篇 |
1999年 | 208篇 |
1998年 | 184篇 |
1997年 | 121篇 |
1996年 | 117篇 |
1995年 | 84篇 |
1994年 | 116篇 |
1993年 | 87篇 |
1992年 | 138篇 |
1991年 | 145篇 |
1990年 | 132篇 |
1989年 | 146篇 |
1988年 | 112篇 |
1987年 | 135篇 |
1986年 | 107篇 |
1985年 | 110篇 |
1984年 | 103篇 |
1983年 | 74篇 |
1982年 | 89篇 |
1981年 | 77篇 |
1980年 | 75篇 |
1979年 | 98篇 |
1978年 | 65篇 |
1977年 | 67篇 |
1974年 | 79篇 |
1973年 | 67篇 |
1972年 | 84篇 |
1969年 | 59篇 |
排序方式: 共有10000条查询结果,搜索用时 8 毫秒
71.
Sandra Wegert John Caprio 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》1991,168(2):201-211
1. Receptor sites for different amino acids in the facial taste system of the channel catfish, Ictalurus punctatus, were determined from in vivo electrophysiological cross-adaptation experiments. 2. Relatively independent receptor sites were indicated for L-proline, D-proline, D-arginine, L-histidine and L-lysine, as well as those previously reported for L-alanine, L-arginine and D-alanine. 3. The functional isolation of two nerve twigs that were more responsive to D-alanine than to L-alanine or to other test stimuli provided further evidence for the existence of D-alanine sites that are independent from those to L-alanine. 4. Under all cross-adaptation regimes, the taste responses to the majority of test stimuli were reduced. Various possible mechanisms accounting for this generalized reduction in action potential activity during adaptation are discussed. 相似文献
72.
73.
Potentiation and suppression of mouse liver cytochrome P-450 isozymes during the acute-phase response induced by bacterial endotoxin 总被引:2,自引:0,他引:2
L A Stanley D J Adams R Lindsay R R Meehan W Liao C R Wolf 《European journal of biochemistry》1988,174(1):31-36
Infection and inflammation are known to affect the metabolism and disposition of drugs and carcinogens. We report a detailed study of the effects of bacterial endotoxin on the constitutive and inducible expression and activities of cytochrome P-450 isozymes from families P-450I, P-450IIB, P-450IIC and P-450III. In general high doses of high endotoxin caused very marked suppression of P-450 isozymes and associated activities. However, this effect was differential, the expression of certain isozymes being only slightly reduced whereas others were suppressed to almost undetectable levels. Low doses of endotoxin also gave differential effects on cytochrome P-450 expression. Of particular interest was the very marked potentiation of the inductive effect of both 3-methylcholanthrene and phenobarbital. In the case of 3-methylcholanthrene the 10-fold induction of activity was increased to 24-fold by concomitant endotoxin administration. In this regard it was interesting that 3-methylcholanthrene was an effective inducer of a wide variety of acute-phase proteins including metallothionein, serum amyloid A, fibrinogen and hemopexin. These data show that endotoxin, and therefore bacterial infection and inflammation, can have profound and differential effects on components of the cytochrome-P-450 monooxygenase system which could result in significant changes in susceptibility to the effects of drugs, chemical toxins and carcinogens. 相似文献
74.
Sandra Knapp 《Brittonia》1986,38(1):89-91
Solanum malacothrix S. Knapp is described from the slopes of the Sierra Madre del Sur in the Río Balsas drainage in the state of Guerrero. It is apparently rare, and has been collected only twice. Its relationship to other species in Mexico and northern South America is discussed. 相似文献
75.
Cardiac hypertrophy was assessed in adrenal medullectomized and sham control rats given ethanol in a liquid diet mixture, intragastrically, in severely intoxicating doses at 8-h intervals for 48 and 96 h. The ethanol treatments produced increases of some 20% in wet and dry proportional heart weights in the control animals, but this rapid development of hypertrophy was less apparent in the medullectomized rats. Hearts of the medullectomized animals given ethanol were heavier than those of pair-matched controls given isocaloric maltose-dextrin in the diet mix. These increases were not statistically significant. The ethanol treatments produced, in addition, equivalent increases in the weight of the adrenal glands of both medullectomized and control animals. Medullectomy was evaluated by analysis of adrenal glands and urine samples for catecholamines. The majority of adrenals from the demedullated group had nondetectable amounts of catecholamines and minimal quantities were found in the remainder. Adrenaline was not detected generally in urine samples from this group. The adrenaline contents of adrenals from sham controls given ethanol were markedly reduced by the ethanol treatments, whereas the quantities in urine were many times greater than those from rats given maltose-dextrin. Urinary noradrenaline levels were increased in both control and medullectomized rats given ethanol. The results of this study identify that adrenal medullary catecholamines participate in the rapid development of cardiac hypertrophy that results from severe, continuing ethanol intoxication in the rat. 相似文献
76.
