全文获取类型
收费全文 | 7640篇 |
免费 | 1667篇 |
国内免费 | 967篇 |
出版年
2024年 | 84篇 |
2023年 | 377篇 |
2022年 | 373篇 |
2021年 | 540篇 |
2020年 | 493篇 |
2019年 | 490篇 |
2018年 | 387篇 |
2017年 | 417篇 |
2016年 | 344篇 |
2015年 | 344篇 |
2014年 | 476篇 |
2013年 | 601篇 |
2012年 | 338篇 |
2011年 | 371篇 |
2010年 | 299篇 |
2009年 | 372篇 |
2008年 | 382篇 |
2007年 | 402篇 |
2006年 | 333篇 |
2005年 | 315篇 |
2004年 | 268篇 |
2003年 | 271篇 |
2002年 | 215篇 |
2001年 | 168篇 |
2000年 | 148篇 |
1999年 | 145篇 |
1998年 | 118篇 |
1997年 | 100篇 |
1996年 | 103篇 |
1995年 | 91篇 |
1994年 | 88篇 |
1993年 | 71篇 |
1992年 | 68篇 |
1991年 | 67篇 |
1990年 | 46篇 |
1989年 | 47篇 |
1988年 | 39篇 |
1987年 | 47篇 |
1986年 | 36篇 |
1985年 | 62篇 |
1984年 | 48篇 |
1983年 | 37篇 |
1982年 | 56篇 |
1981年 | 36篇 |
1980年 | 26篇 |
1979年 | 25篇 |
1978年 | 29篇 |
1977年 | 24篇 |
1976年 | 16篇 |
1975年 | 12篇 |
排序方式: 共有10000条查询结果,搜索用时 250 毫秒
61.
Most previously published electromyographic (EMG) studies have indicated that the temporalis muscles in humans become almost
electrically quiet during incisai biting. These data have led various workers to conclude that these muscles may contribute
little to the incisai bite force. The feeding behavior and comparative anatomy of the incisors and temporalis muscles of certain
catarrhine primates, however, suggest that the temporalis muscle is an important and powerful contributor to the bite force
during incision. One purpose of this study is to analyze the EMG activity of the masseter and temporalis muscles in both humans
and macaques with the intention of focusing on the conflict between published EMG data on humans and inferences of muscle
function based on the comparative anatomy and behavior of catarrhine primates. The EMG data collected from humans in the present
study indicate that, in five of seven subjects, the masseter,anterior temporalis, and posterior temporalis muscles are very active during apple incision (i.e., relative to EMG activity
levels during apple and almond mastication). In the other two human subjects the EMG levels of these muscles are lower during
incision than during mastication, but in no instance are these muscles ever close to becoming electrically quiet. The EMG
data on macaques indicate that, in all six subjects, the masseter, anterior temporalis, and posterior temporalis muscles are
very active during incision. These data are in general agreement with inferences on muscle function that have been drawn from
the comparative anatomy and behavior of primates, but they do not agree with previous experimental data. The reason for this
disagreement is probably due to differences in the experimental procedure. In previous studies subjects simply bit isometrically
on their incisors and the resulting EMG pattern was compared to the pattern associated with powerful clenching in centric
occlusion. In the present study the subjects incised into actual food objects, and the resulting EMG pattern was compared
to the pattern associated with mastication of various foods. It is not surprising that these two procedures result in markedly
different EMG patterns, which in turn result in markedly different interpretations of jaw-muscle function. In an attempt to
explain the evolution of the postorbital septum in anthropoids, it has been suggested that the anterior temporalis is more
active than the masseter during incision (Cachel, 1979). The human and macaque EMG data do not support this hypothesis; during
incision, the two muscles show no consistent differences in humans and the masseter appears to be in fact more active than
the anterior temporalis in macaques. 相似文献
62.
Regulation of the loss of frost hardiness in Pinus radiata by photoperiod and temperature 总被引:2,自引:1,他引:1
Abstract. In controlled environments, the interactive effects of warm (16: 8°C, day: night) and cool (12: 4°C, day: night) temperatures and long (13.5 h) and short (10 h) photoperiods on the dehardening of seedlings of Pinus radiata D. Don were investigated. In another experiment, the effect of four photoperiods from 9 to 14 h was examined. In a third, dehardening at constant temperatures from 5 to 17°C was followed. There was no evidence for an interaction between photoperiod and temperature. Dehardening was temporarily delayed by photoperiods below about 10 h, but there was no other quantitative effect of photoperiod. At constant temperatures, the rate of dehardening was initially constant but declined as the minimum summer frost hardiness was reached. In the initial phase the rate of dehardening was a linear function of temperature, increasing from 0.05°C day−1 at 8°C to 0.30 °C day−1 at 17°C. Temperature controlled the loss of frost hardiness by regulating the rate of dehardening. 相似文献
63.
Nonrandom insertion of Tn5 into cloned human adenovirus DNA 总被引:4,自引:0,他引:4
64.