Isozyme electrophoresis of up to 55 loci, and microcomplement fixation of albumin were used to assess the extent of structural gene divergence among karyotypic forms of Australian Rattus. The results show that the Australian Rattus are monophyletic with respect to R. rattus or R. norvegicus. Within the Australian Rattus, rates of chromosomal evolution have varied enormously, the highest rates being found among members of the R. sordidus group, where extensive chromosomal repatterning has occurred with little or no structural gene divergence. 相似文献
77.
D R Adams 《The American journal of anatomy》1986,176(2):159-170
To determine the extent and ultrastructure of epithelium lining the transitional nasal mucosa of the neonate, gnotobiotic calf tissues were prepared for scanning and transmission electron microscopy. Stratified cuboid epithelium of the rostral 40% of the nasal cavity contained few ciliated cells; the next caudal 10-15%, although ciliated, had extensive nonciliated areas. The predominant type of surface cell was nonciliated, had short microvilli, and contained a multilobate nucleus and numerous pinocytotic vesicles. In some areas the surface of these cells presented a cobblestone appearance. Basal cells contained numerous bundles of filaments, ribosomes, and basal vesicles. Caudally, nonciliated columnar cells included a cell type similar to the more rostral cuboid cell, as well as brush cells and immature secretory and ciliated cells. Goblet cells were infrequently observed. Intraepithelial nerve terminals were abundant. Other intraepithelial cells, often difficult to identify owing to varying characteristics, included lymphocytes. Based upon comparisons of this neonatal epithelium with mature epithelium, observed in earlier studies of other mammalian species, the transitional mucosa is believed normally to occupy an extensive area of the nasal cavity. 相似文献
78.
MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) produces an irreversible parkinsonism in primates. Recent evidence suggests metabolism of MPTP to 1-methyl-4-phenylpyridine (MPP+) is required for toxicity. We have proposed that MPP+ may play a central role in the toxicity of MPTP, but direct assessment of the effects of MPP+ in brain is difficult. Therefore, we have sought to define the mechanism of peripheral MPP+ toxicity in the rat and mouse. Systemically administered MPP+ produced its major pathology in the lung and was typified by perivascular edema. An increase in plasma glutathione disulfide concentrations also resulted, suggesting that MPP+ in analogy to paraquat produces oxidative stress. In addition, the lethality of MPP+ in the mouse was increased by dietary selenium deficiency. These results define in both pathological and chemical terms the potent systemic toxicity of MPP+ and suggest that MPP+, because of its high concentration in primate brain, has the potential to play an important role in the CNS toxicity of MPTP. 相似文献
79.
Sandra J. Walde 《Oecologia》1986,69(2):243-247
Summary A model derived from marine research, and recently applied to stream communities, suggests that community structure is more likely to be influenced by predators in benign versus harsh abiotic regimes. Experiments were conducted to determine if increasing the harshness of a particular regime would alter the impact of a stream invertebrate predator on prey densities in field enclosures. Density of a stonefly predator, Kogotus nonus, was varied in containers exposed to low (benign) and high (harsh) levels of fine sediment. As predicted by the model, the harsher regime eliminated predator effects in two of three experiments. In the third experiment, however, high levels of sediment actually enhanced the impact of the predator on the prey community. A consideration of the possible mechanisms underlying this model led to the conclusion that increasing the harshness of a regime can be expected to produce outcomes ranging from elimination to enhancement of predator effects, depending on how the regime is perceived by the predator versus the prey. 相似文献
80.
Laboratory studies on salmonella-contaminated cheese involved in a major outbreak of gastroenteritis
Dried biomass of Streptomyces cyaneus NCIB 9616 harvested at 1, 3, 7, 10 and 14 d from a liquid culture was degraded by alkaline methanolysis and the fatty acid methyl esters released examined quantitatively by gas chromatography. Isoprenoid quinones were extracted with petroleum ether and methanolic saline, purified by thin-layer chromatography and quantified by high performance liquid chromatography. Data for each component were plotted against time. The results showed that at the beginning of logarithmic growth there was a small quantitative shift in the fatty acid composition but, the fatty acid profiles remained relatively constant during both the logarithmic and stationary phases of growth. In contrast, the isoprenoid quinone composition depended markedly on the stage of growth. These results indicate that valid comparison of isoprenoid quinone profiles requires that sampling take place at defined stages of the growth cycle. 相似文献