Synopsis Laboratory studies were conducted on 15 schools of blackchin shiners, Notropis heterodon, to determine if they altered their structure in response to changing environmental demands. The hypothesis tested was that fish schools should sacrifice a flat, hydrodynamically efficient structure in favour of an unobstructed visual field in the presence of a predator by staggering in the vertical plane. Ten schools were exposed for two weeks to a simple environment with only a current. For the next two-week period a predator was added. Five control schools were exposed to the simple environment for both two-week periods. Six of the ten treated schools increased their staggering in the vertical dimension as predicted while none of the control schools changed. This result was suggestive that hydrodynamic advantages were sacrificed. Respirometer experiments indicated these fish were capable of achieving some hydrodynamic benefits from schooling but these benefits may be a function of fish size. 相似文献
65.
A direct radioimmunoassay for measuring urinary 20-hydroxyprogesterone cross-reactivity to monitor and assess luteal function and detect pregnancy in the lion-tailed macaque (Macaca silenus) is described. Urine samples were collected daily during ten nonconceptive and five conceptive ovarian cycles of five dult female lion-tailed macaques. Urine was analyzed for concentrations of 20α-hydroxypro-gesterone cross-reactivity, estrone conjugates, and creatinine. The strength of the luteal phase in normal nonconceptive cycles (n = 8) is characterized by a maximum sevenfold increase (day 9) in mean 20α-hydroxyprogesterone cross-reactivity over follicular phase levels; the duration, by a 13-day sustained elevation of mean 20α-hydroxyprogesterone cross-reactivity levels. Pregnancy is detectable from 20α-hydroxyprogesterone cross-reactivity values approximately 20 days after the periovulatory estrone conjugate peak (n = 4). Apparent anovulation (n = 1), extended follicular phase (n = 1), and early abortion (n = 1) also are detectable using 20α-hydroxyprogesterone cross-reactivity measurements. 相似文献
66.
67.
Takashi Tsuchiya Atsushi Ichimura Yoshinori Nagai 《Cell biochemistry and biophysics》1987,11(1):109-122
Correlations in the baker map and the tent map as examples of one-dimensional, fully developed chaos are considered. It is
shown, utilizing symbolic dynamical systems derived from these maps, that the vanishing second-order correlation function
is not sufficient to guarantee uncorrelatedness. Importance of the higher-order, especially third-order, correlation functions
is emphasized for chaotic systems. In search of the quantities that grasp correlational behaviors as a whole in chaotic systems,
it is proposed to use the fixed-separation correlation integral, which is a modified quantity of the usual correlation integral
devised to calculate the fractal dimension of strange attractors, for these maps. It is shown that the new quantity contains
all the even-number orders of autocorrelation function that are commonly considered. 相似文献
68.
Abstract Three proteins from Halobacterium marismortui , malate dehydrogenase (hMDH), glutamate dehydrogenase (hGDH) and ferredoxin (hFD) were purified and characterized with respect to their molecular masses, amino acid composition and, for hFD only, primary structure. Striking features of halophilic proteins are: the high excess of acidic over basic residues; acidic clusters in the sequence. Low-salt concentration causes inactivation and changes in structural parameters of hMDH and hGDH. Reactivation of hMDH involves long-lived stable intermediates. The salt concentration optimum of enzymic activity is independent of salt nature. The high capacity of halophilic proteins to retain water and salt is due to unique molecular properties, studied by physico-chemical techniques. 相似文献
69.
The relationship between NADPH-dependent lipid peroxidation and degradation of cytochrome P-450 in adrenal cortex mitochondria 总被引:2,自引:0,他引:2
J Klimek A P Schaap T Kimura 《Biochemical and biophysical research communications》1983,110(2):559-566
The relationship between NADPH-dependent lipid peroxidation and the degradation of cytochrome P-450 has been studied in bovine adrenal cortex mitochondria. Malondialdehyde formation is accompanied by a corresponding decrease in total cytochrome P-450 content. Inhibitors of lipid peroxidation also prevent the loss of cytochrome P-450, further demonstrating a direct relationship between NADPH-dependent lipid peroxidation and degradation of P-450. To differentiate between cytochrome P-450(11)beta and P-450scc, steroid-induced difference spectra were used to evaluate P-450 degradation. These measurements provide the first evidence that both P-450's are degraded during NADPH-dependent lipid peroxidation with P-450(11)beta being much more susceptible to this process. 相似文献
70.
E D Gundelfinger 《FEBS letters》1983,157(1):133-138
The interaction between the three Drosophila DNA-dependent RNA polymerases (EC 2.7.7.6) and the DNA template or the RNA product was investigated by photochemical cross-linking and binding studies, using RNA polymerase subunits immobilized on nitro-cellulose filters. It can be shown that the two largest subunits are responsible for the binding of the enzymes to both template and newly-synthesized RNA. 相似文献